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I wanted to make some hills out of polystyrene and had seen a video about melting the sides with a candle to give them texture. I was wondering whether this would firm up the sides (to stop the little balls falling off) of if anyone could suggest another technique?

That would make the outer edges stronger and less crumbly.

Although do it outside, the fumes off that will be awful!
Hot wire cutter can do a similar job.

Also, just sealing the styrene with pva, then a durable varnish afterwards should be fine. You have to seal it well because lots of the thinners used in tough paints will melt it.

Give them a quick once over with some watered down PVA glue would probably do the trick

Or use the polystyrene that's isnt "puffed" balls and just strictly dense foam.

I rub it over with an old soldering iron, binds and gives texture.

If you had the means to control the temperature finely enough, melting expanded (the white beaded stuff) or extruded (the pink or blue insulation foam sheets - i.e. what Crulo is talking about) polystyrene would essentially yield plasticard (sheet polystyrene). Trouble is, a candle or soldering iron will likely just burn things and make a mess. Any styrene that does hit that sweet spot is going to be paper thin - likely less. With such a thin layer of denser plastic, the squishiness and poor "poof-to-poof" adhesion of the rest of the block means you've gained little, in terms of durability

Much better to coat the stuff with something resilient or simply use a stronger material from the outset. XPS (expanded) is great filler for large forms with a hard shell (paper mache, plaster, plasticard cladding, etc.), but EPS (extruded) is a much better basis for terrain that will only receive texture and paint on top.

I would use a fine or if you prefer - gritty - polyfilla... normally used for walls but great for terrain

Propane torches will do the job much quicker than a candle...

T