I have five gallons of latex interior flat paint which I custom mixed
from several partially full paint cans four months ago.

I went to use the paint last weekend and it had partially solidified
in the bucket.

The bucket was well sealed. It wasn't the paint on the surface that
was solid, it was a large core of paint with thinner liquid on the
sides and top.

I did get the paint liquid enough to use. After painting, I
transferred some into a 1 gallon container and into a cup sized
plastic container with lid.

After a week of being sealed, the gallon and cup containers also
appear to be solidifying from the middle out (or bottom up?).

I've mixed my own custom colors from leftover paint before and have
never had this problem.

Anyone have thoughts on why the paint is solidifying so quickly?

Any way to retard or prevent it?

I want to be able to use the paint in a couple of years to top coat.