Unfortunately, pigments will rub off after a while if you\'ll be using them with gaming. In terms of fixatives, here\'s a few of my experiences:
use turpentine/turpenoid for fixing oil stains, dirt and rust stains/runs. Oil based fixatives are pretty forgiving and will generally produce good results at all times.
for larger oxidated, rusted metal surfaces, use rubbing alcohol. Alcohol produces a different effect, but is a little trickier to use. Lightly apply alcohol to your area, but try not to let it pool, otherwise you\'ll get a funky looking spotted effect and you\'ll have to start over.
In terms of using permanent sealants (gloss and matt varnishes and the like), you can apply these to places where you\'ve used pigments to cover large areas (like rust), but don\'t try to permanently seal dirt and smaller weather markings, otherwise the effect just sort of disappears and you have to start over. If you\'re using weathered models for gaming, then you\'ll just have to reapply pigments every so often.