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Unread 07-08-2013, 10:45 AM   #1
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Layering, gluing, and sanding insulation foam

I'm building a big dinosaur-skull helmet out of blue insulation foam. I'm building it up out of several layers, and it's taking shape nicely, though it still looks blocky like something out of Minecraft. I've got maybe one more evening of assembling the base pieces before I start sculpting in earnest.

Any tips on gluing the layers together? I'm concerned that the glue will be much more resistant to sanding than the foam. I don't want to end up with a striated look, with ridges running along the glue lines between the layers. I'm thinking I should use white Elmer's glue - strong enough to keep the layers intact until I reinforce the outside with paper mache, but soft enough that I can sand and file it.

With the outer layer of paper mache, I can disguise any sanding ridges somewhat. But I'd prefer to get it as close as possible before starting that step.

I guess I can trim down any objectionable ridges with the hobby knife, but I'm sure there are some better tricks out there.

I've used Liquid Nails in the past for a prop out of white bead foam (ugh) but that was a wide, flat axe with minimal sanding needed along the glue lines.

No reference pics, it's an original character.

Last edited by nathancarter : 07-08-2013 at 10:49 AM.
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Unread 07-08-2013, 12:52 PM   #2
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The best thing to use is 3M's spray-adhesive specifically formulated for polystyrene foam. However, I don't think I've ever seen that stuff sold in a brick-and-mortar store.

The problem with white glue (PVA glue) is that it cures by drying, and the foam is waterproof. So white glue takes forever to dry. Many of the air-dry glues have this problem.

Rubber cement works pretty well, though it can slightly dissolve the foam. You brush a thin layer on both surfaces, allow them to dry, and then press them firmly together. When the layer is thin, it dries quickly enough, and the solvents in modern rubber cement are weak enough that it doesn't really get a chance to dissolve much.

Low-temp hot-glue can also work, though it is a bit of a pain to get the layer spread nice and thin.

Nearly everything you use will have a very slight striation problem, but unless you apply the stuff too thick, it is always easy enough to slice through it. The paper mache will totally cover any subtle ring-effect anyway.
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Unread 07-08-2013, 03:30 PM   #3
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Thanks. I'll see if I can find the adhesive spray on Amazon, but might just end up using low-temp hot glue, since I'm impatient... and gotta be done by D*C (under two months).

I'll take some pictures along the way.
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Unread 07-08-2013, 08:31 PM   #4
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On a whim, I bought "Tree House Studios" brand spray adhesive from Hobby Lobby, since I was there for a couple other things anyway. Test piece was successful: It doesn't eat the blue Styrofoam brand insulation foam, and the bond seems plenty sturdy. Comes in a black can with a white cap, says "Bonds paper, FOAM, wood, ..." right on the can, cost me $5.99.
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Unread 07-08-2013, 09:32 PM   #5
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i always use tritebond 3 wood glue and it works fine for me, im glad that worked out for you! i remember the frustration of figuring out how to seal and paint rigid foam
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