Tanky I use a watercolour wash, applied over a coat of Future.
Just get a couple of tubes of artists water colour, I use black and burnt umber, and squeeze some into a dish. the colours you use are up to you, and dependent upon the subject.
I then add some water to the dish to get a thin wash, and simply daub all over the panel lines etc. The beauty of water colours is that because they wash off with water, it doesn't matter if you make a pigs ear of things.
Just let them dry, then wipe off with a damp Q-tip, or piece of kitchen towel, at 90 degrees to the direction of the panel line. This should leave you with paint in the recess, and nowhere else. Any excess can then be wiped off the panels.
Finally seal the whole lot with a coat of the appropriate varnish.
Like Rick said, more difficult to explain than actually do.
Progress Update
Finally finished the wing root joint. Now scribing in all the panel detail that HC left off! Which is quite al lot!!!!
At this rate should be ready for primer by next weekend!
Karl