best way to repair fiberglass on the underbody

[Michael L.]

Guys,

I need to made some minor repairs to the fiberglass on the underbody of my '69 coupe in a couple of spots. For example, the battery ground hole was enlarged at some point in the past and now the grommet for the ground cable wont stay secure. What's the best way to make small repairs on areas like this? What's the best souce for a small piece of fiberglass or similar material I could use as a patch?

Mike

[Mark E.]

Re: best way to repair fiberglass on the underbody

Michael,

No need for a piece of fiberglass to repair a wallowed out hole. Prep both sides by sanding with 30 grit or gingerly grinding the surface to about 1 to 2 inches around the hole. The idea is to chamfer or taper down the surface around the hole so the new fiberglass will be flush. You might want to mask the surface beyond this to prevent errant damage. Cover the hole from the bottom with duct tape. Shred a small amount of fiberglass matting by hand or with scissors. Fix up a couple of ounces of resin with hardener (follow its directions), then mix this with the shredded matting. Using an acid dip brush, apply the resin laden matting on top of the duct tape and about an inch beyond the hole's circumference. Work out any air bubbles using the brush and smooth its surface as best as you can. You have about 4 minutes to do this before the resin sets. Let it harden for a few hours (you can hurry this up using a heat lamp), then grind mostly flush with the surface. Block sand using 30 then 80, then 220 grit. Pay attention to the repair's thickness- make sure it's not too thick or too thin.

You may need to add another round of glass matting to take care of low spots. Use bondo for small imperfections (air bubble divots, etc.). Fog it with semi-flat black to match the rest of the box and the inside is done. Pull off the duct tape from the bottom, rough up any imperfections (like the seam from the old hole) and apply matting as needed. Grind, block sand, finish to match the floorboard. Then drill a hole the size needed.

This is much easier to demonstrate than explain. Search "Corvette fiberglass repair" on YouTube. You'll find a bunch of videos teaching basic repair.

Here's a guy filling in an antenna hole on a C3, which is a similar process to your repair: https://youtu.be/tFpxjAaxGSM


EDIT: If the hole is just a little bit oversized and not otherwise damaged, you might get away with just prepping the circumference, applying some matting, then shaping it to size.

[Bob R.]

Re: best way to repair fiberglass on the underbody

Michael
I made the repair a little differently than above I used a bolt and fender washers to compress the resin and mesh material so when taken apart the new material was flush with the old. I also used a piece of wax paper on each side of the opening to prevent the resin from sticking to anything else. Then you can adjust the size of the hole with a drill or a file. You can purchase mesh and resin at Home Depot. They make different types of mesh some have the mesh running at 90 degrees and others the mesh has a random woven look which more closely resembles the factory appearance when finished.

[Michael L.]

Re: best way to repair fiberglass on the underbody

Thanks guys. I was able to find something on youtube that approximates what you guys mentioned. I picked up a fiberglass repair kit from the auto parts store. will let you know how it goes and will post pics when done.

Mike

[Mark E.]

Re: best way to repair fiberglass on the underbody

Thanks guys. I was able to find something on youtube that approximates what you guys mentioned. I picked up a fiberglass repair kit from the auto parts store. will let you know how it goes and will post pics when done.

Mike

Good deal. Make sure you get fiberglass matting, not cloth.