Getting an even paint job can be one of the hardest thing in making customs. Avoiding brush stroke and "blobiness" can be tough.
Thinned paint is one method. But, especially with white it can be very difficult!
Spray paints, either from a spray can, or from an airbrush work wonders.
To get even lines using masking tape (I especially like the blue tape found in hardware stores) is a big help. Even then you can get bleed through, but this is generally easier to clean up than when you work by hand.
And repainting classic, hard to find, G1 TFs can get you booo-ed, unless they are already broken or something
Good luck in your future painting!