Help with form material to repair fiberglass

[James B.]

My '67 has a hole in one corner of the jack compartment behind the driver's seat. Obviously Bubba wanted a easy way to remove the diff. I will shortly be pulling the body and then I can get to both the top and bottom side of that area. The hole is in the corner and about the diameter of a golf ball. The curvature will be hard to duplicate and I want to get it right. There is a '65 coupe body in the shop that I can use to make a form and I want to make forms for both the top and bottom sides. My question is what can I use to make the forms. Someone has suggested plaster of paris but I will need something that sets up very quickly as I will have to hold the form until the material partially sets up. There is no way to clamp something to that area and I cannot damage the '65 by screwing a form into that area. Can anyone suggest a material that might work? thanks for your help, Jim Baker

[Bob R.]

Re: Help with form material to repair fiberglass

I used plaster of paris in the past and it work really well. It does set up fairly quickly and is probably the best material for your task. I would spray the original part with silicon so you can separate it after it hardens. You could probably use your own car for the mold the opposite corner is probably the same shape. You may want to add something to the plaster to keep it from breaking like fiberglass mesh it will act like a reinforcing material.

[Bill H.]

Re: Help with form material to repair fiberglass

I've done repairs like that using aluminum roof flashing (I keep a roll in the shop). It's thin enough to conform to the tunnel easily. Before you bend it, put some wax on it (I use Maguiar's Mold Release Wax but car wax will do) and buff the wax off. You could attach the aluminum with small sheet metal screws (easy to fill in) or possibly hot glue.

Make sure you bevel both sides of the hole, 2 pieces of mat cut to the OD of the bevel (for the top and bottom) and 2 pieces cut to the size of the hole should come close to the thickness of the tunnel.

Even if you go with the plaster, you're going to have some sanding to do.

[Scott M.]

Re: Help with form material to repair fiberglass

Modeling clay works good for contoured shapes as well.

[James B.]

Re: Help with form material to repair fiberglass

Thanks to all, you have given me some good suggestions. Bill, I like the idea of the roof flashing and hot glue (as I don't think that would hurt the fiberglass on either the example body or mine); and yes I will certainly bevel both sides of the repair area. Bob, if I try plaster of paris I will use fiberglass mesh to strengthen. Only thing can't use in my friends shop is silicon as it plays havoc with painting. Scott, does modeling clay harden or is there a way to harden it (bake in oven)?

[Scott M.]

Re: Help with form material to repair fiberglass

There are different types, not sure what the one is use, but its semi rigid until you work it and warm it, sometimes I have to throw it in the microwave to soften it enough to use then I press it into place to shape it. Coat it the resin side with some Johnson's paste wax duct tape the plug in place and lay down the resin and mat.

[Tracy C.]

Re: Help with form material to repair fiberglass

... My question is what can I use to make the forms. Someone has suggested plaster of paris but I will need something that sets up very quickly as I will have to hold the form until the material partially sets up. There is no way to clamp something to that area and I cannot damage the '65 by screwing a form into that area. Can anyone suggest a material that might work? thanks for your help, Jim Baker

I've spent some time in a Pattern shop and here is how you make a plaster splash...

Get some beewax and mash it up with diesel fuel until it is brushable with a short nap brush. (cut the fibers on one of those cheap 2" brushes back to about 1" long)
Brush a coating of this over your target area
Mix up the plaster and spread some over the area you want to replicate. Try to build up a 1/4" layer. Let this stiffen some but not completely set-up
Back this with hemp soaked in plaster. Spanish moss availible in a hobby store will also work.
Build up to approximately 1-2 inches thick and let this completely set up.
You will be able to pop this off in about 15 minutes.

Once removed, shelac the mold surface and spray 2=3 coats of sealer (varnish or clear lacquer)
Once this is dry, wax the surface with 4 coats of carnuba wax and you are ready to lay down your fiberglass into you new mold.

[Keith B.]

Re: Help with form material to repair fiberglass

Bubba must have been reading the service manual for once.

[Scott S.]

Re: Help with form material to repair fiberglass

James, Are you dying the resin to match the parent (floor pan) material? There is pigments to do this type of color matching.
Scott

[Tim T.]

Re: Help with form material to repair fiberglass

James,
If you want to go the plaster of paris route, there is something that is faster setting & stronger.
Go to your local dentist & ask him if you can buy some fast set dental plaster. It is very cheap & if you are a regular patient, he ought to give it to you.
Mix a small amount to see how fast it sets before you use it. Cold water will slow the set time.
Dr T

[Patrick H.]

Re: Help with form material to repair fiberglass

Rather than silicone or other agents, try Kiwi neutral color (actually any color) shoe polish as the release agent applied to the battery box before putting on your mold material. No, I'm not kidding.

In working on rifles we often use epoxy to bed them to the stock. I'd heard about Kiwi but of course have a wide variety of automotive waxes available here at home. So, one day I ran a test using several waxes and the Kiwi. Would you believe that the Kiwi worked the best??
It allows me to pop the barreled action right out of the stock once the epoxy has cured.
Several of my rifles have been bedded with this and I'm a believer.

Of course, no matter what you use, take wax & grease remover to the area to remove your release agent before actually trying to bond fiberglass to it - or it won't stick.

Patrick

[Patrick H.]

Re: Help with form material to repair fiberglass

Addendum -

I thought about this more today. I would highly recommend you NOT use silicone or any wax that contains silicone. You won't be able to easily wash it off after making your mold, and you sure won't get your repair to stick to it.

[James B.]

Re: Help with form material to repair fiberglass

Keith, yes, I think I remember that- We've reached a new high- Bubba wrote the service manual!!

[James B.]

Re: Help with form material to repair fiberglass

Tracy, interesting and I want to try this but it is all new to me so a couple of questions (and I've never worked with plaster of paris):
1. A "plaster splash" is that a term for a mold?
2. The beeswax and diesel fuel. I assume this is the breakaway agent? Does it also act like an adhesive? could I use mineral spirits instead of Diesel?
3. Is this plaster, plaster of paris from a regular hobby store?
4. Hemp- is this a hemp fabric or strands of hemp.
For the underside mold I will be working under the body, as I build up the 1/4" layer, will the plaster stick to the bottom of the jack compartment or will I have to make some simple form and hold it up until it hardens?
Tracy, Great suggestion and I will try it. Regards, Jim Baker

[Keith B.]

Re: Help with form material to repair fiberglass

Keith, yes, I think I remember that- We've reached a new high- Bubba wrote the service manual!!

don't forget for BB AC cars to drill a hole in the inner fender