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So I'm making some big scenery out of styrofoam (the white kind) and I started thinking, how do I protect it for general wear and tear? Is a layer of glue with sand enough? A layer of plaster with sand? Plaster and glue?

If you must work with it then sand and glue is your best bet as it hides the texture too.
Mix in a little black paint to the mix so hopefully any chips and dings will be black rather than white or sand coloured it'll save you on primer in the long run too.

I'd get some Gesso (kinda like a paint-like plaster) and go all around it. Not only does it texture it to a gritty finish, but also hardens and makes for good protection. If anything can break it, it'd probably powderize the foam anyways.

You might want to try plaster of paris. It's dirt cheap and can be easily mixed in bulk / slathered over the foam to create a hard shell. Only draw back is that it's very brittle so you may experience some chipping.

Wood glue will protect it from melting if you use spray paints.
Plaster, Milliput, caulking (may not be an international word - white DIY wood filler), etc will make it sturdy.
Your main worry is stopping your primer from eating your foam, though.

couple coats of interior or exterior waterbased acrylic house paint ..inexpensive and stands up..pick the main colour at your home goods store..(works great for large peices) ..paint can be thinned and texture can be added ..(note this it not a minatures paint by anymeans)..alternately good olde fashioned paper mache over the top works well then paint (newsprint torn into strips and a gluie made of startch, salt and water heated till thickened) Paper Mache will last for generations if not abused..also very low techa and very inexpensive..

For outright toughness I'd suggest the following;
1) Plaster bandage.Model shops sell it.
2) A more unusual approach is kitchen roll and plaster.I made a whole table top of styrofoam covered in this stuff and it never chipped once! Tricky to apply as you have to brush the plaster on to the kitchen roll;you'll need a lot!

 Novasetri wrote:
You might want to try plaster of paris. It's dirt cheap and can be easily mixed in bulk / slathered over the foam to create a hard shell. Only draw back is that it's very brittle so you may experience some chipping.

Straight plaster is horrible for coating terrain, because it chips so easily. Unless you're super-careful with it, you're very quickly going to wind up with white spots all over your terrain where bits of plaster have chipped away.


A good coat of glue or glue and sand can help, but polystyrene is inherently fragile for terrain construction, so they're going to break or chip sooner or later. The plaster bandage idea would probably help, but I've never tried it.

Slipstream wrote:
For outright toughness I'd suggest the following;
1) Plaster bandage.Model shops sell it.
2) A more unusual approach is kitchen roll and plaster.I made a whole table top of styrofoam covered in this stuff and it never chipped once! Tricky to apply as you have to brush the plaster on to the kitchen roll;you'll need a lot!

Kitchen roll?

 Vulcan wrote:

Slipstream wrote:
For outright toughness I'd suggest the following;
1) Plaster bandage.Model shops sell it.
2) A more unusual approach is kitchen roll and plaster.I made a whole table top of styrofoam covered in this stuff and it never chipped once! Tricky to apply as you have to brush the plaster on to the kitchen roll;you'll need a lot!

Kitchen roll?

Kitchen roll is a uk slang term for paper towel thats twice the length of toilet roll as its mainly used in the kitchen and comes on a roll.

Ps

sand and pva with paint

I have a huge range of home made terrain, i regret some of my plastered terrain(plaster is good but only in the right places, its also better if mixed with pva instead of water), its great but it chips bad, ive put most of it on bases to minimize chipping and im very careful but it still happens. Use very fine sand, coat the poly in pva and paint mix then apply dry sand let dry then repeat pva and paint. This should be repeated if needed. dont use a premixed render of sand n pva its messy doesnt stick well and dries weaker and often strange.

For practical solutions for your region go to the Model Railroad shop/museum/club nearest you they will teach you how .

Don't know if this is an option for your piece, but using some MDF board as a base will go a long way in preserving the edges. Just extend it out about an inch and a half past the terrain piece and it will take the bumps and chips before the Styrofoam; it's easy to repair, too.

After making a LOT of foam terrain over the years, I have two suggestions;

1 - All the things people on here have said; plaster, glue, spackle, etc. in a mix of some kind over the foam.

2 - Or don't. Yeah, that foam is cheap, but in the long run, if you balance out the work vs. result and the costs - I have never found it to be really worth it.

I would suggest abandoning the white open cell foam, and just going with the construction insulating foam (closed cell, sometimes pink or blue). Its more durable and just easier to control when working with it.

Again, I base this upon a lot of experience - some of it quite hard earned. I get it - sometimes you find a great piece of foam - love the shape or size etc. but it can be a headache in the long run.

I would say make a few test pieces, and play with them for a while and see how they turn out before you invest a lot of time and effort into more complex projects.

best of luck!