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Old 07-21-2011, 12:13 PM   #25
Sir Not Appearing
That's no ordinary rabbit
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Joined: Jan 2008
Location: Indiana: Motorcycle Hell
Oddometer: 1,114
Originally Posted by TheOtherBart View Post
I've been thinking about laying up a flattrack-style seat for my XR, but I'm too buried in projects to take the time for the whole plug-mold-part thing. Can I get a decent 20-footer by laying the seat right over a foam core? For a seat like that which is going to have some compound curves should I try to use cloth or is mat okay? It seems like mat would be easier to conform to the shape. Then smooth the outside with Bondo, sand, and paint?
I think for a "one-off" seat, the lost foam method would be a good way to go.

I forgot one of the steps we used when I typed my original post the other night. From my research when I was developing my project above, there was some debate on whether the following technique could cause issues down the road or not. What I had read about online on some forums was the use of 3M #77 spray adhesive when using the lost foam method. So we tried it and had our best success on the vehicles that used it and had some delamination issues between layers on those that were built without it. Here is what we did:

  • Build foam mold by cutting, sanding, etc.
  • Tape the foam mold on all but one side (in our case the bottom)
  • Cut the fiberglass cloth to the size needed
  • Spray the taped sides with 3M #77
  • Lay the cloth on and use squeegee/bondo spreader to get it tight
  • Mix resin and then brush it on
  • Cure
  • Sand first layer
  • Spray with 3M #77
  • Lay cloth, add resin and repeat until you have the number of layers you want

I got the idea from R/C sites and this is how some hobbyists do R/C boat hulls and fuselages for planes that have lots of curves. After the last layer was cured and sanded, we applied a very thin “skim coat” of body filler, sanded smooth, sprayed with high-build primer, sanded, painted and clear-coated.

I’m not saying this is the “proper” way, but it worked very well for us and the spray adhesive held the fiberglass to the curves. About half the kids in my 2 classes produced some very nice, slick looking car/truck bodies. If they had been bike parts, they would have been just fine for a 20-footer or better. Also, when we were done racing, about half the students (mostly the not-so-pretty projects) wanted to “crash test” their project so we fired them off at 40-50mph into a curb. Only one had fiberglass damage and they were all disappointed as they expected them to go out in a big cloud of fiberglass dust. Not so…. After 3 layers, they were much stronger than even I expected.
1998 Triumph Daytona
1991 EX500
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