is there a term used to describe how paint changes when it dries? what i mean is when a paint goes on and looks a bit patchy when wet but then looks realy good (no ptchiness) when it has dried. i'm sure a decorator i used to know used to say a certain paint had really good .......... or really bad .......... to describe this, but can not remebmber the term.

Depends what type of paint your using. Water based is mainly by evaporation, oil based is a combination of evaporation and oxidisation. There`s a few terms applied such as paint film forming, skinning over, the technical term he used could be coalescence. Patchiness is caused by the application of paint unevenly, because during initial drying stages the water in the paint is forced to the surface and then evaporates, so it takes longer where the paint is thick.
This explains why we always mist coat to satisfy the porousity of the surface to be painted, so the water in the paint is allowed to cure, film and evaporate properly rather than being sucked into the painted product.

maybe it was coalescence. the reason i ask is because i tried two different paints, both water based emulsion. both looked a bit patchy when they went on, but one looked much better than the other when it had dried. would it be correct to say it had better coalescence?

Or you could just say, 'it looks patchy now but thats down to it drying.'

"coalesce" - mix together different elements; "The colors blend well"

Coalescence starts to get a bit more technical and refers to atoms bonding etc.