Initial/mid-term/late stage water resistance is evaluated based on the overall water resistance of the coating according to the length of immersion time;
Water resistant whitening is a specialized phenomenon: the coating turns white, loses its luster, and becomes turbid after immersion in water, with a focus on “whitening”, without necessarily causing blistering or paint peeling.
Applicable systems: water-based acrylic, water-based epoxy, water-based 2KPU, water-based alkyd, water-based styrene acrylic and other water-based industrial paints. In many formulations, the water resistance qualification ≠ water resistance white qualification, which is the easiest point for formulations to step on.
1、 Initial water resistance
Definition: Short term immersion test
Test duration: commonly 2h, 6h, 12h, 24h
Assessment objective: Will the coating quickly absorb water, bubble, lose adhesion, and flash rust on the substrate when it first comes into contact with water
Failure symptoms: rapid foaming, edge water seepage, flash rust, and softening of the paint film
Mechanism: incomplete film formation, multiple hydrophilic groups on the surface, rapid dissolution of cosolvents/hydrophilic additives; The paint film has many micropores, and water quickly penetrates.
· Formula focus: lotion film-forming property, emulsifier residue, wetting agent, cosolvent, flash rust inhibitor, cross-linking density.
Typical scenario: Just after the workpiece is sprayed, it encounters rain, condensation, and short-term soaking, causing problems.
2、 Mid term water resistance
Definition: Medium duration immersion in water
Testing duration: 48h, 72h, 120h, and 168h are the most commonly used levels for industrial paint
Assessment objective: After continuous water absorption, the stability, adhesion, and shielding of the paint film structure
Failure symptoms: slow foaming, decreased adhesion, localized rust spots, swelling and softening of the paint film, not necessarily immediately whitening
Mechanism: Water slowly penetrates into the interior of the paint film and accumulates at the interface; Hydrolysis of weak cross-linking points, continuous migration and precipitation of hydrophilic substances; The interface adhesion is peeled off by water.
The vast majority of water-based industrial paints have national/industry standards, and the water resistance index is mid-term water resistance, which is a routine acceptance index.
3、 Later stage water resistance
Definition: Long term immersion in water
Test duration: 28d, 60d, 90d or even longer, belonging to accelerated aging and long-term evaluation
Assessment objectives: Long term hydrolysis stability of paint film, hydrolysis resistance of cross-linking bonds, long-term shielding and rust prevention ability
Failure manifestations: slow hydrolysis, chain breakage of large molecules, continuous degradation of adhesion, extensive bubbling, substrate corrosion, permanent loss of paint film strength
Mechanism: Long term hydrolysis of ester bonds, amino bonds, and weak polyurethane bonds; Interface debonding of fillers/pigments; After long-term water absorption, the paint film undergoes plasticization and stress accumulation.
Commonly used for heavy-duty anti-corrosion, underwater, and humid conditions, ordinary topcoats generally do not require later water resistance.
4、 Water resistant white
Definition: After the paint film is immersed in water, it may appear white, foggy, dull, semi transparent white, but it does not necessarily mean bubbling, peeling, or rusting!
Core feature: Belongs to optical phenomena, not direct adhesion/rust prevention failure, but belongs to appearance defects
Testing method: synchronized with water resistance, observe whether the paint film turns white after immersion in water; Take it out and dry it, some can be cleaned, some permanently turn white
Two types:
1. Reversible water whitening: Water enters the micro pores of the paint film, causing a difference in refractive index between water and resin, resulting in light scattering and whitening; Take it out and air dry, evaporate the moisture, and restore the transparency/luster of the paint film. Cause of formation: the lotion film is not compact, with many micropores and high water absorption. Many single component acrylic acids are the most common.
2. Irreversible water whitening: hydrophilic additives, low molecular weight substances dissolve, or resin hydrolysis produces polar groups; After drying, it still turns white and foggy, and cannot be returned. Causes: excessive emulsifiers, small molecule thickeners, residual cosolvents, poor hydrolysis resistance of resins, and hydrophilic pigments and fillers.
The key difference between water resistance and water resistance:
Water resistance: foaming, peeling, rusting, adhesion, paint film integrity. Water resistance: whether it will turn white, lose light, or foggy after soaking in water. Many formulas: no foaming or rusting after soaking in water for 7 days, but it turns white as soon as it is soaked in water.
5、 Comparison Summary Table
| project | Classification basis | Focus on the key points | Typical Failure | reversibility |
| Initial water resistance | Short immersion time (≤ 24 hours) | Short term anti-seepage and anti flash rust | Quick foaming, softening, and rusting | Irreversible, once foaming occurs, it is difficult to recover |
| Mid term water resistance | Medium duration (24h ~ 7d) | Interface adhesion, shielding and rust prevention | Slow bubbling, rust spots, and loss of adhesion | Bubbling is basically irreversible |
| Later stage water resistance | Long term immersion (> 7 days) | Resin resistant to hydrolysis and long-lasting anti-corrosion | Hydrolytic chain breakage, extensive rusting and delamination | irreversible |
| Water resistant white | Appearance turns white after immersion in water | Film transmittance / refractive index, micropores, and small molecule precipitation | Whitening, foggy, dull, without bubbles or rust | Reversible (drying and cleaning)Irreversible (permanent whitening) |
6、 The logic of selection in formula (high-frequency pain points of water-based paint)
1. Single component phenylpropanoid/pure propanoid: It is the easiest to achieve water whitening, even if it is resistant to water for 7 days without foaming, it is still difficult to achieve whitening when soaked in water; If you want to resist water whitening, give priority to low soap/soap free lotion, reduce emulsifier, improve film compactness, and crosslink appropriately.
2. Water based 2KPU and water-based epoxy: Water resistant whitening is usually better than single component acrylic acid, but poor formulation can also cause whitening.
3. A misconception: high cross-linking density ≠ certain water resistance to whitening. Crosslinking solves hydrolysis, foaming, and rust prevention; Water white is more related to film density, free hydrophilic small molecules, and porosity.
4. Differences in Testing:
Water resistance record: foaming, peeling, rusting, cracking
Waterproof whitening record: whether whitening occurs, degree of whitening, and whether it is restored after being taken out and dried
Post time: Oct-27-2026