I've done fiberglass enclosures before, what are you planning to build?
I seldomly use fiberglass mat, I always used a stretchy felt like material from a faberic store and soak it with the fiberglass resin, It's easier to work with, it stretches for making nice tight shapes and soaks up a lot more resin than fiberglass mat, which makes it more solid with a one-time application.
Here's the steps I do: I build structures/skeletons of what I want with MDF wood, and whatever other material. I then make sure it fits in the location with a little room all the way around to allow for the stretchy material, bodyfill, and covering(vinyl or acustic carpet) Then I stretch the material around the structure, stapling/hot glueing it to the wood, making sure there's no creases. I then mix up some resin in a cut-down empty 4L washer fluid container, I mix it hot(it's pretty humid around here so using more hardener than specified cures it faster) Then I use large dollar-store paint brushes to apply the resin the the fabric soaking it well. I have a beer during the curing time. Then it's ready for body filler to smooth it, and add shape, using body fill shavers, I smooth it off before the body fill fully hardens. Then sand, sand sand, test fit, add body fill, sand sand sand. at the end, it's either ready for vinyl, carpet or even paint.ÂÂ
If you want to copy a panel, or get a certain shape into a cavity in the vehicle, you cover it with 3m masking tape, twice, overlaping your tape edges. The fiberglass resin won't stick to the masking tape(i found using cheap tape, the resin will leak through and stick to whatever is behind there and make a huge mess, I only use 3m now) depending on what your doing you can make a frame from MDF or something, and do the same as above, lay material stretching it, hot glueing it, and soaking it in resin.
