How it forms
An amine curing agent is meant to react with the epoxy resin. Amine molecules near the surface can react instead with water vapour and carbon dioxide from the air, forming carbamate and carbonate salts that collect on the surface. The risk rises whenever cure slows down while moisture is available: low substrate temperature, high relative humidity, condensation on the fresh film, and air loaded with combustion products. Some curing agents are more prone to it than others, and some data sheets specify an induction time after mixing to reduce it.
Where it shows up
- Winter shutdowns and unheated enclosures, where cure stretches overnight while the steel cools
- Tank interiors, sumps and confined spaces with poor ventilation
- Enclosures heated with direct-fired propane heaters, which release both moisture and carbon dioxide
- Any film that dropped close to the dew point during cure
The cold-weather coating application article covers the heating and humidity controls that prevent it.
How to detect it
Blush can show as a greasy or waxy feel, a whitish haze or bloom, a surface that stays tacky, or water that beads instead of wetting the film. On dark coatings it may not be visible at all. Field checks include the water-break test, wiping with a damp white cloth, and test kits that react with amine residues. When application or cure conditions were unfavourable, the safe assumption is that blush is present until a test shows otherwise.
How to deal with it
Carbamates are water-soluble, so removal means washing with clean water, often warm, with scrubbing, followed by rinsing and complete drying. Solvent wiping on its own is not an effective method and can spread the residue. If the recoat window has passed, the surface is then abraded as the data sheet requires. The coating manufacturer's instructions govern both the removal method and the recoat conditions.
Prevention is cheaper than removal:
- Keep the substrate above the dew point with the margin the specification sets
- Heat with indirect-fired equipment and control humidity during application and cure
- Respect the minimum cure temperature and the induction time on the data sheet
- In cold or humid service, choose a curing agent formulated for low blush