As Randy says, you need to clean the surface. However, I have never been able to make a satisfactory contour with the "wet-finger" technique anyway. So I have to sand with abrasive wrapped around an appropriately sized dowel. This has always removed the surface stuff, so far as I have seen, although I've sometimes used alcohol or acetone anyway, since it takes only a few seconds and removes dust. Smooth fillets don't really need much paint. So the process I recommended above doesn't take much time. For what it's worth, here are pictures I took this month of an unfinished Yak-9 (begun ca. 1961, fillets and Brodak dope applied in 2002) which has just been hanging in the basement for a few years -also, my flapless original's tail, during construction revision. The dope has shrunk some on the Yak, as you can see from the edges, but there's no lifting.
SK