Ask the Expert Question-and-Answer Archive

by Ron Joseph

March, 2004

CARC Topcoat Over Epoxy Primer

Q. I would like to know how long one needs to wait for primer (MIL-P-53022B) to dry in order to apply CARC paint on it? Is there minimum or maximum time-frame for application of CARC on the primer or one just needs to wait for primer to be dry and then apply CARC?

A. You can apply the CARC epoxy topcoat MIL-C-46168, or MIL-C-53039 over the CARC primer MIL-P-53022 and soon as it is dry. If you recoat the primer too soon, then you have a potential to entrap solvents in the coating and ultimately this can cause blistering or even worse, lifting. Therefore, if you are in a rush to overcoat the primer, at the very least pass a stream of warm air over surface to quickly draw the solvents out of the film. You can do this by means of large industrial fans. I would not apply the topcoat over the primer while it is still tacky.

On the other end of the scale, you do not want to leave the epoxy primer uncoated for too long before applying the topcoat because you might encounter intercoat adhesion problems. The maximum recoat time depends on the ambient temperature and perhaps also on humidity. If I were running a paint shop I would want to apply the topcoat within 24 hours after applying the primer.

A. Earlier in the week I sent you an answer to your question about CARC primers. A friend of mine, Tom Burke who has worked with CARC coatings for many years has just offered the following additional opinion:

It is difficult to measure dryness of the epoxy primer MIL-P-53028B. Recoating the primer in a production shop environment is assessed by ambient temperature and the thickness of the primer coat. A rule of thumb is low temperature heavy coatings 4 to 5 wet mils can take 4 to 6 hours before recoating is acceptable. At normal temperature, 72 °F and 2 to 3 west mils of primer recoating with CARC topcoat within 30 minutes is acceptable.

Another measure is if the primer is tack free, when you touch the surface, there is no transfer of the primer to your finger it is ready for CARC topcoat.

It may take a little trial and error with the finger test. But, after a couple of tries, you will get the point.