How to Seal and Waterproof a Concrete Pool

Keep Clear Patio Sealers Away From Your Submerged Pool Basin

The biggest mistake pool owners make when sealing a concrete pool is grabbing a jug of clear masonry sealer from the hardware store. Clear silane or acrylic sealers work well on dry pool decks, driveways, and patios. They shed rain and stop salt damage on flat ground. But they cannot survive underwater. When you submerge standard patio sealers under thousands of gallons of pool water, the chemical bond breaks down fast. Within weeks, the clear film turns cloudy, lifts away in slimy sheets, and drifts into your skimmer basket. Those gummy sheets can plug your pump impeller and ruin your filter.

A swimming pool shell deals with forces that a normal patio never faces. On the inside, water exerts constant downward and outward weight against the walls. On the outside, damp soil creates hydrostatic pressure (water pushing from the ground outside against the buried concrete walls). To seal gunite (a sprayed concrete pool shell) or concrete plaster, you cannot rely on a clear invisible film. You must fill open gaps with hydraulic cement (a fast-setting waterproof patch mud) or pool epoxy, and then cover the bare shell with an underwater-rated epoxy pool coating. These materials cure into a solid barrier that stays bonded even under constant water pressure.

Drain, Wash, and Etch the Concrete Shell Completely First

Waterproof pool sealants only stick to concrete that is completely clean, rough, and bone-dry. Skipping surface prep is the main reason new pool coatings peel off in large sheets after just one season.

1. Drain the pool safely. Never drain an in-ground concrete pool right after heavy rain or if your yard has a high water table. Without the weight of water inside, hydrostatic pressure can lift an empty shell right out of the ground. Open the hydrostatic relief valve (a small threaded plug inside the main drain) as soon as the water level drops to let groundwater seep safely into the basin. Before you pump out thousands of gallons, check local rules on how to dispose of pool water safely and legally so you avoid fines or flooded neighbor yards.
2. Pressure wash every square foot. Rent or buy a pressure washer rated for at least 2,500 pounds per square inch. Blast away stubborn algae roots, peeling paint, and loose mineral scale. Start at the shallow end steps and push all dirt down toward the deep end drain where you can scoop it out.
3. Etch the bare concrete with acid. Smooth concrete will not hold a thick pool coating. Mix one part muriatic acid into four parts fresh water in a plastic watering can. Always pour the acid into the water, never the other way around, to prevent violent chemical splashes. Pour the mix across the walls, scrub with a stiff plastic brush, and watch it foam. The acid cleans out deep oils and opens the concrete pores until the shell feels rough like medium-grit sandpaper. Rinse the entire pool with clean water right away.
4. Wait for the shell to dry completely. Concrete acts like a sponge. Even when the surface looks chalky, deep moisture stays locked inside. Wait three to five sunny days before sealing. To verify it is dry, tape a one-foot square of clear plastic wrap tightly to the deep end floor with duct tape. Check it the next morning. If droplets form under the plastic, the shell is still wet and needs more drying time.

Chisel Out and Pack Cracks With Fast-Setting Hydraulic Cement

Hairline surface checks in old plaster are common, but any crack that goes through to the gunite shell will leak water into the surrounding soil. Smearing patch paste over the surface will fail because water pressure will push it loose from behind. You must mechanically lock the patch into the concrete.

1. Widen the crack with a chisel. Grab a cold chisel and a three-pound hand sledgehammer. Chisel along the crack until the groove is at least a quarter-inch wide and a half-inch deep. Cut the channel into an inverted shape that is wider at the bottom than at the pool surface, like a dovetail joint. This backward angle physically traps the hardened patch material so inward water pressure cannot push it out.
2. Clean out the groove. Run a stiff wire brush along the chiseled channel to break free any loose concrete fragments. Use a wet-dry shop vacuum to pull out all dust, sand, and pebbles. Rinse the groove quickly with clean water. The concrete should be slightly damp when applying the cement, but you must sponge out standing puddles.
3. Mix small batches of hydraulic cement. Hydraulic cement hardens within three to five minutes, so mix only what you can press into place with one hand. Add cold water to the powder in a small plastic bucket and stir with a margin trowel until it reaches the consistency of heavy clay.
4. Pack the cement deep into the crack. Wearing heavy rubber gloves, pick up a handful of the mixed cement, roll it into a sausage shape, and jam it hard into the groove. Push firmly with your thumbs or a flat trowel for two full minutes until the cement stiffens. Trowel the top flat so it sits flush with the pool plaster.
5. Leave expansion joints alone. Do not pack rigid cement into an expansion joint (the flexible gap between the pool shell and the concrete patio deck). That joint must move as temperatures change. Seal expansion joints with flexible polyurethane pool mastic, not cement.

Apply an Underwater Epoxy Pool Coating in Controlled Layers

Once you patch every crack, you seal the entire basin using a two-part epoxy pool coating. This coating forms a thick, waterproof skin over porous gunite and patched plaster.

1. Mix the two-part epoxy base. Two-part epoxy comes with a resin bucket and a separate hardener container. Pour the hardener into the resin bucket. Stir with a low-speed drill mixer for at least five minutes to ensure complete blending. Let the mixed bucket sit untouched for twenty minutes out of direct sunlight so the chemical curing process kicks off properly.
2. Cut the edges and corners with a brush. Use a four-inch solvent-resistant brush to cut along the waterline tile, around the light niches, around return jets, and across step edges. Take extra care around the main drain pot to seal the seam between the concrete and the drain flange.
3. Roll the body of the pool. Pour the mixed epoxy into a metal roller tray. Use a short-nap roller, about three-eighths of an inch thick, on an extension pole. Start at the shallow end and work backward toward the deep end so you never step on wet epoxy. Roll in cross patterns, going horizontally first and finishing with light vertical passes to smooth out roller lines.
4. Respect the recoat window. Almost all pool epoxy coatings require two coats for a true waterproof seal. Check the can instructions for the recoat window, which is usually between 24 and 48 hours. If you roll the second coat too early, the bottom layer will wrinkle. If you wait too long, the first coat cures into a glass-slick shell that the second coat cannot stick to without full sanding.

Let the Shell Cure Fully Before Running Fresh Water

Rushing the refill process will ruin an expensive coating job. Epoxy does not dry by water evaporating; it cures through an internal chemical reaction that requires consistent warm temperatures.

• Wait five to seven warm days. In temperatures around 75 degrees, let the pool sit empty for at least five full days. If temperatures stay in the 60s, extend that wait to seven days. Keep children and pets away from the pool area while it cures.
• Pump out rainwater immediately. If a sudden shower hits while the coating cures, do not let rainwater sit in the deep end. Pump out standing rainwater right away so it does not leave dull, milky stains on the fresh epoxy finish.
• Fill the pool in one single run. Once you put the hoses in the deep end, let the water run continuously until it reaches the midpoint of the skimmer opening. Never turn the hose off halfway through the fill. If you pause the water flow for several hours, you will end up with a permanent bathtub ring line etched into the new finish.
• Balance the water chemistry on day one. As soon as the pool fills, fire up the pump to start filtration. Follow the correct process to how to balance a newly filled pool in the right order. Pay close attention to pH and calcium hardness. Highly acidic or aggressive soft water will pull minerals straight out of fresh cement patches and soften a new coating.

Stop and Call a Pro When You Spot Structural Movement

Surface patches and DIY epoxy coatings solve minor plaster leaks, crazing, and stable cracks. They cannot fix a shifting pool structure. Look out for these red flags before spending time and money on sealants:

• Offset cracks. Run your fingers across the crack. If one side sticks out further than the other, the concrete shell has snapped and the ground underneath is moving. Surface cement will crack again in weeks.
• Cracks wider than a quarter inch. Gaps that you can slide a pencil into usually mean broken steel reinforcement bars inside the gunite. A contractor must install structural carbon-fiber staples to pin the shell back together.
• Wide areas of hollow plaster. Tap the pool walls with the plastic handle of a screwdriver. A sharp click means solid concrete. A dull, hollow thump means delamination (plaster separating and peeling off the raw shell). If large sections sound hollow, the old plaster must be chipped off completely and replastered by a crew.
• Rapid water loss that drops below the fittings. If your pool loses several inches of water every day and the level keeps falling past the light niche and returns down toward the main drain, you likely have a broken underground pipe beneath the shell.

Frequently Asked Questions

Can I use clear driveway sealer to waterproof my concrete pool?

No, clear driveway and patio sealers are not designed for submerged conditions. Under water pressure, these sealers turn milky, lift off the concrete, and peel away into floating strips. Those strips will quickly clog your skimmer, pump impeller, and filter system. Only use two-part epoxy pool coatings and hydraulic cement inside a pool basin.

Can I seal concrete pool cracks without draining the water?

You can patch minor underwater cracks using specialized two-part underwater epoxy putty. You mix the putty by hand at poolside, dive down, and press it firmly into the chiseled crack. This works as a temporary patch to stop an active leak, but a permanent, clean seal requires draining the basin and packing the groove with hydraulic cement.

Why is hydraulic cement better than regular concrete mix for pool repairs?

Standard concrete shrinks slightly as it dries, which leaves microscopic gaps where water can seep through. Hydraulic cement contains special additives that cause it to expand slightly as it cures, locking it tight against the surrounding walls. It also sets in less than five minutes and blocks pressurized water immediately.

How many years does an epoxy pool coating last?

A properly applied epoxy pool coating usually lasts between five and eight years. Its lifespan depends mostly on how clean and dry the concrete was during application and how well you manage your daily water chemistry. Keeping your pool water balanced prevents the epoxy from chalking or blistering prematurely.