Finishing on Polystyrene foam

Currently I am working on a large model about 60" X 20" large and the material chosen for CNC is Polystyrene foam.
The problem I am having with this is after the model is cut from the CNC machine the foam is very porous, the next step is making a plaster mold, but I want to maintain accuracy from the foam model to the plaster mold without hving the sand the mold after. So my question is, Is there anykind of Spray coating or finishing process that I can do to such a large porous object before going to a plaster mold?

Any help or suggestions please >.<

Polystyrene (traditionally blue or pink in color) will melt at the site of any kind of solvent, so make sure to stay away from anything that may contain alcohol. I think I remember people who have sealed it with a few layers of watered-down spackling compound…

Polyurethane foam (usually yellow in color) is a much better model-making base, though it is much more expensive than its polystyrene counterpart.

Are you sure you want to machine polystyrene? I think it will tend to tear and grab onto to your endmill making tears in your project. Polyurethane foam machines very nicely, but it’s dusty. I’ve had success sealing up polystyrene with Bin primer spray or Kilz spray. I’ve even sprayed light coats of laquer over that.

Machining styrene foam is fine, it’s just like urethane foam, you set your machining speed according to the density.

Back when i was a lowly student forced to use deadly materials for my models, i was taught to thin joint compound with water and run it through a spray gun to hard coat the blue foam models. Then prime and paint with water based finishes.

There is nothing worse than massaging foam for a week then hardcoating it and sanding it to a polished surface, just to spray some krylon primer on and watch it find a nanoscopic hole in your top coat and just eat you baby right in front of you.

The Main reason is because of the large quantities we do, Polystyrene is a cheaper material but its lead to us to doing alot of finishing work on the Plaster mold it self. Hence the reason why we are looking at cleaning up the Polyfoam before going into the molding. I think I am gonna try the Spackle meathod but if that takes too long I may look into some sort of spray coating. Thanks fo the input! any more suggestions are welcome :slight_smile:

Wanger, remember that polystyrene is a thermoPLASTIC (as opposed to ureathane which is a thermoSET). thermoPLASTIC = will distort with heat.

Make sure to build up a good ‘slip coat’ (slowly) before you start laying on the heavy stuff.

Drywall guys use a ‘hopper gun’ to blow on spackle. But you will end up doing a lot of sanding on a delicate surface (the polystyrene).

Something to keep in mind with any pattern work is that materials are generally cheaper than labor.