I have some Testors paints that are much older than two years.
Try stirring with a toothpick, after a few minutes do you still have
sludge on the end of the toothpick? Can you see actual chips or flakes
of paint when you first start spreading (especially the flat colors)?
If so the paint is probably past it's prime.
Testors silver, gold, (any metallics actually) seperate quickly while
using. I have to constantly shake the bottle if it's an extended
painting session.
The flat colors seem to have a shorter shelf life, especially once
opened. Flat paint also dries very quickly, yes, even while brushing
it. So working it less is the trick with flat paints.
In general enamels, especially the metallic's, will seperate in the
container I think a bit quicker than other types of paint, because the
pigments are rather heavy.
Adding thinner to the paint can cause it to dry a bit quicker, giving
you less time to "work" the paint with the brush. So be careful how
much thinner you add to the paint.
Try laying down a wet surface of paint and then quickly go over it with
a dry brush (some paint, not much) in one long smooth stroke per area.
Also use an appropriate sized brush for the area so you can cover it
with minimal brush strokes.
A good little trick or habit to get into, is don't paint directly from
the bottle. Wiping the excess paint from the brush on the edge of the
bottle is the worse thing to do. You end up knocking off the dried
flakes of paint back into the mix. I either dip my brush into the cap
after shaking the paint (short guick painting sessions), or I poor a
little into a small container about the size of the cap (longer
painting sessions). That way any dried paint doesn't end up back in the
bottle.
Remember to have a thick enough layer of paint once you smoothed it
out, it will tend to self level a bit. Especially the gloss paints.