Can I start by asking what base is best to use extruded foam? Or is there maybe a cheaper alternative?
Not totally sure what you're asking David. There's all sorts of foam board available ranging from thin pizza bases - Ron's (Spanner570) favourite- up to thick slabs of insulating foam and the thinner high end plastic or card covered foam. The type you need will depend on what you're building. For basic groundwork, just about any sort will work although I'd recommend using a wooden board as a base under your foam for larger dios. (My local DIY store cut up a sheet of 6mm MDF for me last year. The sheet was 8'x4' and I got 4 pieces 2'x2', 4 pieces 2'x1'6" and 4 pieces 1'x1' for about £10 - enough for plenty of dios for single vehicles or small groups of buildings or large aircraft).
The thinner stuff covered with either plastic or card can be quite expensive but it does allow for really accurate work if needed. The cheaper expanded polystyrene stuff is great for rough brickwork.
Any local builder's skip is a likely source of foam as it is used in a variety of types and thicknesses for packaging. Obviously as cheap as you can get but I'd always ask before just sauntering up and picking it out from the skip!
I bought a batch of blue insulating foam a few years ago. I got an assortment of sheets 600x1200mm in 6mm, 10mm and 20 mm thicknesses - I think I got four of each. At my rate of building that's a lifetime's supply right there but I can't remember where I got them from other than it was a site on ebay.
This is the kind of stuff I mean, although this is only the first site I found - there are lots of others:
Find many great new & used options and get the best deals for Styrofoam Insulation Sheets 10mm for caravan/motorhome wall/floor ren. 8/pack at the best online prices at eBay! Free delivery for many products!
www.ebay.co.uk
Those thicknesses allow me to build normal brickwork, rough stone for castles and the like, and I use the 20mm stuff for my groundwork, building up to the required thickness in layers. It fixes well with ordinary PVA although I've found you need to keep some pressure on the join until it sets (A few books resting on top of flat pieces and I use cocktail sticks or dressmaking pins as 'nails' to hold more awkward shapes together. These can be left in place or removed as required after the glue has set).
The cheap expanded polystyrene - the sort that breaks into little round balls - can be used to create rough ground and stone walls - have a look at some of Ron's builds as he's rather a dab hand at doing just that!
Can't help with your questions about washes but I hope this helps.
PS Welcome aboard.
Mainly for thinning paint, usually for airbrushing. You can also use it to make acrylic paint that has thickened in its pot, usable again.