Glassfibre Filling Issues (non-stag)

[V Mad]

I have a special 'racing' glassfibre bonnet on the Tiger which I bought new. I wanted to improve the fit at the front, so I fitted a metal rim that had the right edge, then bonded this to the bonnet, and used a small amount of filler to blend in the edges. It looked great for about a year but then the filler appeared to be shrinking, and there is now a deep ridge around the adge of the join.

I thought that fibreglass mouldings are made with epoxy resin, therefore an epoxy resin filler should work, but it clearly doesnt. Can anyone explain why this might have happened? Hw can I repair it so it doesnt happen again?

[dasadrew]

Chris,

build up the filler in very shallow layers and let each one harden before applying the next one. Make sure it is all done at a good warm temperature or even use an infra red lamp or similar*. I used to build museum models and it is amazing that even epoxy fillers continue to shrink after years, especially if the thickness was more than a couple of millimeters.

*Catch 22 is unfortunately that, when the layers are thin, the hardener needs additional temperature as it doesn't develop enough on its own.

[V Mad]

Interesting, but I dont recall ever having this problem with filler on metal. I thought it was due to the fact that I was filling over glassfibre.

[Jeff]

Interesting, but I dont recall ever having this problem with filler on metal. I thought it was due to the fact that I was filling over glassfibre.

What brand are you using Chris?

Maybe worth changing to another. I have used many different fillers and they vary quite a bit, shrinkage wise.

Jeff.

[KOY 23]

I thought that fibreglass mouldings are made with epoxy resin,

Chris this maybe where the problem is, fibreglass is in fact polyester resin not epoxy.

[V Mad]

Chris this maybe where the problem is, fibreglass is in fact polyester resin not epoxy.

Apparently John fibreglass can be either epoxy or polyester.

[KOY 23]

True but in the context of automotive and many marine applications fibreglass = GRP (Glass Reinforced Polyester). Sand it and your nose will tell you if its epoxy or polyester.