Common Causes for Irregularities and Finish Defects
Polyamide/Amine Blush or Surface Enrichment
Occurs when the proper cure cycle of catalyzed epoxies is interrupted or slowed. The “lighter” polyamide or amine hardener separates from the epoxy and floats to the surface where it oxidizes and turns yellow or brownish in color.
The polyamide/amine blush can be confirmed by rubbing the surface lightly with a clean rag saturated with MEK or Reducer R7K54. The yellow appearance will be removed, but may later return.
- Areas rich in polyamide or amine content may exhibit yellowing/blushing and/or an oily exudate
- Areas rich in epoxy content will appear normal in color for a period of time, but may eventually discolor
- May result in color variations along the same coated surface
- Drying and curing times may be affected
Epoxy coatings are formulated with optimum levels of a hardener for a given level of epoxy resin. This ratio differs from product to product. The use of an improper hardener may result in an undercatalyzed or overcatalyzed product.
- May appear dry but will not fully cure
- May appear soft and gummy
- May be hard and brittle
- Both will result in discoloration, yellowing, and reduced performance properties
Insufficient Induction (Sweat-In Time)
Many epoxy coatings require a specified induction time in order to fully “compatibilize” the epoxy resin and the hardener. When required, this pre-reaction will reduce the likelihood of poor film appearance and poor coating performance. Lower temperatures and/or high humidity will generally require longer induction periods.
Improperly inducted epoxies will exhibit:
- A lower gloss than expected
- Yellowing becoming progressively worse over time
- Polyamide/amine blushing
- Poor color acceptance
- Reduced chemical resistance
- Softer film
- Color float and/or color variation
It is possible that the coating material applied initially may develop low gloss and discolor, while the rest of the surface, painted with the same material, will appear normal as induction time has increased.
Most epoxies are formulated for application at temperatures above 50–55ºF. Temperatures below 50ºF will stop the rate of the chemical reaction between the epoxy and the hardener. The evaporation of the solvents in the coating are also inhibited.
- Both of these conditions will increase the likelihood of blushing, discoloration and down-glossing.
- Be aware that surface temperatures may be much lower than the air temperature. Epoxy coatings should never be applied below the recommended temperature range, or when temperatures are expected to fall below the minimum recommended temperature during the first 72 hours of cure.
High Humidity, Moisture Condensation, Stagnant Air
- Most epoxies are designed for application up to a maximum 85%RH
- High humidity and/or the presence of moisture may cause discoloration or a “greasy” feel and poor uniformity of appearance
- The polyamide/amine may separate and cause discoloration. Maximum effort should be made to increase fresh air circulation to minimize this effect.
Insufficient Curing of Water-Based Primers or Fillers
If water-based primers are not sufficiently cured prior to topcoating, moisture migrating through the fresh epoxy film will inhibit the drying and curing of the epoxy. This will cause discoloration, low gloss and possibly delamination.
Discoloration from Exposure UV Light (Sunlight)
- This is a surface phenomenon attributed to the natural darkening of epoxy resin upon exposure to UV or sunlight. This problem occurs most often in areas with intermittent sunlight, creating a “checkerboard”effect.
- The discoloration may be confirmed by scraping away the surface layer with a knife to expose the underlying film which will appear lighter in color.
Discoloration from Chemical Exposure
- Certain chemical compounds, such as carbon monoxide and carbon dioxide, are known to react with freshly applied epoxy films and cause discoloration. This discoloration will be more pronounced in areas of high film build and in areas with little or no air circulation.
- Check area for cleaning agents that may contain these compounds. Animal kennels or shelters are also suspect because of waste byproducts.