Okay, I haven't finished yet, but thought I'd post some results. I'm trying to make a seat mold from an existing commercial boat seat. I started from Jay's suggestions, and made some procedural changes to fit the situation.

The seat has some concave sections (the side 'wings' where it mounts to the boat), so a full plaster cast wouldn't work if I wanted to extract the cast after it had set. I needed something a little more flexible.

Failed attempt #1: I bought some 'Rigid Wrap', those rolls of plaster-coated cheesecloth that they sell at art stores. After using Saranwrap to cover the seat, I lined the seat with strips of the wet cloth and left it to dry. This turned out to be far too flexible. I was afraid that it would crack if I lifted it off the seat, so I mixed up some quickset cement patch and smeared it all over the inside. This made it rigid enough to lift out. After flipping the mold over, all of the grooves and folds left by the Saranwrap were very evident. I had tried to keep them to a minimum when applying it, but apparently didn't do a very good job. Still trying to salvage the situation, I smeared the surface with drywall joint compound with the intention of sanding the whole thing smooth. While waiting for this to dry, I had another idea.

Pretty decent attempt #2: After some experimentation with some mold release (that's great stuff!), and a scrap strip of fiberglass cloth applied directly to the seat, I decided that a mold formed from the fiberglass cloth would be best. The side next to the seat would be perfectly smooth and would require no further surface preparation (the plaster method requires the plaster be sealed somehow). The only problem was that the single layer of cloth was so flexible when it came off the seat that some sort of support was necessary. I decided to use expanding foam. First I covered the seat in mold release, then glassed in a layer of cloth. After this was dry, I sprayed-in the expanding foam to form a semi-rigid backing for the mold. After the foam was cured, I removed the mold from the seat. This took a little patience, working round-n-round pulling back the glass. After I got the mold separated, I discovered the problem with this approach: the foam formed many voids that didn't support cloth. These places are easy to dent-in, but if I'm careful, I think I can still use this mold. I'll be trying tonight. But, this let me to conclude that the following approach would be even better (and I should have thought of it earlier, doggonit).

Projected attempt #3: I think I'll try this for a multi-use seat mold. It starts the same as #2, but instead of using foam, just use a few more layers of cloth/epoxy. It will be flexible enough to remove from the seat, but won't be so flexible that it puckers and dents with a small push. I think that should make a good mold.

Cost for the learning experience: $15 for the rolls of Rigid Wrap, $10 for foam, $35 for the Happy Bottom seat that doesn't make my bottom happy, and whatever cost of cloth/epoxy that I'm using. Oh well, at least I'm having fun.

Ross