Fiberglass repair, Proper surface prep?

[Mighty Thor]

I am replacing the stringers on my CVX20 and have to do some repair work in the bottom of the hull as well. Some of the layers of glass have delaminated and I am having to grind down to the solid stuff. So what is the proper way to prep the existing glass so that the new stuff will stick to it?

[Tom Brown]

Get a back pad for your angle grinder and run 24 grit paper on it. That will rough the surface up nicely for laminating to. If you can't find 24 grit paper, 40 is OK but 24 is better.
Don't wash the surface with liquid. That includes acetone. Just vacuum it up and lay the new material on top. If you're going to use woven roving for your stringer gussets, use a layer of matt to interface with the old glass.
The idea is that the course paper will leave a rough surface for the new material to grip to. Also, the course paper will open up the old glass and you will have glass strands hanging out of the old material like rebar hanging out of concrete for the new material to bond to. The vacuum will pull the strands up and make for a better bond with the new material.
Try to keep your resin use to a minimum. The goal is to use the least amount of resin that will wet out the glass material.
I replaced the stringers in a few 'Trons myself. Fun boats. :cool: