Tech Tips: Blister & Pinhole Troubleshooting
THE CAuse of blisters & pinholes
When applying fast-set coatings, the cause of blisters and pinholes can be difficult to diagnose. You may spray the coating system for an extended period and never experience a blister or pinhole. Then, blisters or pinholes may suddenly appear on the very next job. In this article, we hope to give a better understanding of the reasons blisters and pinholes occur.
Moisture Blistering
Moisture blisters are probably the most typical form of blisters applicators will experience. Moisture blisters result from excess moisture reacting with the isocyanate component of the coating system. This blistering may not show up until 10 to 24 hours after the spray application. This can be caused by applying the coating in high humidity or excessive moisture in the compressed air lines to the spray gun.
While it is true that the polyurea systems are not as sensitive to moisture during application as polyurethane systems, polyurea can be affected by moisture if there is an off ratio application (iso rich) or isocyanate is spitting or leaking in the spray gun.
Thermal Blistering / Delamination
Thermal blisters develop between the coats or layers of the spray application. These blisters typically occur when a coating is applied to a cold surface. The initial pass or layer cures slower due to the heat sink effect of the cold substrate. The initial coat serves as an insulator, and the second coat cures faster, which results in stress shrinkage being applied to the first coat that is softer from the slower curing resulting in blister formation.
Thermal blisters can also be caused by excessive heat or exotherm buildup within the layers of the spray application. This causes the next layer to cure too rapidly, not allowing it to wet out, and no bonding action occurs, and delamination will result.
Polyurea coatings have high thermal stability compared to polyurethane coatings.
Pinholes
Pinholes are generally caused by moisture entrapped in the substrate that is being coated. The heat induced by the exothermic reaction of the coating causes the moisture trapped in the substrate to grow and force its way up through the still uncured coating (outgassing), resulting in pinholes or blisters. You can experience the same thing if spraying over primer that is not fully cured.
Blister Characterization
If you have encountered blisters during the coating application, you must determine what type of blisters you have before the issue can be addressed. The only way to do this is to cut a blister open and examine the area.
Troubleshooting the blister problem
→If the backside of the blister is smooth, the blister is generally induced by contamination of the substrate or moisture trapped gas in the substrate.
→Suppose the backside of the blister is rough or furry looking. In this case, the blister is generally induced by moisture reacting with the coating during the curing process or an off ratio (iso-rich) application.
→If the backside of the blister is gummy, the blister is generally induced by spraying over primer that has not fully cured or an off ratio (resin-rich) application.
