I used polystyrene from the packaging around TVs and stereos for a base then use tile grout for the plaster it drys fast and comes in many colours then it is weathered works well.Cheap too!

Great tip Sean.

Perhaps you could try writing a build feature explaining to us how you do it?

Here's a how to without pics as I don't have a camera at the moment.
1. When the kids or yourself have opened up the appliance box there is normally all that polystyrene around your new toy (or go to the shops and ask if you can have some)
2. You can cut it up into the thickness and length you require(for that rubble look drag the back of the knife along it )
3. Cut windows or doors into it and for corners just use pva glue to join don't use solvent base as it will melt.
4.Then it's ready for the plaster work,I use a small plaster blade to mix the grout and apply either thin for stucco look or thicker for brick work.For the brick work look just use a toothpick and draw lines in it.
5. depending how thick is how long it will take to dry(usually 5-10min)
then it's ready for what ever paint job you like.
This works well and takes only minutes really and the grout will not crack either. This is really good for 1/35 scale and looks great!!
CHEERS SEAN