Ask the Expert Question-and-Answer Archive

by Ron Joseph

October, 2004

Oil Primer Over Latex Encapsulant

Q. I am researching a paint adhesion failure where the paint is failing in some locations, not others (vapor drive does not seem to be a factor). The project was originally specified to remove lead paint with a chemical stripper, clean it and then apply a latex topcoat over a oil-alkyd primer. I came across a purchase order in the project records for a lead encapsulant product that is a high solids, elastomeric-thermoplastic, water-based copolymer. (It was not in the original specification and believe it may have been applied where not all of the lead paint could be removed). I am wondering if this water based encapsulant was applied and then if the oil based primer was applied over it, would that contribute to failure? The stripping and repainting was also performed in late fall, in a northern climate. Your thoughts would be appreciated.

There are many possible causes for paint delamination. The first and most common cause is due to the substrate and its preparation prior to painting. Other causes are incompatibility between one coat of paint and the next, such as the primer and the topcoat. Another possible reason for failure is that the older coat of paint has oxidized and the upper layers of this aged coating are inert and do not allow the next coat of paint to adhere.

One can apply a waterborne paint over and "oil-based" solvent-borne coating and visa versa, but it is important to allow the lower coating to fully cure before applying the upper one. If this condition was not met this can be a cause of the failure.

None of the coatings you mentioned is likely to be totally impermeable, but it is possible that either solvent or water was retained in the coating for a long period, and this might have interfered with the adhesion.

If the waterborne coating was applied on a cold humid day, curing would have been compromised, because the cosolvent would have evaporated before all the water was able to leave the film. This could be the cause of the failure.

Without seeing the problem and performing on-site and perhaps also some laboratory tests it is impossible to give you a reliable answer. On-site inspection will determine where the failure is occurring, why the failure is localized and not uniform, etc.

You might need to find a consultant in your area to perform such an inspection. Alternatively, if you would like us to assist you further, please get back to me.

Best wishes,

Ron Joseph