working with fiberglass body panels

[juvius]

Hoping some of you body guys can provide me some of your insight. I have fiberglass rocker panels and fender that have some minor problems.. mostly paint chips. As far as sanding and body filler, do i just treat it like any other panel? I'm using USC Feather Lite filler. The msds says it's good for fiberglass. but not to put it on top of paint. So, i'm nervous about sanding the paint down to the fiberglass. Will it hurt if i end up sanding some of the gel coat off? I'll try not to, but shit happens, right?

there are some hole in the rockers from where they were previously rivited on, should i just fill them with filler or should i put a little fiberglass cloth and resin behind then fill it with filler?

any tips or tricks would be appreciated when it comes to working on these.

once i get the fender back on the car, there is one spot at the nose, at the front edge facing the hood that looks like they filed back for some reason. that will need to be built up, but i'll take pictures and ask for help with that when i get that far.

thanks

[azruss]

bondo is polyester resin with a clay type filler. Fiberglass is a polyester resin with binders to adhere to the glass fibers. These are very compatible products. Even more compatible than bondo and metal. If the fiberglass has gelcoat then that is a polyester paint and will be a great surface for bondo as long is it is sanded with something around 80-120 grit. The bond will be mechanical as the gelcoat will be impervious to any solvents. Try to avoid sanding through the gelcoat and getting into the raw fiberglass. Here you will find tiny air pockets that dont want to fill with anything. They are a pain. If you have any stress cracks or "stars" they go entire through the fiberglass and cant be covered up as they will eventually broadcast through your paint. Most fiberglass parts have a lot of wax contamination as parting wax was used in the mold. This wax needs to be completely removed to keep your paint or primer from fish eye. the best way to repair the holes is to sand the back side of the fiberglass at the hole tapering about an 1" from the hole to the back surface. You want the area by the hole to be as thin as possible and taper to full thickness at the 1". stick a piece of clear vinyl box tape on the finished side of the hole. Now fiberglass on the inside with layers of fiberglass to build to the original thickness. After it cures, just re-drill the holes and remove the box tape.

[juvius]

thanks... i appreciate it. i will do as you say.

if i run into spider cracks, how to i take care of them?

[DanD]

If you have severe gel cracks, you grind them down and lay a thin layer of fiberglass cloth over the grind bonding it with resin. then you sand and bondo as usual.

[azruss]

I have tried laying a thin layer of glass over stars and it eventually broadcasted through. this will occur much faster in a vibrating format like a Fiat going down the road. Here it the issue. the white streaks you see from the back side is actually fractured fiberglass with de-lamination at that point. The result is anytime the panel vibrates, heats and cools or flexes, it will find the weakest spots to move and that is where it is already damaged. What I do is tape the finished side with clear vinyl box tape. grind the glass away from the backside as thin as possible at the crack, tapering away from the crack for and 1" or so. Nice to get it clear to the gelcoat if you are that brave. Now when you glass, you removed all the stress cracks except whats in the gelcoat. You can sand that away to the fresh glass and recoat. now you are back to the pin hole issue, but you wont have to worry about the stress crack coming back. Fiberglass doesnt like sharp corners and that is usually where you find the problem.

[juvius]

thanks guys.... i'm hoping i don't find any cracks like that. i plan on cleaning them off tomorrow for a good inspection...