Painting over existing Waterproof Membrane?

[KatieLou]

I bought a house 18 months ago. It has a 1970's era swedish spa-type bathroom that I will eventually replace (in 3-5 years). But for now I'd like to tidy the bathroom up - patch up a bit of plaster and repaint it.

It has a bath with a shower over it. Above the bath on 2 sides, there are large slate tiles cemented to plasterboard - they go up to about chest height. The plasterboard that surrounds the bath/shower area above the tile has been coated with a waterproof membrane by the previous owners. There are no water leakages in that area, and the wall surface is in good condition. (When I eventually renovate the bathroom, I will ensure tiling goes to a more suitable height but for now re-tiling is out of the question as I'd have to take out the whole wall to remove the cemented slate tiles.)

I want to patch up some plaster on another wall that is not near the bath, and repaint the whole bathroom without damaging or reapplying the waterproof membrane above the bath. Can I simply clean the wall surface and paint over it (ie that part of the wall above the bath that's coated with the water-proof membrane)?

[Shedhand]

Just wash down with Sugar Soap (good scrub) and paint over when its dry.

[KatieLou]

Many thanks.