I have not experienced this - however when I mix up the epoxy with microballoons, I would then redope with some tissue or carbon tissue. So set the surface. There could be something inside the epoxy causeing a reaction.

Even after sanding.  Take a rag saturated with acetone and wipe down the fillet.  Even alcohol will take off the oily surface.   

I too agree that allowing dope to set longer is best and that carefully and briefly wiping with acetone can be benificial. I've used acetone, but sometimes just wiped with rubbing alcohol to eliminate the dust after sanding.

One thing I forgot to mention is that 30-minute epoxy has a much greater capacity for microballoons than one might think. I continue to add them until it becomes quite difficult to continue. This lightens the material, makes it more sandable, and probably makes a better surface for the dope to grip. Although it makes contouring a bit more difficult when the epoxy is liquid, it also helps it hold the shape you give it, as does holding the wings at a 45-degree roll angle, upright or inverted (I alternate them as the epoxy sets). Then sanding with your abrasive sheet wrapped around something cylindrical, with appropriate radius, completes the shape more easily.

SK

I think it's cleanliness issue.

Just a thought? Hum what do I know.  Two things first is I have found epoxy and Microballoons to act as if they are greasy. Second never paint fillits. What I mean is let the dust cover them. Lifting comes from shrinkage and air trapped. So a dust coat with long dry times with lacquer base paints is desired.

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