Help! I removed some wall paper over my shower and there is some damage. The wallboard has some black mildew and the wall board is stained with water. My question: Di I need to replace the wallboard- or can I kilz the mildew and then mud over the existing wall board? Also, Do I mud all the way to the shower top and then caulk or do I leave about 1/8" gap. Thanks for any suggestions.