For what its worth, in Formula 1 we did a lot of studies into the effects of rain on the cars wings and bodies.
the basic rule now as a result is that when its dry we use a hydrophobic coating to resist water and dirt as much as possible.
however when its wet, we use a Hydrophillic coating. The reason being that the Hydrophobic coating causes the beading together of droplets which cause greater drag with them having to be pushed off the surface as beads. The beads also cause more surface disruption and them moving about under vibration disrupts the optimum wing tuning frequencies.
BUT with the hydrophillic coating we found that the water forms a nice thin (wetted ) slippery film over the surface with lower drag and better boundary conditioning.
Not flying i know but thought someone might find it interesting.
GB