Causes and Mechanism Analysis of Delamination During Sanding of Water-Resistant Putty

Introduction

After water-resistant putty has dried, sanding does not always produce uniform powdering or an evenly flattened surface. In some cases, the putty is lifted off in patches by the sandpaper, with localized peeling, delamination, or shelling. Many people simply attribute this to the putty being too hard, the sandpaper being too coarse, sanding being done too late, or poor product quality. These factors may all contribute, but the underlying cause is that weak points already exist within the putty layer or at an interface. During sanding, the lateral friction and shear force generated by the sandpaper do not uniformly abrade the putty surface; instead, they tear the putty layer open along its weakest point.

Delamination during sanding of water-resistant putty is not simply a matter of a “surface that is difficult to sand.” It is the result of the combined effects of material hardening, interlayer bonding, substrate condition, application thickness, and sanding timing. Among common water-resistant putties, lime-calcium-based and cement-based systems are two typical inorganic binder systems. Both can provide relatively high water resistance and surface strength, but they develop strength in different ways, so the causes of sanding-related delamination also differ.

1. Powdering, Difficult Sanding, and Sanding Delamination Are Not the Same Thing

Problems that occur during sanding may all appear to be related to sandpaper on the surface, but their underlying causes are different.

Powdering: The overall binding strength of the putty is insufficient, so it turns into a large amount of powder as soon as it is sanded.

Difficult sanding: The surface hardness of the putty is too high, resulting in low sanding efficiency. The surface is hard to sand, or sanding proceeds very slowly.

Sanding delamination: The putty is not uniformly abraded away; instead, it is lifted, peeled, or removed in sheets along a weak layer. When the shear, peeling, and vibration generated during sanding exceed the internal cohesion of the putty or the adhesion at an interface, the weak interface fails first. Delamination depends on whether the putty is internally uniform, whether the layers are firmly bonded, whether the substrate is stable, and how much shear force is generated during sanding.

2. Lime-Calcium-Based Water-Resistant Putty: The Surface Tends to Carbonate First, While Internal Hardening May Lag Behind

The binding strength of lime-calcium-based water-resistant putty mainly comes from calcium hydroxide in the lime-calcium component. After carbon dioxide in the air enters the pores of the putty, it must dissolve in pore water or a water film, and then react with a small amount of dissolved calcium hydroxide through carbonation to form calcium carbonate crystals. Calcium carbonate gradually fills and bonds the internal structure of the putty, allowing the putty layer to develop hardening strength.

The overall reaction is:

Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O

Although water is not consumed on a net basis, in the actual putty layer it serves as the medium for CO₂ dissolution, Ca(OH)₂ dissolution, and ion migration. When the putty is too dry, the reaction medium is insufficient, and the carbonation rate may decrease. When the putty remains excessively wet for a long time, CO₂ diffusion is hindered, and internal carbonation also slows down.

This reaction depends on CO₂ from the air entering the putty layer, so carbonation in lime-calcium-based putty usually progresses from the surface inward. The surface layer is the first to contact the air, the first to undergo carbonation, and the first to form a hardened calcium carbonate structure. As the surface gradually becomes harder and denser, the continued inward migration of CO₂ and moisture slows down, causing internal carbonation to lag behind. As a result, strength development inside the layer is slower than at the surface.

CO₂ in the air

↓

[Surface layer: relatively sufficient carbonation, increased CaCO₃, higher hardness]

[Middle layer: insufficient carbonation, delayed strength development]

[Bottom layer: affected by substrate water absorption, humidity, and application thickness, resulting in uneven hardening]

When the putty is applied too thickly, the lime-calcium content is high, the site humidity is high, or ventilation is insufficient, the difference in hardening between the surface and the interior becomes more pronounced. The surface may already be very hard, while the interior has not yet formed a sufficiently continuous and stable strength structure. During sanding, the sandpaper first acts on the harder surface layer. The harder the surface, the more difficult it is to cut uniformly, and the greater the lateral pulling force generated by the sandpaper. Once this force is transmitted to a weaker area inside the putty layer, the putty may be lifted along the weak layer, resulting in peeling, delamination, or sheet-like detachment.

The “hard outside, weak inside” condition of lime-calcium-based putty is not necessarily caused only by differences in carbonation depth. It may also be related to rapid water absorption by the substrate, insufficient water-retention system, drying-shrinkage gradients caused by thick application, insufficient polymer film formation, and unreasonable filler grading.

3. Cement-Based Water-Resistant Putty: Hydration Hardening Increases Sanding Resistance

The strength of cement-based water-resistant putty mainly comes from the hydration reaction between cement and water. After cement clinker minerals react with water, hydration products such as calcium silicate hydrate gel and calcium hydroxide are formed. Among them, C-S-H gel is an important source of strength in cement-based materials. Here, C, S, and H are shorthand notations in cement chemistry, representing CaO, SiO₂, and H₂O, respectively.

The mechanism can be represented as follows:

Cement clinker minerals + H₂O → C-S-H gel + Ca(OH)₂ + other hydration products

When moisture conditions are suitable and unhydrated cement particles remain in the system, cement hydration can continue for a period of time after application. As hydration products gradually increase, the putty layer becomes harder and denser, and sanding resistance also increases. If the environment is too dry, the putty layer is very thin, or water is lost too quickly, later-stage hydration will also be significantly slowed.

When cement-based putty has just become fully dry, it is usually still relatively easy to sand. However, if sanding is carried out several weeks or even months later, especially in systems with a higher cement content or more sufficient curing moisture, the putty will usually have hardened further, and the abrasive resistance against the sandpaper will increase noticeably. At this stage, sanding is no longer just a light cutting action; it generates stronger friction, vibration, and shear on the putty surface. If weak points exist within the putty, between layers, or at the substrate interface, delamination, peeling, or sheet-like detachment can easily occur during sanding.

The typical path of sanding delamination in cement-based putty is:

Continued hydration reaction → Increased hardness and density of the putty → Increased sanding resistance and shear force → Weak interfaces exist within the putty, between layers, or at the substrate → Sandpaper-induced shear force is released along the weak interface → Peeling, delamination, or sheet-like detachment occurs

If the cement content is too high and the formulation lacks sufficient polymer powder, water-retention system, and flexibility design, the putty is likely to become hard, brittle, and difficult to sand. During later-stage sanding, the shear force generated by the sandpaper increases significantly. Once the substrate has not been properly treated, the bond between two putty coats is poor, or a single coat has been applied too thickly, delamination can easily occur along a weak interface.

4. Identifying the Source of the Problem Based on the Delamination Location

Delamination of water-resistant putty does not necessarily occur only inside the putty layer. It may occur in three locations: within the putty itself, between two putty coats, or between the putty and the substrate.

Shear force generated by sanding → The putty surface is subjected to lateral pulling → The force is transmitted inward and to the interfaces → The weakest location cracks first → Delamination, peeling, or shelling occurs

If delamination occurs within the putty itself, it is usually related to excessive thickness in a single coat, asynchronous drying between the surface and interior, uneven carbonation of the lime-calcium component, uneven cement hydration and shrinkage, or a formulation that is too hard and brittle.

If delamination occurs between two putty coats, it is usually related to an excessively long interval between applications, an overly hardened surface on the previous coat, unremoved floating dust, or the failure to form effective interlayer bonding.

If delamination occurs between the putty and the substrate, it is usually related to substrate powdering, floating dust, moisture return, poor adhesion of old paint films, abnormal substrate water absorption, or inadequate interface treatment.

For on-site diagnosis, the condition of the peeled surface can be observed. If putty powder is present on both sides of the peeled surface, the problem is usually more likely to be insufficient internal cohesion of the putty or a strength difference between the surface and the interior. If the upper layer comes off in sheets while the lower surface is smooth or covered with floating dust, poor interlayer bonding is often the cause. If mortar, old paint film, or a powdery substrate is exposed after peeling, the problem is often related to adhesion failure at the substrate. When necessary, small-area trial sanding, tape pull-off testing, substrate powdering checks, and moisture content checks can be combined for a more comprehensive assessment.

5. Why Is Putty with Stronger Water Resistance Sometimes More Difficult to Sand?

Water-resistant putty needs to have sufficient water resistance, adhesion, and strength, but this does not mean that the harder it is, the better it is. When a formulation relies too heavily on inorganic binders such as lime-calcium or cement, the surface hardness of the putty may increase, and its water resistance may also improve, but its sandability may decrease.

When the putty is too hard or too brittle, the sandpaper cannot easily cut the surface uniformly. The sanding process then generates greater lateral pulling force. As long as weak points exist inside the putty or at an interface, “difficult sanding” can easily develop into “sanding delamination.”

A putty suitable for wall systems needs to strike a balance among multiple properties: water resistance; adhesion; cohesion; flexibility; shrinkage control; sandability; and compatibility with subsequent coatings. Base leveling materials may place greater emphasis on strength, water resistance, and adhesion, while finish putty needs to give greater priority to fineness, sandability, shrinkage control, and coating compatibility. When latex paint, decorative coatings, or high-flatness finishes will be applied afterward, the final coat is better suited to a finish-leveling putty with good sandability, low shrinkage, reasonable alkalinity control, and compatibility with the subsequent coating system.

6. Key Factors That Commonly Lead to Sanding Delamination of Water-Resistant Putty

6.1 Excessive Thickness in a Single Coat

The thicker the putty layer, the more likely it is that the drying rate, carbonation rate, hydration process, and shrinkage state will differ between the surface and the interior. The surface may appear dry and hardened, while the interior may still lack sufficient strength. During sanding, a structure that is hard on the surface but weak inside can easily be torn apart by the shear force of the sandpaper.

6.2 Excessive Lime-Calcium or Cement Content

When the lime-calcium or cement content is too high, the putty can easily develop high surface hardness. However, if water retention, bonding, flexibility, and shrinkage control are not optimized at the same time, the material can become hard, brittle, and difficult to sand. The greater the sanding resistance, the stronger the shear force, and the higher the risk of delamination.

6.3 Insufficient Water-Retention and Bonding System

Putty systems usually require additives such as cellulose ethers and redispersible polymer powders to improve water retention, workability, adhesion, and cohesion. Insufficient water retention affects the reaction medium required for lime-calcium carbonation and the stability of cement hydration. It may also cause the surface to lose water too quickly and lead to uneven strength development within the layer. Insufficient bonding and flexibility systems reduce the putty’s ability to resist sanding-induced shear.

6.4 Inadequate Interlayer Treatment

If the previous putty coat has been left for too long, its surface may become excessively hardened, covered with dust, or contaminated. If the second coat is applied directly in this condition, a weak interlayer interface can easily form. During later sanding, the sandpaper may lift the upper layer along this weak interface.

6.5 Unstable Substrate Condition

Substrate powdering, floating dust, oil contamination, loose old paint films, weak mortar layers, and moisture return in the wall can all weaken the adhesion between the putty and the substrate. Many problems that appear to be putty delamination are actually rooted in the substrate.

6.6 Unsuitable Environmental Humidity and Ventilation

Carbonation of lime-calcium-based putty requires an appropriate amount of moisture, but long-term excessive wetness can hinder CO₂ ingress and moisture migration. Hydration of cement-based putty requires water, but long-term humidity, moisture return from the substrate, or insufficient ventilation may cause delayed drying, leave the interface in a weakened state for an extended period, and increase the risk for subsequent coating application. High temperatures and strong airflow can cause the surface to lose water too quickly, which may also result in the surface hardening first while the interior lags behind.

6.7 Improper Sanding Timing

If sanding is carried out too early, the overall strength of the putty is insufficient, making it prone to powdering, surface roughening, and delamination. If sanding is carried out too late, especially in cement-based or high lime-calcium systems, the putty hardness may have increased significantly, sanding resistance becomes greater, the shear force from the sandpaper increases, and potential weak interfaces are more easily torn open.

A relatively safe principle is to sand the putty promptly after it has fully dried, avoiding both premature sanding and excessive delay. In common construction practice, 24–48 hours may be used as an empirical reference, but the specific timing should still be determined based on the product instructions, temperature and humidity, ventilation conditions, application thickness, and the actual through-dry condition on site.

6.8 Overly Aggressive Sanding Method

Overly coarse sandpaper, high-speed mechanical sanding, and heavy-pressure forceful sanding can all significantly increase sanding shear force. When working with relatively hard water-resistant putty, a small-area trial sanding should be carried out first. Based on the powdering condition, surface hardness, and whether peeling occurs, the sandpaper grit, sanding pressure, and sanding method should then be adjusted.

7. Classification Tables of Representative Reagents and Model Materials Related to Mechanism Studies of Sanding Delamination in Water-Resistant Putty

The products listed below are mainly intended for scientific verification, mechanism analysis, or formulation model studies. They are not equivalent to recommended additives for engineering putty formulations. Actual formulations should be verified based on industrial-grade raw materials, system compatibility, application performance, and relevant standard requirements.

Table 1. Products Related to Binding Reactions and Hardening Mechanisms

Category	CAS No.	Aladdin Catalog No.	Name	Specification or Purity	Product Features and Applications
Reaction precursor for lime-calcium systems	1305-78-8	C420198	Calcium oxide	Reagent grade	Used for studies on lime-calcium preparation, slaking reactions, residual free calcium oxide, and volume stability; related to the analysis of later-stage expansion of putty, abnormal surface hardening, and internal layer stress.
Main binder in lime-calcium-based systems	1305-62-0	C491881	Calcium hydroxide	≥99%	Used for studies on carbonation hardening, alkalinity changes, surface calcium carbonate structure formation, and the “hard surface but weak interior” phenomenon in lime-calcium-based water-resistant putty; suitable for experiments on carbonation depth, strength gradients, and sanding delamination mechanisms.
Carbonation product and carbonate filler	471-34-1	C432736	Calcium carbonate	≥99.95% metals basis	Used for studies on lime-calcium carbonation products, carbonate fillers, filling compactness, and sanding resistance; related to the evaluation of putty whiteness, hardness, powdering state, surface densification, and sandability.
Early-strength hydration mineral in cement-based systems	12168-85-3	T926519	Tricalcium silicate / Tricalcium silicate (C₃S)	—	Used for studies on early hydration, early strength development, and hardening rate in cement-based systems; related to the analysis of sanding window after through-drying, surface hardening, and shear-induced delamination risk in putty.
Later-stage hydration mineral in cement-based systems	10034-77-2	D139919	Dicalcium silicate	Biomedical grade, ≥98%, <30 μm	Used for studies on later-stage hydration, continuous strength development, and age-related hardening in cement-based systems; related to hardness increase, sanding resistance changes, and weak-interface peeling after cement-based putty has been left standing.
Cement setting regulation and gypsum system	10101-41-4	C101878	Calcium sulfate dihydrate	AR, ≥99%	Used for studies on cement setting regulation, gypsum-involved setting processes, filling structures, and early strength; related to open time during application, interlayer bonding, and sanding-window stability.
Fast-setting gypsum-based binder system	10034-76-1	C302031	Calcium sulfate hemihydrate	≥97%	Used for studies on setting and hardening, early strength, and drying shrinkage of gypsum-based materials; related to differences between surface and internal hardening, sanding timing, and interlayer delamination in fast-setting systems.

Table 2. Products Related to Filler Framework, Fineness, and Sanding Structure

Category	CAS No.	Aladdin Catalog No.	Name	Specification or Purity	Product Features and Applications
White hiding and finish-layer filler	13463-67-7	T431947	Titanium(IV) oxide	Premium grade, ≥99%	Used for studies on whiteness, hiding power, surface fineness, and coating compatibility of finish putty; related to post-sanding appearance quality, finish uniformity, and evaluation of finish-layer systems.
Lamellar lubricating filler	14807-96-6	T109493	Talc powder	Pharmaceutical grade, PharmPure™, ≥325 mesh	Used for studies on application lubricity, fineness, filling properties, and sanding feel of putty; related to sandpaper abrasion resistance, surface flatness, and powdering-state evaluation.
Aluminosilicate framework filler	1332-58-7	K299133	Kaolin	Filler grade, kaolinite content ≥80%	Used for studies on putty filling structure, suspension stability, hiding performance, and application properties; related to filler gradation, drying shrinkage, internal strength uniformity, and sanding delamination analysis.
Lamellar reinforcing filler	12001-26-2	B302554	Biotite	100 mesh	Used for studies on the influence of lamellar minerals on crack resistance, shielding properties, and dimensional stability of putty; related to shrinkage-stress dispersion, internal structural stability, and interface peel-resistance evaluation.
Siliceous hard filler	7631-86-9	S104604	Silicon dioxide	≥99.9% metals basis	Used for studies on filling compactness, surface hardness, wear resistance, and sanding resistance of putty; related to filler fineness, hardness gradients, and sandpaper abrasion behavior.
Bentonite thixotropic stabilizer	1302-78-9	N431707	Nanoclay, hydrophilic bentonite	—	Used for studies on thixotropy, suspension stability, sag resistance, and thick-application stability of putty; related to single-coat thick application, differences between surface and internal drying, internal weak layers, and application stability evaluation.
Fibrous mineral thixotropic filler	63800-37-3	S304245	Sepiolite powder	≥400 mesh	Used for studies on thixotropy, water retention, anti-settling properties, and structural support of putty; related to thick-application stability, internal strength uniformity, and sanding delamination risk.

Table 3. Products Related to Water Retention, Thickening, Workability, and Polymer Bonding Modification

Category	CAS No.	Aladdin Catalog No.	Name	Specification or Purity	Product Features and Applications
Water-soluble polymer bonding modification	9002-89-5	P434371	Poly(vinyl alcohol) (PVA)	Mw 89,000–98,000, 99% hydrolyzed	Used for studies on putty adhesion, film formation, cohesion, and interlayer bonding; related to weak-interface peeling, powdering, surface peeling, and delamination under sanding shear.
Anionic cellulose ether for water retention and thickening	9004-32-4	C104986	Sodium carboxymethyl cellulose (CMC)	Viscosity: 1000–1400 mPa·s, USP grade	Used for studies on water retention, thickening, dispersion stability, and slurry uniformity of putty; related to moisture retention, internal strength uniformity, differences between surface and internal hardening, and sanding powdering-state evaluation.
Starch ether for workability modification	9049-76-7	H304935	Hydroxypropyl starch ether	Viscosity (5% aqueous solution, 20°C): 500–20,000 mPa·s	Used for studies on application feel, sag resistance, application-thickness control, and thixotropy of putty; related to thick application, differences between surface and internal drying, internal weak layers, and sanding delamination analysis.
Ethylene-vinyl acetate copolymer for flexibility modification	24937-78-8	P432376	Poly(ethylene-co-vinyl acetate) (PEVA)	Vinyl acetate 12 wt.%; melt index 8 g/10 min (190°C/2.16 kg)	Used for model studies on polymer film formation, flexibility modification, and interfacial bonding; related to sanding shear, weak-interface peeling, and crack resistance evaluation in hard and brittle systems.
Nonionic cellulose ether for water retention and thickening	9004-62-0	H434474	2-Hydroxyethyl cellulose (HEC)	Average Mv ~90,000	Used for studies on water retention, thickening, open time, and surface-state regulation of putty; related to moisture migration, hardening uniformity, sanding powdering state, and application rheology evaluation.
Nonionic cellulose ether for water retention and open-time control	9004-65-3	H434480	Hydroxypropyl methylcellulose (HPMC)	Average Mn ~86,000	Used for studies on water retention, thickening, workability, and open time of putty; related to cement hydration, lime-calcium carbonation, application-thickness control, and internal strength development.
Vinyl acetate polymer for film-forming bonding	9003-20-7	P304881	Poly(vinyl acetate) (PVAC)	Approx. M.W. 500,000	Used for studies on film formation, adhesion, cohesion, and interlayer adhesion of putty; related to sanding shear force, weak-interface cracking, sheet-like peeling, and interlayer bonding evaluation.
High-viscosity cellulose ether for water retention and thickening	9004-67-5	M112869	Methyl cellulose (MC)	100,000 mPa·s	Used for studies on water retention, thickening, workability, and slurry stability of putty; related to drying rate, differences between surface and internal hardening, sandability, and internal structural stability evaluation.

Table 4. Products Related to Setting Regulation, Retardation, Hydrophobic Assistance, and Failure Detection

Category	CAS No.	Aladdin Catalog No.	Name	Specification or Purity	Product Features and Applications
Alkalinity and carbonation indicator	77-09-8	P108714	Phenolphthalein	Indicator	Used for qualitative indication experiments on alkalinity changes and carbonation zones in lime-calcium-based and cement-based materials; related to the assessment of surface carbonation, delayed internal carbonation, carbonation depth, and alkalinity changes.
Cement-based retarder and calcium-ion complexing agent	527-07-1	G278703	D-Sodium gluconate	≥99%	Used for studies on retardation, calcium-ion complexation, and hydration-rate regulation in cement-based systems; related to application open time, early strength development, hardening rate, and sanding-window analysis.
Acidic complexing and setting-regulation auxiliary	77-92-9	C108869	Citric acid, anhydrous	AR, ≥99.5% (T)	Used for studies on calcium-ion complexation, setting-time regulation, and early reactions in cement or gypsum systems; related to hardening rate, interlayer application interval, early strength, and sanding timing evaluation.
Hydrophobic, lubricating, and interface-influencing additive	1592-23-0	C113301	Calcium stearate	Ca 6.6–7.4%	Used for studies on hydrophobicity, lubrication, water-resistance assistance, and surface-performance regulation of putty; related to water resistance, interlayer adhesion, surface sanding resistance, and interfacial bonding evaluation. Excessive use may affect wettability and interlayer adhesion, and should be verified within the specific system.

Note: The above products are representative Aladdin products related to scientific research and formulation studies. For more product specifications, grades, and COA information, please search by “product name/CAS/catalog number” on the Aladdin website.

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