A swimming pool filter traps dirt, grime and all forms of contaminants that otherwise would end up in the water that people are swimming in. A gasket surrounding the filter has the singular job of refusing entry to water that tries to seep around the filter. A damaged gasket can be sealed with waterproof sealant so that it can continue to be used effectively. The gasket will need to be removed from the pool using a few common household tools. The pump that circulates the water in the pool will also need to be temporarily disabled.
Go to the electrical junction box that supplies water to the pool's pump assembly -- typically the box will be in a shed on the pool's deck. Trip the circuit breaker in the electrical junction box to discontinue power to the pump -- alternately, you can flip the master power switch to "Off" to discontinue all power to the pool, which includes the pump.
Go to the side of the pool where the filter grill is located -- typically the grill is on the inside wall at the deep water end, although it could be on the shallow water end instead.
Remove the screws from around the grill with the Phillips screwdriver. Pull the grill off. Pull the filter out of the compartment it is in. Place the grill and filter aside on the deck. Pull the rubber gasket out of the compartment and place it on the deck in front of you.
Dry the rubber gasket with paper towels. Continue drying the gasket until it is completely dry on both sides.
Open the tube of the waterproof sealant. Push one of the pointed ends of a toothpick into the end of the tube to make a hole. Rotate the toothpick clockwise in the hole to enlarge it. Remove the toothpick and throw it carefully away in the trash.
Place the open end of the tube of waterproof sealant against the gasket, at one end of the tear that is to be sealed. Press in on the tube to force out the sealant as you drag the tube across the length of the tear. Lift the tube off the end. Apply another line of sealant to the tear in the same manner as was just done. Put the cap on the sealant and place it aside.
Smooth the sealant on the gasket with the edge of a wooden craft stick. Dispose of the stick carefully in the trash.
Let the sealant dry for the amount of time proscribed in the directions that come with it. Place the gasket back into the compartment so it occupies the space as it did before. Place the filter back into the compartment, followed by the grill placed over it. Reattach the screws.
Restore the electric power to the pool's pump or to all of the electricity-using parts of the pool.