The old bottles were not airtight. So over time (years) the thinner
in the bottle evaporates. Adding new thinner will revive the "paint".

Also, I have a large jar of Accupaint thinner for stripping. Many old
Front Range and McKean models were painted with this same stuff.
They can be incredibly hard to strip but when I drop them in this the
color just washes off.

In fact we owe Accupaint and Star to Fred Becker, who discovered it
in the first place, and shared the information with George Bishop,
whose Accucals and Accupaint were transformed. Previous to that
time, George used a different vendor -- Floquil -- for his paint.

Tim O'Connor