it won’t. You need something much stronger and leakproof.

Usually sunlight doesn’t cause a ‘stain’, it’s actually faded out some of the pigment so there is no stain to remove.
sanding and painting will work, though it’s difficult to get it looking like the original.

I can, I’m just lazy.

Photos please.

And if the drywall is already messed up, yank it off and see what is underneath, and take more photos. We can’t assess the seriousness without seeing under the drywall.