Once the binder in enamel paints (water-based paints are usually enamels) turns into gelatinous mass, or even starts to harden, that chemical reaction is not reversible. You can't just add some thinner like you can do with lacquer paints. No way to save it.
Well, you can add thinner and stir or blend the heck out of it, but all you will do is to pulverize the semi-hardened enamel into tiny chunks. It will not be like paint you expecting.
While I agree with p. that it seems a hopeless task; if you continue to pour liquid on the subsequent emulsion, it can work.
I've resurrected an old PollyS green and others of the era to useable format-
do what he says- but use luke warm water to soften, and stir the hell out of it; a portion at a time, till the whole bottle eventually. I also put the jar in the mcrowave to soften for about 5-6 seconds, perhaps twice as I feel the substance temperature.
Obviously you need to use careful judgement about how much 'liquid' you are applying to rewet the paint. Insuffucuent 'mixing will simply give a coloured water type solution. More grinding required.
I normally use just discarded paint brush ends for my mixing, but knowing I was trying to break down particles, I also used a stainless steel screwdriver to provide adequate friction and force.
- I tried ISO and various thinners over the years but refused to throw the paint away.
Now, what has cracked the mystery, and made a passable soluble solution (!), is the thinner from Games Workshop/ Citadel (now Warhammer?)- called 'Lahmian Medium' in their 'Technical' range. Wacky names made for juvenile fantasy gamers. But hell it worked!
All best, dave
