What causes imprinting through the paint??

[Otee453]

The title says it… but maybe the question is better put, what was done wrong or insufficiently so that imprinting shows through on hot days?

I did my own body and paint. Black. I think it turned out fantastic (better than I hoped). But today, hot as hell, I noticed a line appear in the nose. It was clearly imprinting from the fiberglass as I felt underneath and I could feel a ridge in the fiberglass.
I followed the popular formula… 80 grit, bodywork, 3 coats poly, 3 coats 2k urethane, 2k sealer, base, clear. Block sanding the grits commonly outlined by the Jeff’s.

The imprinting doesn’t bother me, but I’m curious, what was deficient in my methods and what would prevent the imprinting in the heat?

[GoDadGo]

The material expansion rates for gel coat and body filler are different so the mold parting lines needed to be completely taken down prior to the body filler being applied...This was something that Jeff Kleiner advised me about over and over and having built fiberglass boats gave me an advantage as a first time builder.

Shown below was my OCD attempt at knocking them down:

Mold Parting Line Removal:
https://youtu.be/_3sLamdkIFg

Mold Parting Line Smoothing:
https://youtu.be/meBYeI96_A8

Completed Car Shown Below:
https://youtu.be/9WEe6-wdNtA

Hope This Helps & Good Luck!

[Otee453]

Hmm. I thought I took down the parting lines quite well, ground out the voids, filled the voids with HSRF, then rage gold. BUT… it had to be something. I wonder what I could’ve done differently.

I also have seen a little very subtle imprinting area (size of a silver dollar) show up by the windshield on the driver side. Sure enough… underneath you can feel a thick area of fiberglass underneath.

[j.miller]

Yep, What Go Dad said. We treat them differently depending on if its a tight parting line, gel coat in the parting line or gel coat under the first two layers of glass. I won't go into all of it but I will say you chose the most reflective color and hardest to hide imperfections. That being said , you are getting beat by the heat. Black gets hot, swells and twist the glass showing the difference in substrates as Go Dad stated...da Bat

[Chris @ Forma]

An additional caution. We studied at the school of hard knocks and came away with another potential issue. If there are air bubbles trapped in the gelcoat/resin at the mold seams, the bubbles can expand in the heat and pop the paint in hot weather. We grind beyond flush and create a V-groove at all of the mold seams. We then fill this groove with epoxy resin and then grind off flush. This is time consumeing and laborious but much better than having to redo paint.

Chris

[Otee453]

Yep, What Go Dad said. We treat them differently depending on if its a tight parting line, gel coat in the parting line or gel coat under the first two layers of glass. I won't go into all of it but I will say you chose the most reflective color and hardest to hide imperfections. That being said , you are getting beat by the heat. Black gets hot, swells and twist the glass showing the difference in substrates as Go Dad stated...da Bat

Thanks. Not sure I’ll do another FFR car again in the future, but I think my next resto build I will do my own paint. Like anything we do, I would like to get better as I do more of it.

[Otee453]

An additional caution. We studied at the school of hard knocks and came away with another potential issue. If there are air bubbles trapped in the gelcoat/resin at the mold seams, the bubbles can expand in the heat and pop the paint in hot weather. We grind beyond flush and create a V-groove at all of the mold seams. We then fill this groove with epoxy resin and then grind off flush. This is time consumeing and laborious but much better than having to redo paint.

Chris

Yes, I found a heat pimple in an area where there was no mold line. I spent a LOT of time searching for voids and pinholes, so finding the pimple when it was hot was a bit of a frustration.

[narly1]

Yes, I found a heat pimple in an area where there was no mold line. I spent a LOT of time searching for voids and pinholes, so finding the pimple when it was hot was a bit of a frustration.

What are everyone's thoughts on doing the required part line repairs, priming the body and then letting it cook in the sun for a time to see if any flaws emerge?