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Old 06-12-2018, 12:00 PM
cliffrod cliffrod is offline
Join Date: Dec 2014
Location: Spartanburg, SC
Posts: 1,608

Originally Posted by kcoffield View Post
I've worked with expanded and extruded polystyrene extensively in lost foam patterns for metal casting and resin/cloth lay ups. I'm sort of surprised to see the 3M 45 and Bondo mentioned because I've found they both attack polystyrene. When you see "extremely flammable" on the spray can it usually means toluene which aggressively attacks PS. The carrier in polyester (Bondo) also attacks PS. That said, if you mist the 3M 45 on, there just isn't much solvent to attack the PS. If you mix the bondo hot so it kicks quickly you can get away with it too. If you apply polyester resin directly to PS in an attempt to lay glass cloth on it, it will rapidly reduce the PS to a gooey mess.

Most spray contact adhesives designed for PS foam are latex based (3M 77 I believe). The carrier doesn't attack PS but it dries slower. I hate using contact cements of any kind to bond PS because when you try to sand or work the seem it balls up, loads up your tools and abrasives, and also doesn't sand evenly at the joint, especially a glancing/feathering joint.

Most Epoxies (try a sample first) don't attack PS so they work well for glue. If you coat the PS surface with epoxy laminating resin or epoxy paint first it seals the PS and you can usually apply just about anything on top of it.

For laminating PS sheet, I've found shellac works well. The solvent for shellac is alcohol which does not attack PS and still dries relatively fast. If you paint a layer of shellac on the foam, let it dry, and then paint on a second layer, it reactivates the first coat. When it gets tacky, it will stick sheets together like contact cement and if you time it well, still allows a little re-positioning before it sets. It machines/works better at the joint than the contact cements because it hardens but it still will cause an imperfection when shaping the foam across the joint because of the difference in hardness. Clamping the sheets in a press helps this by keeping the joint thin.

Plaster adheres fairly well to PS and though heavy, is an easy to work top coat. Do yourself a favor and build a hot wire to rough things out....keeps the dust and mess down.

Great info, Kelly. Thanks for sharing.
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