Epoxying and Painting.

I am wondering how long should I wait before painting over some epoxy fiberglass work I have done. I plan on using polyurethane for the paint, specifically PPG Concept as the top coat with K38 as the primer.

Thanks for any help.

Kevin

 

Soon as the epoxy is cured is ok...but with some caveats.

a) I presume you are using some sort of filler? The cloth/resin matrix may show shrinkage over a couple of days if you paint straight on top. Not that this is likely but I'll state the fact since you are asking a question
b) no idea what K38 primer is. Is it epoxy based (either epoxy-amine type crosslink or epoxy-isocyanate)? You may get adhesion problems with it if it is polyester type primer and you aren't filling

You are generally best to leave the epoxy cloth matrix to cure off for a few days just to ensure that any solvent from primers don't get sucked in. Of course if you use filler this comment isn't relevant as the filling time with be enough normally unless you are using some very slow cure resin in cold temps.

On general note: if you don't use peel ply, make sure you clean off any possible amine sweat with warm soapy water and scotchbrite - before sanding.

 

reply

Colin,

Thanks for the quick reply. K-38 is a high build primer and I assume it is an epoxy paint, I have to add a hardener. I can paint an epoxy seal coat on but want to avoid this if possible. It is a 20' catamaran and I don't want to end up spending $500+ on paint if I can help it. These are all PPG products recommended for the top coat, an acrylic urethane.

Most of the work I have done is covered with some epoxy/ micro-baloon fairing compound I have mixed up. It sounds like you are saying that since I have used this fairing mix I do not need to worry about any primer sovents getting absorbed into the fairing compound, is that correct? I am in Hawaii and the temp is warm.

All the work I am doing is being sanded prior to any new coats of fairing compound, would that take care of the amine sweating? I am not really familiar with amine sweat, could you expound on that Why would peel ply prevent this?

I assume I just need to finish sand the fairing compound and then prime it, without washing it?

Thanks again,

Kevin

 

Contact you epoxy manufacture (paint compatibility issues), but I'm reasonable sure (80%) you don't need a primer on most epoxies, unless the topcoat requires a special "bonding agent" or you need to further smooth the surface with a building primer. BTW, not all two part paints are epoxy, in fact most aren't. Read the label, if it mentions alkyd or enamel anywhere in the product description, it's not epoxy, but likely a modified alkyd or a polyurethane.

 

The K-38 is a good 2K urethane filler primer for automotive use. Look at the ingredients on the catalyst can. If you see the word 'isocyanate' anywhere, it is urethane for sure. Now they often disguise the word as 'proprietary HDI' or 'HDIM' (Hexamethylene Diisocyanate (HDI), Hexamethylene Diisocyanate Monomer (HDIM). PPG has especially attempted to make their product labels devoid of information other than draconian health warnings.

If this fiberglass work is not on a boat it should do just fine. If it is on a boat, I'd rather see you use an epoxy based filler primer as they are far more water resistant.

Jimbo

 

Thanks Jim,

Now you tell me.

It is for a boat but it is a beach cat and won't see the water that often. Maybe 10 hours every few days, the rest of the time it will be high and dry. Do you think that will be okay? or should I return it and look for some of the epoxy primer you talked about?

I would just hope that the PPG DCC Concept paint I had mixed up would be compatible with the epoxy primer.

Thanks for your help.

Kevin

 

Concept (DCC) is a great system with no bad habits at all and is compatible with just about anything. It's not the toughest paint around being about he equivalent of DuPont Centari. It should work OK as long as it's not on the bottom of the hull. One little problem with DCC is that it is not very flexible once fully cured, so if you apply it to something even a little bit flexible, it will crack. You could easily fix this by mixing in a flexibilizer additive. Use about half of what they call out for use on a urethane bumper.

PPG makes both sanding and non-sanding primers that are epoxy based. The DP are non-sanding primers (though you can sand them after a thorough cure, but they sand hard.) The DP line fills pinholes and self-levels like few other products I have used, though I suspect you will need some 'prime and sand' action before you apply your paint over bare glass work. I don't use any of PPG's sanding epoxy primers so I can't comment on any of them, using instead marine and aerospace specific products from Awl-Grip and the like.

Awl Grip 545 is almost an industry standard for a marine duty filler primer and you really could not go wrong by using it. Like DCC it has no bad habits and the DCC will go over it perfectly.

Jimbo