-Thin with mineral spirits for humbrol enamel. What you want is to
have the paint flow across the surface and level out after a brush
stroke. If it's too thick it will leave ridges. Multiple coats are
required.
-Never use straight from the bottle, even if you don't need to thin it.
-The larger the surface is the large the paint brush should be. This reduces brush stroke marks.
-If you have a spot with raised paint lines you can sand it down with a very fine grit sand paper/stick.
-When
finished you will still be able to see the brush marks but a top layer
of Dull Coat (a clear coat that makes the finish look duller) will
remove these lines. I have not done much myself with gloss coats.
I never found a need to prime plastic models when using humbrol enamel, but if you do I suggest Tamiya Fine Surface Primer or a good automotive primer.
Also
for future reference, I find Model Master acrylic (thins with water,
clean up with soapy water & maybe isopropyl alcohol) paint next best
alternative to humbrol enamel if you don't want the fumes.
Mongol General: Conan, What is best in life?
Conan: To crush your enemies, see them driven befor you, and hear the lamentations of the woman!