Sealing Foam for Painting

[elmayerle]

I've got a "Tu-26" display model (well, most of it, I'll need to replace the missing inlets - going to build up new ones out of card) that's cast in foam.  Any advice on how to seal it for painting and such?

[Rafael]

Hola, Evan.

I use to work a lot with EPS (Expanded polystyrene) foam, the one used for packing stuff. It's sandable, light, and easy to cut. What I use to seal my foam is a layer of white glue. I've found the carpenters types very good. I also use a layer of a "mud" made with common plaster and white glue. Good for sealing and also for building up/filling, depending on the ingredients proportions. With too much plaster added, is prone to develop cracks after drying, but a second layer can fill problematic areas or, better yet, a good pass of wet sanding paper can reaccommodate the entire surface. Any of these is a good seal for painting with other than acrylic paints. For a safety measure glue with white glue whenever possible. First try on a piece of scrap foam and use the one most to your liking.

These were made with cardboard, foam and mud (note the cardboard inlets) and painted with white foam-eating enamel spray paint:

[deathjester]

The way I was taught to do it was to glue small strips of plain white paper to the model with white glue.  Then, when dry, paint the whole thing with watercolour white paint.  When that is dry, you can paint it with what you like, as it is fully sealed, with a nice smooth surface.

[Weaver]

I think I've seen a spray-primer for foam in a hobby shop before now. Can't remember the specifics though....

[Rafael]

Yes, DJ and Weaver. You're both right. Foam is susceptible to almost anything non-water-based. The U-212 submarine hull in the last pic is being treated to a papier maché surfacing (more pics when ready in its thread) with the simplest formula: newspaper strips soaked in a 50-50 water-white glue solution (I prefer brushing it generously instead of submerging it in a pool of the stuff) The result is a 5-to-6 layers papier maché surface. Hard, very hard and sandable/Fillable. Any combination glues/paints will then work on that material.

[jcf]

Evan,
Is it the yellow to light-tan expanding polyurethane foam? I remember an old
Alpha-jet kit in 1/48 that used EPU foam for the main airframe components.

If so it forms a skin as it expands and cures, and can then be laminated over
with polyester or epoxy resins, it does not melt like polystyrene foam.
So the surface is pretty much sealed already, and is actually resistant to
most solvents, meaning it can be a bitch to work with when gluing part to part. 
 

Give it a light scrubbing with a scotchbrite pad, to improve adhesion, and hit it with
the primer of your choice.

http://www.shopmaninc.com/foam.html