The end result can be very bubbly and not ideal. If you cast your shape from a mold, the answer is easy: spray into the negative shape and let that form contain it. That's basically how the people at the Monster Shop built their armatures. My problem is that I'm lousy at molding (despite vinamold) and I am working on a positive shape.
My solution: corralling. Way back when, I was a First Aid Attendant. When something horrible happened and large chunk of a person, well, fell out, you would have to keep it from travelling. But, you didn't want to put it in a vice grip (hey! I need that to live!). But you still needed to contain it. The answer was corralling. You use a firm substance (like bandages) to keep migration down to a minimum. Then apply the grip to the bandages.
So, I laid down the spray foam in an even pattern. Then, I let it begin to expand-- that can be hazy. It takes 8 hours to stop expansion. In about 10 minutes it can be close to full size, or at least in the ballpark. Taking a page from my first aid manual and a dredging up the racial memories from Red-Green show, I then began to layer duct tape over the foam, starting an anchor point when the foam with dry or there was no foam. I carried on past the foam in place and anchored the other side of the tape. Then overlap a little and repeat. The end effect is something like a silver mummy, but it can limit bubbling and migration.
As the foam expands, it gets resistance from the tape. The tape will gain rigidity and resist the foam, the will push back and out to the side, thereby getting rid of any valleys you may experience in surface. After the entire process of expansion and setting has occurred, I will pull off the tape and we will have something much closer to the ideal silohouette. If you're trying to repeat my steps, be patient: Even after it seems that the foam has set in 90 min., it will keep slowly expanding over the course to several hours. Give it time; sleep on it. My method isn't the fastest. History will be the judge of whether or not it works.
Is duct tape too intense and there will be too much of it in use? Yeah, but cellophone (glad wrap) is too flimsy; tin foil may work (I will try this in the next chapters). Ideally, I would like to work this into a negative mold and just spray it in. This sculpture is a one-off: no "1 of 200"; no "made from luxurious resin"; no "you can't cast that deep undercut" and no "buy a warehouse to house your molds." At the beginning of this process, I went through the catharsis of tossing five garbage bags of molds. I'm not going back there for a while.
I have more of the steps detailed on my Flicker pages.
***Update***
The corralling worked! I pulled off the duct tape to find that the foam underneath had a) been containing; and b) expanding towards its fellow chunks of spray foam. The net effect: a lot more even shape and a lot less need for sanding. It does use alot of duct tape (sorry, Mother Earth; luckily we have a spare planet now. whew.) So, the next experiment (expect and update next Sunday afternoon), will use tin foil with a small sum of duct tape (as a teaser: I will be building hands with spray foam. I may make a video of it as the process will take a maximum of 10 minutes each! Stay tuned!).
