I have a small bottle of polyurethane touch up paint that has started to get too thich to use. Whats the best thing I can add to it, to bring it back to usuable staus?
Replacing it is not really an option. Thanks.

The only thing you can do is add a small amount of paint thinner and mix it really well. When you are done mixing, spray a small amount out and if there are any paint chunks coming out the paint is bad and I recommend not painting with it.

this is not paint that I would spray...just a very small bottle for brush-touch-up.
Is there a specific type of thinner that will work better, given that this is polyurethane??

I appreciate the advice.

Polyurethane paint requires a catalyst to harden and a reducer to thin it out. The reducer will thin it out for you but it takes foever to dry without the catalyst. May cost $35 or so to get these other two from a paint store.
Roger