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Old 10-31-2013, 12:29 AM
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Primer on fenders

I put fiberglass fenders on my '41 Chevy and the mfg. said to use polyester primer on the fenders, which I did. Now that I have done that, do I have to use polyester on subsequent coats ?

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Old 10-31-2013, 12:58 AM
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poly is recommended to hide usual imperfections in fiberglass. It's good for that. The problem with Poly primer is that it's porous and is like a sponge so it's not ideal for final prime. You want a dense primer like epoxy or even a 2k Urethane is better for that, but they are not as thick so do your straightening with the poly primer, whether it be 1-2 rounds of priming and blocking to get there. Once you get to that point, THEN spray your final primer. This is pretty important cause during the life of your paint job your first and best line of defense is your clear, yet you will have water vapors that travel thru your paint and then they work themselves out. If you have an epoxy it's a better barrier against water vapors due to density of the primer.
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Old 10-31-2013, 08:37 AM
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Polyester in Slick Sand or Feather Fill...I'm not a big fan of these primers, as Henry mentioned, they are porous and moisture will find its way through them. For me, the ideal way to treat any fiberglass part is to let them sit in the warm sun (I see your from California so hopefully warm and sun is something that you have an abundance of) for a few days and let all of the gasses escape from the fiberglass. After that, block the gel coat with 180 grit paper and again let it sit for a few days.

If you then apply 2 coats of a quality Epoxy primer allow proper flash times and then either follow that with a quality 2K Urethane primer or more Epoxy, you will have a much more durable substrate than using a polyester primer......BUT

Your question was "do I have to use polyester on subsequent coats", and the answer is can use a either a 2K Urethane or an Epoxy primer over the Polyester Primer and I would recommend either instead of a polyester primer. If you are going to use an Epoxy primer over top of the Polyester primer, make sure that the Epoxy primer has good sanding qualities...all Epoxy primers aren't the same, some don't sand well at all...check with the manufacture of the Epoxy primer to make sure it does sand and isn't designed to be top coated with in the time window where sanding isn't required for adhesion.

Hope this helps.

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Old 11-01-2013, 01:26 AM
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Thanks Tech, thanks widetrack. I appreciate your help. I was headed in that direction, but I was afraid I was over-thinking it. I tend to get lost in the forest sometimes.
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