by Ron Joseph
Paint Wrinkling When Applied Over Itself
Q: An oil based Finish Paint (single pack) with some thermoplasticity in it is applied on a two-pack solvent borne Aromatic Polyurethane based zinc phosphate primer. The finish paint was applied after overnight curing of the primer.
When a second coat of the same finish paint was applied over the already finsih painted component, the following day, (i.e. after overnight curing of the first coat of the finish paint) the fresh wet paint started reacting and showing defects like wrinkling, cracking , shrinking.
What is the reason for the above phenomenon? (The first coat was already found to be tack free with the required nail hardness achieved, prior to application of the second final caot of the finish paint).
How then should one apply the second coat of the finish paint, if the first coat of the finish paint could not give the desired appearance?
A: Some coatings, such as single component alkyd-modified acrylics are prone to wrinkling when they are applied over themselves. If you read the product datasheet carefully the paint manufacturer should have warned you about a recoating “window”. For example, you might need to apply the second coat between 2-6 hours after applying the first coat. If you apply it too soon, you will get wrinkling, and if you apply it after 6 hours you will get the same paint defect. I have given you a hypothetical example; therefore you should consult the product data sheet for details.
If you don’t have a product data sheet, you can establish the guidelines for yourself by applying the first coat of paint and then waiting for varying time intervals before applying the second coat.
One last point: Film thickness also affects the recoating window; therefore when you conduct your tests insure that you apply the same film thickness that you intend to deposit during production.
Hope this helps.