I never thin if the only point is to make it stretch. See it all the
time in new construction. go buy another gallon.

If a can of paint is left open too long it skims over before it
thickens. Paint in a closed can that's been sitting a while will
thicken. If stirring doesn't thin it out I'll add a bit of water. Other
than that i don't use water.

I do use a latex paint additive (soemone said Floetrol) at times. Some
latexes set up quickly -- not good for the brush or the finished
product. A latex paint additive slows the dry time, so the finish will
smooth out something like oil-based does..