I bought them from dungeonstone so I'm not sure what they are using though they say its very dense and porous.
There are several materials which can be used for the casts, from plaster of Paris, to Hydrocal, to dental stone. Certainly, plaster is very porous. It also suffers when the mix has pockets of unmixed plaster, which has no adhesion and can lift off from the surface with just a slight tug. I haven't tried painting the other materials, but the guides say they take paint better than plaster.
For all but the most minimal of painting, I suggest a primer. On the plus side, you should make it appropriate as a base color, which cuts down on the number of painting steps. Also, you can just slop it on with a large brush (but not too thickly!!), and mostly not worry about areas which will need a different color. For many of the projects, there are large areas which will all be painted with the same scheme.
See:
http://www.hirstarts.com/casting/casting.html and
http://www.hirstarts.com/painting/painting1.html.
For ensemble pieces, say, one of the bridge projects (
http://www.hirstarts.com/plans/bridgeplan.pdf), the painting is made easier because it is done after the pieces are assembled. That way, you aren't painting individual pieces, and the painting goes quickly. However, if you are doing a project which has a lot of separate parts (say, this new project for Zombiecide walls:
http://www.hirstarts.com/zombicide/zombicide.html) there is a lot more painting to do, since each piece needs its own attention.
Extra overhead with Hirst is having an area setup for casting, plus the time spent doing the casting. Most of the projects need a *lot* of casts. For example, the smallish 8" reound fieldstone tower requires 26 casts of the #78 RoundTower mold.
http://www.hirstarts.com/plans/8fieldplan.pdf. I'm thinking that folks who do much of this get multiple of the same molds to reduce the calendar time to do the casts.
To say, there are folks who will do casting for a charge, which is a decent intermediate cost way of getting pieces.
In any case, you would very much want a dedicated area to work. Most partners won't tolerate your tying up the kitchen for casting and painting!
Edit: This is so awesome I had to point it out:
http://www.hirstarts.com/watermill/watermill.html
With:
20 castings of mold #226 wooden beam mold.
16 castings of mold #225 half-timber mold.
15 castings of mold #253 brick panel mold.
10 castings of mold #263 rubble block mold.
10 castings of mold #230 clay tile roof mold.
8 castings of mold #227 tavern window & door mold.
Thx!
TomB