If you're making a gas tank, you really ought to use epoxy resin and Caswell fuel tank liner, if you don't want a leak. If you're just doing a seat, polyester resin is fine.
For the tank:
I first separated the tank bottom from the top and sides:
The plan was to keep the bottom portion which would keep the stock mounting points and petcock bung, saving me some time. I got it how I wanted it and set it on a foam base and molded a little portion of foam to extend the tank to the rear:
Trimming to size:
Then I put it back on the frame and sculpted the tank shape on top of it:
All glassed up:
The inside:
Next, I gave it a layer of Kevlar for abrasion resistance against asphalt...hah:
The seam where top half meets bottom was sealed with two layers of Kevlar, and the gas cap was epoxied in:
The tank was filled and puttied to make smooth...I'll get to the seat next:
Then primed:
In hindsight, I wouldn't have used the Kevlar. During the smoothing process you will have to take the high points out of the glass. Fiberglass simply sands away, staying smooth. Kevlar only frays if you try to sand it. Which means building up your filler layer thicker than you would like. And God help you if you sand through the filler and hit the Kevlar...to cover up the fray you have to add even more filler, because it "puffs" up when it frays.
