I use Squardon white and here's a trick that I haven't heard mentioned here yet.
I apply tape to either side of the seam that I'm going to fill with putty. I place the tape as close to the seam as I can and burnish it down. I apply the putty and immediately go over the putty and seam with a q-tip dabbed in nail polish remover (currently using the non-acetone based stuff).
The nail polish remover removes excess putty and leaves the seam filled nice and flush. The tape prevents excessive amounts up putty being applied where it doesn't need to be. When you're finished, there's minimal sanding required (usually 600 grit wet then dry) and you lose very little of your surrounding detail (rivets, access panels, raised lines, recessed lines, whatever)