There are several ways to block a water stain on drywall or trim. Here are three possible methods:

Use a Stain-Blocking Primer: One of the easiest ways to block a water stain on drywall or trim is by using a stain-blocking primer. These types of primers are specifically designed to block stubborn stains such as water stains, smoke stains, and crayon marks. Before applying the primer, make sure the surface is clean and dry. Apply the primer with a brush or roller, and let it dry completely before painting over it.

Try using spray on shellac: this is the secret ingredient in many stain blocking primers. It goes on clear and does a great job of blocking most stains. In addition, it dries VERY fast.

Bleach and Water Solution: Another method to block water stains on drywall or trim is to use a bleach and water solution. Mix one part bleach with three parts water, and apply the solution to the stained area using a sponge or brush. Let the solution sit for about 10-15 minutes, then rinse it off with water. Once the surface is dry, apply a coat of primer and let it dry before painting over it.

Replace the Damaged Section: or in the worst case scenario you may just need to replace it. If the water stain is severe, and the drywall or trim is damaged, you may need to replace the affected section. Use a utility knife to cut out the damaged area, making sure to cut along the edges of the drywall or trim. Cut a replacement piece of drywall or trim to fit the hole, and attach it using drywall screws or nails. Once the replacement piece is in place, use joint compound to fill in any gaps or seams. Sand the surface smooth, then apply a coat of primer before painting over it.