Before painting would seal, especially in the restroom, there are some spray on items. Careful if working with areas if the paint is really old, don't want to get into the lead paint etc. When your working on the area benind the commode make sure to cover the plumbing drain hole, those sewer fumes are not good!
Checked the DIY and This old House when, worked on fixing some broken drywall corners, sure some of the other, hardware co's have resources these helped a lot and couldn't believe it worked, sure did feel good when it was done.
Good Luck on the project !! :)