Hey Mick,

That is an interesting thought to use foam saturated with plaster to make a pattern. Since you are asking for some thoughts, I will just kinda fire some out and see where it leads...

The open cell foam has a structure is like one of those 3D molecular models you see on the science shows. I have never tried it but it looks like the foam structure is open enough that the plaster should hold together after it is set. I think that sitting on the plaster saturated foam would squeeze out the plaster leaving the foam to rebound when you release the pressure. (I used to use layers of burlap saturated with plaster to build and strengthen molds but this is a different approach.)

The plaster may have be forced into the foam with out making it too thin. Plaster doesn't "dry" - it sets just like epoxy, water is the catalyst. The strongest plaster is made by (slowly) adding enough water to a bowl of plaster powder until the water starts to well up to the surface. Adding too much water causes the plaster to be weak and soft.

While it is curing, the plaster goes through the stages: liquid > "plastic" > set > cured. The change is fairly fast but you would have to sit very still until the plaster was well set or it would be likely that the cross linking would be disturbed and the mold would be fragile until it was glassed.

In your suggestions you are trying to make a mold by containing the plaster with foam or plastic sheet. This can be done but it can also be very messy. A quick frame made of cardboard or wood is often easier.

The easiest way to make a mold with out a frame would be to make an impression in sand and pour a 1/2" layer of plaster over the whole area. After that has set, add a couple of layers of plaster saturated burlap to give the casting some strength. Clean the pattern and trim to size. The bottom of the kayak could be copied a similar way. Then make a fiberglass copy of each side and epoxy them together. A couple of stiffeners or some "great stuff" might be required inside for strength.

The approach that I used for my seat was to make a frame to contain the mortar that was also an integral part of the mold. The wood allowed me to shape the sides easily. I could have used plaster for the casting but I did not need that quality of finish. A $3.00 bag of pre-mixed mortar was good enough since I had to fiberglass the mold anyway.

Pursuing the idea of using foam to make a pattern... what if you carved a piece of foam to fit the kayak and roughed out the contours for your butt. Saturate the foam with plaster and let it set (might need a bit of support on the bottom while setting). Once the pattern is set it should to be strong enough to sit on carefully, put the pattern in the yak and make adjustments to the contours as required. After the pattern has cured, apply a coat or two of epoxy. You could work with that directly or make a negative mold from it. It would probably take an hour or so to make the initial pattern and a week to cure. Might be fun to try.

Hank