I heard from my friendly games workshop manager, that screen wash is good for it to. I used water up until recently and now I have started using up my 'eavy metal medium (cus now there is lahmaim medium to replace it), I don't think I could go back, the medium is basically paint without a colour. And if found when thinning mettallics with water I found that if i put in too much water it would separate and u could start to see white bits in it, medium doesn't do this, also medium helps make it smoother, even though water does this too, medium helps give a more even coverage.
That said I wouldn't be dashing to the shops anytime soon to get it, even though medium is really useful u can only start to tell the difference if u r like doing something like non-metallic metal, water is just fine there won't be a huge change in ur painting quality if you use thinner, but if you can afford it get some
Me: To be honest im amazed there isn't a chaos god of not revising or at least chaos god of procrastination
MidnightSun: There will be, when enough people do it. Y'know when the Eldar were all engaging in their hedonism and Slaanesh ripped open the Eye of Terror with his creation? Slaanesh will have NOTHING on the God of Procrastination, and his vile minions will carve the galaxy asunder in a thousand year reign of blood.... Tomorrow