[Bicycle Bob]

Hmm, I was chasing a linkback, and found this old unanswered query.

Building up a foam block is popular. The glue lines can be a problem when sanding, but latex-based contact cement, preferably aerosol, is not too bad. To create a big blob at the same price, you can mix Part A and Part B by sloshing them around in a garbage bag which becomes the rough mold.

Snap-off blades make great carving tools but dull very quickly. Sanding is good. You can buy urethane foam, which is impervious to polyester, and use scraps to sand the rest, as blocks that shape to the correct contour.

The foam surface must be filled smooth to release a mold. Candle-wax (Parafin in America) might work, applied with a hair dryer, but I've never heard of it. I have used a mix of microballoons, wall repair compound, and latex paint, which dries to the same sanding-hardness as insulating styrofoam as a filler and top coat. It, in turn, has to be sealed and polished, using either Shellac or Polyvinyl Alcohol (PVA - also handy as a water-soluble mold release)
Then it gets waxed with mold release wax, which is very thin and hard, to preserve details, good or bad. 3 coats are always used on a new mold, along with muttered incantations and/or other safeguards. (OTOH, I knew a guy with a whole shelf full of oddball resins, with the recommended release agent for each. One day, he used Pledge furniture polish on a countertop, and a sample of each resin released just fine.) A "soft" master made of foam and filler will break up during release, so it is probably better to just use regular mat and gel coat for a surface. You can protect styrofoam from a 1st layer of polyester with a layer of masking tape, and use easy-to-sand, quick-to-patch polyester all the way to the 'way-easier-to-polish final surface. It can be good practise with the stuff, too.

Usually, we make a master model of a part, and pull a mold from that, to get the shiny side on the outside of the final part. For a shallow shape like a fender skirt, it might be OK to use a male mold, and finish the outside. With a re-useable mold, you might be able to do the other side by starting with a thin layer, and then turning it inside out. Then add a second layer to give final strength, and tape it on while still "green" to take out any warpage. That won't work for a flanged edge, of course, just a spoon shape.

To create hard points for mounting hardware, you can add layers to make a thicker pad, and then encapsulate a bolt head or nut, visible only from one side. The layers of cloth or mat with the hole have to be well bonded, and spread out to a long tapered edge.

Adding material from a second kit sounds fine. Just remember to thoroughly sand off any wax before making secondary bonds, and leave it rough from 100 grit. While epoxy is quite wonderful, and on average is tougher than polyester, you should not be relying on the resin for strength in any composite part. It is only there to keep the strong fibers lined up.