HERE I COME
TO SAVE THE DAY!!!!....sorta....
So i have good and bad news for the both of you.
Good news! You are not alone!
Bad news! You are screwed either way!
Snoop you are pretty close in your assumption with it being the pigment, but its rather, the lack of it that causes this problem. Yellows and whites are very VERY bad at drying out, this is in all paint ranges, not just
GW, albit,
GW is a wee bit worse because it has latex in the paint, but thats a whole other issues. If you water it down, as you found out, it gets really runny which no one likes, but, its kinda the only thing you can do. So, you may ask your self, well what can i do to actually fix this problem?
More good news there are a few things, ill go from the worst to the best solutions
1)Dont paint in white, Yeah not really helpful i know sorry :/
2) Air brush your white, get some Air brush paint which have extremely small ground pigment and it works much better. Down side, you need an air brush, up side you have an air brush now
3) Thin your paints and have patience, paint very thin layers of white, over and over again to get the effect.
4) Dont paint in white* Paint off white, honestly you should never NEVER paint in pure white unless its a flair or edge highlight. Pure white looks very cartoony and not that great, if you want to stick with
GW i would HIGHLY suggest you start using a color called Palyd witch flesh, for your whites, its an off white and does not have the drying out effect you get with most whites.
5) Dont use
GW whites,
GW, while i love them, have pretty crappy white and yellow paints, this again is due to the latex in the paint.
As for the spreading bristles thats not just the paint, thats you. If you are dealing with paints that dry out very quick like yellows, whites, and metallic, you HAVE to wash your brush constantly, I use a winsor newton brush and still you metallic, HERETIC! the enthusiasts might say, but as long as you keep the paint wet and keep it from drying on the brush you are golden.
Hope this helps guys!