I'm making a plug to form a fiberglass cowl for a scratch build of my own design. This is my first attempt at fiberglass construction. I have formed the plug from the green florist foam (oasis), coated it with finishing epoxy to stabilize the surface, sanded and filled with spackle to get a smooth shape, and coated that with finishing epoxy for a smooth finished surface. I'm almost ready to start using it to form the mold.

The question: Before I wax and put on the gel coat, should I wet sand the finish epoxy coating (400 then 600 grit), or should I just go ahead and wax the finish epoxy without additional prep?

Brad

It's hard to say without some pictures but the one thing to remember is that the mold finish will only be as good as the finish on the plug..

There is no fast easy way to make good glass parts. It is a three part process. Plug, Mold, Part. The surface finish will not improve at the completion of each stage. The finish on the plug determines the finish on the part. The high gloss finish in the mold is not just for show. The gloss finish is a big factor in the release of parts from the mold.

So, paint the plug with a good quality paint, enamel, auto, etc etc. Polish and buff it to a high gloss. Wax the plug with Mold Release wax, Then apply the gelcoat.

The picture shows a reasonably good mold surface.

Ed S

Ed,

I've seen those molds many times now, and am curious how you did your parting plate with the different angles?

I took another look at it tonight, and wasn't satisfied yet. so I sanded with 220 to knock down the high spots, and used a credit card edge to scrape finishing resin across the surface to fill in the low spots. It works very well. Tommorrow night I 'll hit it with the 400 wet sanding and do it again. I'm not planning on painting the mold, since the finish resin is creating a nice high gloss finish. Once I'm satisfied with the finish on that, I'll wax. Looks like a few more nights sanding.

Brad

I second what ED says, all the work on the finish of parts coming out of the mold happens on the plug. If you are going to make two or three parts, having to do a little touch up on the part isn't a big deal. If you want something that comes out of the mold ready to rock and roll, spending the extra effort on the plug is very worthwhile. Here's a plug I worked up just before molding. I finished this with tinted polyester gelcoat - I find that it is a terrific finish product with good filling properties that can be sanded aggressively but still brought to a terrific sheen (it just doesn't hold this kind of sheen in the outdoors as long as some other finishes).

That plug started life as a hand-carved plug that was given to me looking like this:

Hi Dave,

It was a bit of a task. There are three different surfaces on bottom of the fuselage. Under the wing the fuselage is flat and parrallel to the wing. There is a slope up to the nose and up to the tale end. If I had any sense I would have made the fuselage flat all along the bottom.

The fuselage has a 1/4" rad all along the bottom corner. So the bottom of the mold is 1/4" deep with the rad in the corner. For the parting plate I clamped a strip of aluminum to the bottom of the fuselage that followed the shape. I prefinished some 1/4" wood and glued them to the aluminum plate. the wood pieces followed the shape of the fuselage sides. The first half of the mold to be made was the deep three sided part. When that was done I removed all of the parting board material and made the bottom mold.

The best thing I did with that mold was to have the removable plate at the firewall. I join everything in the mold while it is all still wet. Removing the firewall plate prevented the finished fuselage from locking in the mold.

Ed

Wow! Amazing mirror finish! That's too good to go through a molding intercourse!