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Thread: Do you have to sand fiberglass first?

  1. #1
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    Do you have to sand fiberglass first?

    I have an AD355 kit and will be bonding a few pieces together. Does anyone know if I have to sand or grind the old fiberglass first before applying more fiberglass to it? This is all on the non gel coat side of course. For example I need to bond the upper door to the oem style lower door. Hate to spend the time and mess grinding he surface first if it is not required.

  2. #2

    Re: Do you have to sand fiberglass first?

    Quote Originally Posted by Ferrari Berlinetta
    I have an AD355 kit and will be bonding a few pieces together. Does anyone know if I have to sand or grind the old fiberglass first before applying more fiberglass to it? This is all on the non gel coat side of course. For example I need to bond the upper door to the oem style lower door. Hate to spend the time and mess grinding he surface first if it is not required.
    Glenn, I normally take an air grinder and just quickly run it across the area to be bonded with a rough grinding bit on it just to rough it up. It only takes a few seconds, and seems to make things hold better. I also never use regular resin for bonding, I use some type of epoxy bonding agent. Plexus is my favorite, but 3M has a few good ones too. Once it is set, it will never come apart.

  3. #3
    Senior Member Don's Avatar
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    Re: Do you have to sand fiberglass first?

    Glenn;

    Always grind or rough sand all fiberglass surfaces before mating parts together. Also go to the MacGyver tutorial thread and see what I just recommend for the outside seams as well. Always seal both sides of any pieces mated together or there is a very good chance they will crack on you in the future.

    As for the back side, if the panels were made with non waxed fiberglass resin, you don't need too much sanding but if it was built with waxed resin, then you have to sand the wax off or you will have long term bonding problems. You will easily tell if the resin was unwaxed as when you start to sand, your paper will get bunged up pretty quickly as the unwaxed allows you to lay fiberglass over the existing so it stays ready to mate. It doesn't affect daily wear and tear or long term strength at all, it just allows for additional fiberglass bonding after the fact. Waxed fiberglass will seal the fiberglass under it so that you need to sand that layer off.

    I know about these various issues as I have had all these problems in the past that I had to fix and now sand everything and seal the top seam on all mated parts or new pieces being bonded into existing panels.

    Cheers
    Don

    308 Ferrari replica
    Prova Countach 5000QV

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