Once water-solubles are dry, they are like any other oil paint. There are many magical incantations for going back into dried oil paintings up to and including rubbing them with a cut onion… but the best is simply to scuff the surface with fine sandpaper or wet-or-dry paper. This provides a roughed-up surface that has enough tooth to hold subsequent layers of oil paint. I prefer the wet-or-dry, used with water, as you accomplish two goals: roughing the surface and washing it.
It’s actually the same process that used to be done when re-painting houses or interior walls, in the days when the paints were oil-based. The motto was that preparation was 9/10ths of the paint job, and preparation began with washing, then dulling the surface to ensure that the new layer can bond to the old.
If you’re using toxic water-solubles (true cadmiums, for example), you probably want to wear a dust mask while roughing up the surface. In fact, a mask is a good idea in general while sanding anything, paint, wood, metal, etc.