There is epoxy based resins, and polyester based resins.
Epoxy resin although a little harder to work with as far mixing, you have to be a little more precise on the amounts. It does give you a little longer work time. It is more stable when dry, it is also more waterproof, and bonds to itself both cured, and uncured better than polyester based resin. Epoxy resin is a fair amount more expensive too.
Only some of the epoxy resins leave a wax type emulsion after they dry. If you happen to use one that does it washes right off with vinegar, and wipe clean with water. Then you can sand it without clogging sand paper.
Another popular misconception is the more resin you use the stronger. The strength is in the glass. All the resin for is to make it hold together, hold a shape, and give it a structure. When fiber glassing if you are going to cover a form be it a permanent form such as wood, or foam which is a great way to make a shape, then remove it later leaving the glass in that shape. You should brush a thin to medium coat depending on the weight of mat, or cloth you are using onto the forms surface. Then lay the glass on to the wet form dry. Use a short bristled paint brush, or a fiber glass roller to force the resin up through the glass. Working any air bubbles out in doing so. You should only be able to saturate the glass with the resin no more. Sometimes you need to add a little here or there. or mop some up with the brush.
Once your first layer has dried, if its not completely cured with epoxy resins you can do one, two, or more layers over it in the same fashion.
When using polyester based resin you will need to let it dry enough to give it a half way decent sanding to create a mechanical bond for the next layers.
You shouldn't brush on polyester based resins as top coats. They can shrink later on as cause blemishes. or have air bubbles that lead to pin holes when you sand them.
As far as epoxy based resins they make fillers you can mix with them to make them more like a body filler. Fine sawdust works for this as well. Like the type you would get from a belt, or palm sander dust bag.
Once the glass is done you should only need filler to hide the rough surface of the glass. Or to blend it in with old glass if its a repair.
As far as working with wood. You should be able to shape the edges and corners or the wood any way you want. If its plywood, and leaves craters because of the layers use body filler to fill in holes, etc.
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