Fiberglass/gelcoat repair 101

[Alex]

I'm starting to learn fiberglass/gelcoat repair. Using this video as my guide

YouTube - How to Repair Fiberglass


Let's say you have a flat piece of fiberglass/gelcoat that is roughly 1/4'' thick. It has a hole in it that you want to repair.

Okay, so are these the rough steps? (questions are in bold)

1) On the backside, grind the area around the hole to form a good bonding area (the scarf), then clean with acetone

2) Cut out a template of the repair area on mylar, tape it on and put some object (e.g., styrafoam) against it to hold it solid in place

3) From the backside, mix your gelcoat (mine says 10drops of 1%mekp to 1oz of gelcoat), then apply ~20mils of gelcoat to the area that is being repair. Question - do I just want to do like a dozen strokes with the brush,let it dry, then move on, or do I want to do multiple layers (i.e., brush it on, wait to dry for 2-3hrs, brush on more, wait to dry 2-3hrs, then brush on a final layer)?

4) Cut out your repair area from the matt and cloth. Do 2 layers of matt, then 1 layer of cloth, then matt, cloth, matt, cloth, etc..., finishing with cloth once the area of the repair has been built up to the original area. 2 questions here

a - Between layers (e.g., matt, cloth, matt, cloth, etc...), do I want to wait for the resin to dry, or do I want to apply resin, apply matt, apply resin, apply cloth, apply resin, apply mat, etc... until it's built up, or do I want to wait, like resin, matt, wait; resin, cloth, wait. If so, how long to wait?

 

[Jim Rosenthal]

I'd say that you should try all these techniques out on scraps as practice before you actually do any repairs on a car. You'll need some fiberglass panels for your learning curve- you can either use scrap parts, find something in a boatyard (there are usually abandoned boats that you can saw a piece from) or order flat fiberglass panels from McMaster-Carr.

All other things being equal, I'd brush on some, wait a bit for it to tack up (until it isn't sticky, but it's soft), brush on some more, wait a bit, etc. Two-part resins such as polyester and epoxy cure by polymerizing into one large cross-linked molecule. Once the resin is completely set up (polymerized) it can't cross-link to fresh resin. So you want to catch it at the point where it is still not totally cured, but it is firm enough that your added resin isn't going to remove what you initially deposited.

I would also point out that polyester resin isn't the best material at all with which to do repairs on fiberglass laminates that were constructed with polyester. A much better choice is vinylester resin- polyester doesn't bond as well to previously cured polyester laminates. Boatbuilders and boat repairers use vinylester or epoxy resins to repair polyester structures. If you live in a maritime area, you might consult a boatyard where someone is willing to help you out and show you how to do all this. Generally speaking, boatyards are better at repairing fiberglass structures than autobody shops, although autobody shops tend to do very nice paintwork over the repairs.