The Saab Link Forums - View Single Post - Composites: so easy, anyone can do it wrong.
View Single Post
Old 08-13-2016, 12:58 PM   #1
Avid TSL User
Join Date: Apr 2011
Location: Mountain View, CA
Posts: 194
Composites: so easy, anyone can do it wrong.

Near the beginning of my blue car's life, back when it was still going to be a lemons racer for some friends, there was a hood pin "incident" at a test day. The hood flipped up, took a huge dent, and shattered the windshield. Obviously the windshield had to be replaced right away, but the hood was left with the gnarly dent because screw it, lemons car.

Since inheriting the car I'd always meant to replace the dented hood. And fix the sketchy, extremely front-heavy weight distribution. Hey, maybe there's a way to get these two birds stoned at once.

Like a lot of you, I'm crazy about carbon fiber. Nobody makes carbon fiber parts for saabs (except Kevin Yankton, many years ago, and those parts were unobtainium). You can't just go on amazon and prime up a ng900 hood. It's either get someone to build you a one-off at extreme cost, or do it yourself.

I had previously experimented with fiberglass molds and a carbon fiber sunroof delete panel for my c900 7-8 years ago. I didn't have a shop back then, so it all had to be done in the hatch of the car itself. It turned out terrible. It only managed to keep rain out because it was half glue and caulk, and I eventually replaced it with stainless steel in the same build thread where the cage went in. As a learning experience though, I thought I had paid a lot of my dues. This time, things would be different.

What actually happened was a four month odyssey of finding every way to screw up composite fabrication. All I can say in my defense is that this time, NEXT time will be different.

Starting point: junkyard 9-3 hood. Pick n pull had a sale that weekend where everything was half price, so I think I paid $30.

Making a mold sounds straightforward, you just put release agent on your part and slather a bunch of gelcoat, fiberglass, and resin over it.

And end up with junk. Lesson one is that the big sheets of thick chopped strand mat don't like to lie flat. Air gets trapped and the gelcoat and glass don't bond right, so they delaminate and flake apart.

No big deal, all this stuff is sandable so I can just throw more gelcoat on the thin areas and sand it smooth right?

Lesson two is that sanding is an infinity of work. If the part is a little off, yes, it can be patched. If it's majorly screwed up from the start, fixing it is impossible because there is not enough reference for where the surface should be sanded down to. This was a waste of chemicals and time and produced a hurricane of nasty white plastic dust.

Rather than throw it away immediately, I used failed top skin mold #1 to practice trimming to fit, and to test fit the aerocatches that would replace the exposed hood pins. Other than the fact that this is not structurally rigid, it's half believable.

jerrit is offline   Reply With Quote