Many ' hot ' products will melt some paints...that's why you see so many going with water based poly as it is not a 'hot' product to make some types of cresting paints run.
When I messed with wood arrows I sealed first, crested, then covered the crown or it would smear badly, as others have stated. If you want your arrows to look nice more than a week you need to seal before to give a good surface to apply the cresting, then after to protect, as many have already stated above.
Legolas, when I built arrows in the 80's/early 90's I always hated that 'yellowing' effect that some clear seals cause, and looked long and hard to get away from that. Now there are so many products available with no smells, no yellowing, it's THE GOOD OL DAYS NOW!!!!!