Sanding is one way to fix things, but I agree that sanding isn't a good option. I will outline the options I would present to a potential customer. Choose the one that sounds like something you can do. Each option assumes the bathroom has either no texture or a very light texture.
Before you choose, the bathroom needs to be prepared for repair and painting. Washing the walls and ceiling is a good idea, especially if mold is present. Now cover and mask all items that won't be painted and remove the outlet and light switch covers. All loose paint and any loose drywall or plaster needs to be scraped until all areas are firm and sound.
OK, now you are ready to prime the entire bathroom. I like to use fast drying stain blocking primers. The oil base primers tend to stick better and cover more stains but smell bad, while acrylic primers have lower odor and easy clean up. Choose a very good primer as this is the foundation for any repairs and painting. Apply the primer correctly, no reason to create more work.
Each of the options below is doable by almost anyone, but some will require a little more experience. Choose an option you are comfortable with.
Option 1) Apply drywall mud to the bad areas then paint.
This is the easiest and quickest way to fix your problem bathroom, but works only with a smooth wall (no texture). All bad areas will need a minimum of two coats of drywall mud. You can use either regular all-purpose or setting types.
The all-purpose drywall mud is the easiest to work with but might require an extra coat and dries slowly. While the setting types of drywall mud dry much quicker but are a powder, which is difficult to mix, and dry harder, meaning harder to sand.
This option requires that all surface imperfections be skimmed over with drywall mud. It will take several coats each drawn out a little further than the last. After the drywall compound has fully dried, sand the edges and face smooth.
Start with a 6 inch drywall knife and progress to a 10 or 12 inch knife. The amount of surface imperfections and bad roller marks will dictate how much of the walls and ceiling will require skimming. After sanding apply a good primer to seal the drywall mud before painting.
Option 2) Apply a texture before painting.
This is usually the better option because it doesn't require as much patching and skimming of the surface. A texture can cover a lot of imperfections and look very good doing it. Many textures can be applied over existing textures, the success of retexturing a wall depends on a lot of factors including the old texture and the desired new look.
You have a wide variety of textures to choose from. Hand textures are fairly easy to do and won?t require the rental of a machine. Varieties to consider are;
Brush Textures – A texture brush can be purchased, most hardware stores and home improvement centers, for just a few dollars. With a texture brush you could do a stomp texture, stomp and drag or swirls. Be creative.
Skip Trowel – This look is accomplished by using either a trowel or flat drywall knife and applying small areas of drywall mud then drawing it out across the wall. Overlap each area into another. The effects can be heavy like Spanish stucco or light like Venetian plaster.
Be inventive. You can use rolled up newspaper with one end cut into 1 inch strips about 1? inches long. I have used this method to give the impression of roses on a ceiling. Almost anything can be used to create a texture out of thinned drywall mud.
Machine textures require the rental of a hand held hopper and air compressor. Textures possible are orange peel, light to heavy, knock down, light to heavy and Spanish knock down, which is a very heavy texture. Using a texture machine can be tricky and requires a little experience. I recommend the hand textures if you are not confident of your abilities.
Experiment using a scrap piece of sheetrock or heavy poster board. Apply a little texture, if you don't like it simply scrape it off before it dries and try another technique. All textures will need to be primed after drying, typically 24 hours.