by IBBoard at 03 Jan 2011, 19:54
I'd definitely say that the new pots need thinning before use (anything in a dumpy pot with shiny plastic sides), but if you get hold of older paints or other brands then it can vary (I've got paints that I can get away without thinning). In general, though, it is advisable to thin paints, if only because accidentally making it a little too thin just requires an extra coat, where as accidentally applying it too thickly is more difficult to correct.
As for how to do it, it depends. A white tile is good (if you have one lying around), or I've used a CD jewel case over a bit of white paper, or if you've got money to burn then GW do a shield-shaped pallet (or most arts and crafts shops will sell conventional ones cheaper - ditto for "accessories", really). Put some paint on your pallet and water it down there. I normally load the brush with water, deposit the water next to the paint, then dry the brush to make sure paint doesn't shoot up the bristles, then mix.
The alternative (bad habit) way is to put the water into the middle of the lid of the pot. It is quicker, means you don't need a pallet and stops you leaving unused paint on a surface to dry (which is why I do it a lot) but it does mean that you can't see the consistency as well and that you're mixing it in the pot, so you may over-water the entire pot with time. Given the state that my Bleached Bone paint was in (i.e. drying out and thickening to gloop within a couple of weeks) then that last item wasn't always a problem for me, though!
As you worked out, washes don't need to be watered down (although they can be for some effects - like getting a lighter wash on High Elf armour so that it keeps more of its shine, but still gets shaded in the creases).