"moldless" fiberglass/cf construction? Options

[obaa996]

With all the expertise on this list, I thought this might be a good place to start....
I'd like to build a new backpad for my car; I have aftermarket seats, so the stock pad doesn't fit, and besides, I'd like to embed a subwoofer box into it anyway. Since this is going to be a one-off, I don't really want to go through the effort of building a mold, etc. for it either. What I'd like to do is build some sort of form (in the car) which looks like what I want. Hopefully, II can then cover it with fiberglass to form the basic backpad, and then cover it with a layer of carbon fiber (because I like the way it looks). When it's all cured, I'd then remove it from the car, remove the form from behind, and then add more fiberglass to build up the box.
It all sounds good just talking about, but how hard would it be to actually do? What can I make the original form out of? How do I seal/finish off the foam so I can lay fiberglass over it (and be able to remove it later). And how do I get the different fiberglass/cf layers smooth enough so that the finished product will be nice appearance-wise?
All the sites I've found online all discuss building a mold first, but I can't imagine that all the custom pieces I've seen at shows, etc. resulted from a plug/mold/finished part process just for a one-off piece.... Any suggestions here would be appreciated!

[Kevin@ojai.net]

Ok, here's one way (very quick and dirty and incomplete) on how to do this...

Get a big chunk of foam and shape it to how you like it. Then cover the foam with a layer of resin mixed with microspheres -- this will seal the foam, allow you to sand it to get however smooth of a finish you want and allow you to pull the part in the future. Once the "plug" is finished being prepared apply a liberal coating of wax and spray it (don't paint it on -- it won't work) with PVA release agent. You can then make a glass mold.

The trick to getting a nice finished product is a nice mold, and the trick to getting a nice mold is having a nice plug, so spend your time on the prep work. When making the mold apply a little extra resin to the first layer of glass to get a nicer finish, and to have a little room to remove any imperfections you find. You should also apply pressure, to remove any air that is trapped between the layers of glass, and to push the glass firmly against your plugs. The easiest way to do this is with a vacuum bag. Once finished, you can apply a liberal coating of wax and PVA to your mold. You are now ready to lay up your part. If you can, incorporate a lip so you have something to grab a hold of to pull the part out of the mold -- this lip can be very easily cut off after you pull the part.

The reason for making a plug and then a mold is so you can control the surface finish of your finished part. Also, when selecting a resin make sure it will maintain its mechanical properties at the temperatures seen inside a car during the summer. Vinyl-Ester resins should work – you may be able to get away with using a cheaper resin, but it would really suck to have your back pad deform on a hot day…

HTH,

-Kevin