Paint Defects--Water Damage

1．Description

Craters and water marks on the surface of the paint film caused by rain or heavy condensation.

Figure 1 Water Stain/Water Damage Diagram

2．Probable Causes:

Rain falling on a freshly painted surface, due to dissolution, rain wash, alkali efflorescence, etc, causes craters to form on the surface before it has hardened sufficiently.

3．Prevention:

Allow coating to surface harden prior to exposure to rain or heavy condensation. For example, the construction temperature is higher than 3℃ dew point, do not work when it rains

4．Repair:

If the situation is relatively mild, use a special cleaning agent to wipe. Abrade, clean all surface damage areas and apply overall undercoat and or topcoat.

5．Interpretation of professional terms:

① Dissolution: paint usually contains a large number of surfactants, such as various additives (including wetting agent, dispersant, defoamer, thickener, etc.), lotion (if any), color paste, etc. These surfactants will exist in the paint film. Most of the surfactants show hydrophilic properties because they carry rich hydrophilic functional groups. When they contact with rain, they are easy to dissolve or wash off these surfactants, resulting in different gloss and color of the paint film, and various strip or dot marks and bright spots;

② Alkalinity: The substrate usually contains a large amount of alkaline substances or salts. Due to the infiltration of rainwater, it will migrate to the middle or surface of the paint film, thereby damaging the internal structure of the paint film or changing the color of the paint film, especially on alkaline cement substrates. The main component of cement is calcium silicate (CaSiO₃), which is a weak acid strong alkali salt. When it comes into contact with water, calcium silicate hydrolyzes into alkalinity, and its alkalinity is proportional to the content of CaSiO₃. The chemical formula is shown in Formula 1. In the presence of a large amount of water, it becomes a flowing carrier, allowing a large amount of Ca²⁺and OH ？ to penetrate through the pores and cracks of the paint film to the surface of the film, reacting with air to form alkali, which can cause changes and damage to the color of the paint film. Its chemical formula is shown in Equation 2: