Problem with Print-Through

General Composites Discussions Composites Talk
#1

Hi everyone
been lurking here for a while and finally decided to post about a problem I am having with a car I am building. I have some marine experience with composite work but only with the basic stuff, nothing like the high-tech work I have seen on this forum. All my work is done using basic hand-lay up with molds or moldless techniques and some cold-molded based stuff on the body. (yeah I know quite a variety :slight_smile: ) I use mostly epoxy, west-marine or US composite.

After I finished the body I left it in the sun for a few days, sanded and painted a finish coat thinking that the print through was over with. Now I discover that the print through comes out when the sun shines on it and heats it past about 120* (it can get up to 170 sometimes), then disappears again when it cools down.

The body is highly finished with gloss black, the composite consists of layers of epoxy and cloth, with the top layer being epoxy mat and/or a surfacing veil with some thickened epoxy used as a fairing compound. That is then covered with duratec primer - 2 or 3 coats and a two-part polyurethane paint. As you can see in the photos the surface is flawless until the sun heats it up. The parts with the mat look a little like orange peel but the front fenders which are simply cloth with lots of fairing compond and duratec have the usual fabric print through look.

I have tried sanding down and recoating the duratec with no success I am hoping someone might have some suggestions that don’t involve remaking the part!

Thanks for any help.

I was hoping the close-ups would show the print through a little better. You can just see the mottled effect on what should be straight lines in the reflections.

#2

The print through is caused by the materials used in the construction of the part. It most likely won’t go away with surface coatings.
Strange that the parts “go back” to being smooth when cooled. If the parts are post cured to the highest temp they’ll see in service, any print through should appear. Then it can be sanded smooth, and shouldn’t be seen again at anything less than post cure temps.

#3

Boat builders use VE barrier coats after spraying their gelcoats. they build up a 60mil thickness on top of there 20 mils of gelcoat then they laminate. I have never used them and question the thickness, but CCP claims that they actually get a tuffer surface gelcoat because of it. Boat builders love the stuff apparently, originally they used the stuff below the waterline to help protect against blistering but found the surface finish was much better than above the waterline. If you really want to try to get a better finish you could do a give it a try, however I cant make any personal claims on it.

#4

What oz cloth are you using? Is your resin puddling?

#5

Thank you for all replies.

I expected to see some print-through after the post cure but thought that it would be permanent, which is why I finished it twice. It is becoming apparent to me there are two kinds of print through. One that is permanent and comes through in the post cure process and can be sanded out, and another that is caused simply by temperature and the different expansion rates that occurs between the resin and glass. Does this sound right? Do you think putting some heat lamps on it and taking temps up higher would do anything?

I have seen the barrier coats used on the bottom of a boat but never on finished surfaces, sounds interesting but wouldn’t a few coats of duratec to the same thickness achieve the same thing? I would be willing to try it on the next part but not on this one. I spoke to West System and the only thing they would suggest is to use their new high-temp resin, not much good to me unless I want to refinish the part which doesn’t really sound attractive to me at the moment plus I am sure is big money.
Thanks again for all input.
BTW I am using 9oz cloth and I think the mat was 1.5oz.

#6

That is exactly wht I first thought, since it looks more like wrinkles then fabric. Granted, pictures are never easy to show the real problem, but it looks like wrinkles, which can easily be caused by the resin/fiber mismatch, ESPECIALLY if it happens with the sun’s heat.
Maybe try to heat up a section indoors.

No idea how to fix it though.

#7

You said you used Duratec Primer… which is a polyester primer, correct? Perhaps using polyester primer over epoxy is a bad combination? Maybe use urethane primer instead.

All the body guys I know do not like using polyester primer because of the shrinkage factor.

#8

the problem is from the resin and fibers expanding and contracting differently. Its a classic problem. Doing as TET said and post curing will help the problem by raising the TG, but not if the resin system is cheap. A good VE or epoxy will have much better surface qualities. also using a surface veil or a very fine glass like 105 or 120 glass will help minimize the print. Other than that you must creat a much thicker barrier between the surface and the fibers by using a barrier coat. Of course there could be other methods I am unaware of.

#9

For future reference, you can use a skin coat behind the gelcoat.
A high quality polyester gelcoat is fine. When at the “fingerprint” stage, wet layup fiberglass veil with VE resin. Let cure.
Then lay up with a good low-shrink resin, preferably more VE or epoxy. This will achieve a great surface finish that stands up to wide temperature fluctuations.

I’ll see if I can find and take pics of some parts in the shop that have been out of the mold a while but not painted. From 0* to 350* in the oven, there’s no noticeable print through and the surface is smooth.