The best way to deal with humidity in the bathroom is to install an exhaust fan.

But on to your question -- you are re-painting walls that are already painted, is that right?

I don't think plaster walls require anything special -- I use Kilz primer followed by semi-gloss paint. If semi-gloss is too much for you, a satin or pearl finish should be fine. There are also additives you can add to the paint to avoid mildew (ask at paint store). My dad does that since he refuses to use anything except flat finish paint. I go with semi-gloss and no additive needed since it is very scrubbable.

If there are mildew spots now, I would spot treat with bleach first. If not, you should not even need primer unless you need it to hide the old color.

Plaster is actually superior to drywall IMHO in dealing with moisture. It won't grow mold like drywall.

Good luck!

Plaster is a wonderful material. I understand it is more sound proof too. As Evelynann said a fan is the best thing to do whether you have plaster or drywall. Also if you keep the plaster you want to be sure it is solid on the lathe. If you cut more holes for outlets and switches be careful to protect the walls integrity.

I used Benjamin Moore Bath & Spa for my 1st bath remodel and plan to do so again. It is designed for bathrooms as the name indicates.

When I've seen mold on my remodel it was on the dry wall material. So dry wall isn't any guarantee. it has been on the dry wall paper where it rest against another surface like more dry wall, cupboards, or wall paper for example.

You can post over on the Remodel Forum too for helpful information.

The walls should be a type of plaster called 'Keen's cement.'

It is especially hard.

It was standard in homes with plaster walls for use in wet areas like bathrooms for many years (and is still available from USG)

Thanks for the help.

It sounds like if I get a good bathroom paint, I should be fine.

Also need to figure out a vent.

Make sure you thoroughly wash down and then rinse before painting.

Bathrooms tend to accumulate all sorts of things on the wall surfaces (everyone wipes down tile frequently, few wash the walls).

if the wall is not very clean ad the plaster is Keen's a new layer of paint can curl and pull off from the humidity in a bathroom.
The humidity and condensation penetrates latex paint but can then often go no further (especially if there is old alkyd (oil based and possibly lead containing) paint already on the wall).

The moisture behind the paint then blisters the new layer off the wall.

This is one of those jobs that preparation before applying the first drop of paint really maters.
Luckily bathrooms are not usually all that large.

We just painted our plaster bathroom walls and used Zinsser Bullseye 123 followed by BM Aura Bath & Spa on rec of paint store. Considered Perma-White but it was going to take too long for the color match--if mildew recurs we'll consider going that route, though. Only two days in so much too soon to tell! Washed walls with soap and then straight white vinegar pre-priming, but also too soon to tell if that was effective. We'll see! BM also makes a Kitchen paint in satin that is good for moisture if you don't want a matte finish.