Try some distilled water. I have used that in the past with Tamiya paints. Although eventually that bottle of paint will break down and cease to be usable.
Or you can transfer what you need into a smaller container for each session, and thin that as needed.
F is for FIRE, That burns down the whole town!
U is for URANIUM... BOMBS!
N is for NO SURVIVORS...
- Plankton
LSM