Joseph Meehan wrote:

Paint thinner is a description of the use, not the material. As you
have noted many different things can be a paint thinner, even water, which
would not of worked at all in your use.


One of the big problems with many retail brands of paint "thinners" and
solvents is that they contain quite a bit of dissolved water. When used
with moisture-cure polyurethane paints (almost every one-part PU is this
type), the water in the solvent can cause bubbles to form or premature
gelling in paint guns. Professionals who use PU paints also use
de-watered thinners to avoid these problems.