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Ask the Expert Question-and-Answer Archive

by Ron Joseph

November, 2008

CARC Compatibility

Q. I've read several of your posts on paint compatibility but my problem seems to be the reverse of previous posts re: CARC/enamel paint compatibility. We have a base coat of an oil or solvent-based red oxide primer and want to apply a top coat of CARC (one-part, water dispersible, aliphatic polyurethane). The primer will have been fully cured by the time of top coat application; sometime in the spring. Are the two finishes compatible or must we re-prime?

A. Since you will be waiting until the spring before you apply the CARC water-dispersible topcoat, it is most likely that the top coat will adhere to the red oxide primer. However, bear in mind that if your customer requires you to apply the CARC coating system, you will not be meeting that requirement. The CARC system comprises a CARC epoxy primer followed by the CARC top coat. If the red oxide primer is, in fact an alkyd or alkyd-modified resin (as implied by your email) it does not have the chemical resistance properties of the CARC system.

Q2. Thanks for getting back to me on the CARC over red oxide primer question.

We're doing the restoration of a military vehicle for an organization's Citizen Soldier Museum. While the CARC finish is appropriate to the vehicle's era, it will not be exposed to field tactical conditions and as such does not require that all of CARC's properties be met. It is my understanding that the vehicle may be operated in parades and that it may be moved under it's own power to static display sites for veteran's events. If so, then the worst it may see is vibrations from over the road operations, outside exposure to the elements, and the occasional camera flash.

We have the MSDS and painting instructions and will observe all application and exposure-to-personnel criteria during painting and curing. Again; our biggest concern is that the CARC will adhere properly; or if we would be looking at getting to repaint it in the near future. We would prefer that it not scatter a trail of paint chips in it's wake, like autumn leaves.

For my own personal clarification; I've been under the impression that the CARC topcoat itself is the chemical agent 'shield' (similar to dipping the entire vehicle in polyurethane plastic) and that the specified primer coat is just to give the best adhesion for the CARC. Have I been laboring under a misconception?

A2. Under the circumstances you mentioned, the CARC topcoat can be expected to perform well over the red oxide primer, but you can test for adhesion by coating a small area in advance of the major repaint.

While the CARC topcoat does bear the brunt of protecting the vehicle from chemicals, the primer must also play this role in areas where the top coat had been breached, such as at scratches, voids, etc.

Q3. A test spot is usually an excellent way to check paint compatibility, but in the case of one-part, moisture-activated CARC; once you crack the can open, the curing process kicks off. I think we'll paint in the spring and just hope for the best. The other alternative being batted around is that we apply a polyurethane/epoxy primer over the red oxide primer but if incompatibility is an issue, that fresh layer of primer won't stick any better than the CARC top coat. Ahh, decisions, decisions... what's a young boy to do?

A3. Your approach seems fine to me. I didn't suggest the epoxy primer because is adds another cost, but if you are willing to apply it, all the better.

When I open a can of a single component moisture curing polyurethane, I immediately transfer only a small amount into another clean, dry container, and then firmly close the can and turn it upside down! I was taught this trick of the trade many, many, (many) years ago. Then I only use the paint in the small open can. Whatever remains is tossed as hazardous waste. The paint in the can does not continue to cure and the shelf life remains excellent.


Ron Joseph

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