Okay, I've worked with insulation foam a few times and I have a few things to add that I've had success with.
1) Adhesives - Water based contact cement is what I've always used to attach foam to foam or foam to anything else. The stuff I've used before comes in a little pot that looks like a minature paint can and the label is yellow and green. I forget the brand, but I can find that out for you later. But the biggest part to remember is that it HAS to be WATER BASED
. No if, ands or buts about it. I once tried to use these cheap little tubes of contact cement from the dollar store because I was low on time and money and THIS
is what happened to it. The contact cement was apparently alcohol based and ate through the foam right before my eyes!
As for using contact cement you'll need a paintbrush (a larger painters brush from the dollar store is perfect). Contact cement (or at least anything I've used) is very watery, so you'll need the paintbrush to spread on an even, decent amount on BOTH surfaces you're going to be sticking to each other. Then you wait perhaps 5 minutes (depends on how thick you applied it) or until it is almost dry and very tacky. You then put both tacky surfaces together. While they're drying it's best to either put something heavy (like a text book) on top of it or clamp the pieces together. My suggestion is to leave this for as long as you possibly can (24 hours would be great, but I've started working with it only after a few hours in the past).
BTW, once you're done with the paintbrush for the contact cement, if you want to keep it/use it again, CLEAN IT RIGHT AWAY!! Do not let it dry! It'll cake onto your brush like rubber and be almost impossible to clean up after the fact, so do yourself a favour and don't use any fancy, expensive brushes.
2) Strength - I completely agree with you that Gesso is the perfect top coat (as you can also sand it to a degree to get a much smoother finish), but as the ONLY thing protecting the foam from dents and scratches ... no to much. What I've used for both Mr. Pointy
(my Rogue sword) and my Lute
has been a combination of wood glue, water, cheese cloth and tissue paper, which I usually hear referred to as Scrim. Take wood glue (as it is stronger than regular white glue, but if white glue is all you have, go for it) and basically mix a ratio of 1:1 glue and water (ie, as much water as you have glue). If you'd like it to be thicker, just add less water.
You're going to use this combination exactly like paper mache. The cheese cloth will be your base (you can get this in the dollar store here in Canada ... not sure where you'd get it anywhere else) and the tissue paper will go over top to make the surface smoother. First spread a thin layer over the surface (so that it sticks better), then take your cheese cloth (which you need to cut into pieces before hand. Really, and size or shape will work, but the bigger you make them the harder it will be to make it smooth, so I would suggest roughly 2"x2" or smaller) and lay it flat on your surface piece by piece, then take your paint brush and cover the dry parts of the cheese cloth with some scrim until they're completely covered.
Continue this until you've covered your entire surface, overlapping pieces is a good idea for strength, but do your best not to have any bumps in the cheese cloth, as itís hard to get rid of those after the fact. 2 or 3 layers is best for strength. Once you're satisfied with the amount of cheese cloth you've covered your surface with, switch over to using tissue paper. Helpful hint with the tissue is to rip
your pieces instead of cutting them with scissors, as this will make it easier for the edges of each piece to disappear into each other instead of seeing straight cuts all over your surface.
I'd just like to take a moment to explain why
you've done all this. Cheese cloth is a fiber, therefore making it harder to tear apart, unlike tissue paper. This makes a strong under coat for your foam. The thicker the layer the stronger it will be. Problem with the cheese cloth is that it's a fiber, so it has texture. For your own sanity (as in, I skipped this step on my Rogue sword and lived to regret it) do a top coat with the tissue paper. What this does is add another layer of strength to your prop, plus smoothes out and covers the texture of the cheese cloth, which means you can use less gesso on top of the scrim (because you have less texture to hide/even out). Yes, you can skip the cheese cloth coat and go straight to the tissue, but it will also make your prop less resistant to dents and scratches.
Then gesso and paint. ^_^