Type of Plasterboard to use in a bathroom

If there is moisture still there, then mold and mildew will return. If the problem was not fixed, then it will return. Bathrooms are particularly bad in this respect, since moisture is a natural consequence of using water. I would expect to need to redo things like caulking every few years to fix this. It will not last forever.

1. Scrape/pull off the old silicon.

2. Apply a solution of bleach to kill the mold. Be careful when you apply it, as bleach can damage the clothes you are wearing. Keep the area ventilated well.

3. Let it dry. If you are in a hurry, a fan blowing on the area will help. A hair dryer can also speed that process considerably.

4. Apply new caulk in a smooth bead. I use my finger to spread it along the joint. Definitely use a high quality silicon caulk with a mold/mildew inhibitor. These are caulks that are designated for use in bathrooms.

5. Allow it to fully dry before you get it wet again. Give it a day or so. It will skin over in an hour or so, but make sure it has properly cured before use of the shower.

Expect to redo this process in some number of years. The exact time it will last depends on how well the area is ventilated.

Of course, there may be mold in the wall itself. This is difficult to deal with unless you tear the wall down to replace it completely, and even then water tends to attract the stuff. Mold spores are everywhere.

There are a few options that can be used to repair this damage. You can use standard joint compound (non-lightweight), and apply it in thin coats to avoid cracking due to shrinking, joint compound mixed with plaster, or professional grade oil based drywall spackle.

The first step would be to dry the wall and inspect it for mold. If there is any surface mold present, you can kill it with a bathroom cleaner that has bleach in it. After the surface has been cleaned and dried out thoroughly, you would want to apply a stain blocking primer like Kilz. This will give you a good surface to bond the repair to. You can then apply the drywall filler of your choosing. You may need to apply it in several layers to get a good result.

Once you are satisfied with the repair, then you can begin to prepare the wall for paint. I suggest priming the wall with an interior/exterior grade primer that will be able to stand up to the moisture. 2 - 3 coats should be enough for good coverage. I also recommend that you extend the primer past the damaged area to ensure that the same problem won't reoccur. If you can, prime the whole area around the shower. After it has been primed, then you can apply the top coat. A good idea would be to repaint the entire bathroom with paint that is made for kitchen and bathrooms. This kind of paint will resist water, and can easily be cleaned. Many brands are also resistant to mold.