Painting on compound curves?

You might try Micro-mask if you have a steady enough hand

For really delicate stuff, I have found nothing better than Bare Metal Foil.  Yes, it's expensive and fussy to remove, but it conforms to compound curves beautifully, and it's very thin.  Work it down gently with a toothpick and trim with light touch and a very sharp blade.  I use strips of it around the edge and fill in the middle with regular white glue.  I remove it with a sharp toothpick and tweezers.  use a piece of masking tape to clean up the goo that's left.  Practice on scrap first.

Don

On compound curves I mask with the narrowest tape I have.  I have some of the Tamiya 6mm stuff, and Pactra even has 1/16 width.  Then I use regular masking tape to cover more extensive areas, but the narrow stuff follows the curves nicely.  

Sometimes, when it is neccessary to paint around very small details that are dark colors, but the detail has either raised or engraved "panel" lines, I put a piece of masking tape over the area and use a brand new #11 blade in my hobby knife and trace around the panel or whatever, leaving the cutout piece to mask the panel or part.