Whether your pool is made out of gunite or concrete, it will most certainly require maintenance as it ages. Part of that age-related maintenance is repairing cracks, chips and hollow spots, which are very common. With time, the pool's plaster starts to wear away, exposing the gunite, which is below the plaster. Luckily, this is maintenance that can be done with ease, as long as you take the proper steps.
First, drain all the water out the pool. Wait a day or two for the basin to dry completely. You'll be using electric power tools to fix the cracks and an empty, dry pool is necessary.
The next step is to take a saw or grinder with a diamond blade and on the first crack, cut into it a half-inch deep and extend the length of the crack one inch on each side. Let the crack dry (there will most likely be water that seeped inside the crack) and clear all dust and debris from the site.
Using your caulk, apply a thin strip of caulk along the length of the crack. Because you will be applying plaster over the caulk, be sure to leave enough space (roughly a quarter inch) for the plaster so the surface remains smooth. Allow the caulk to dry completely.
Next, it's time to mix your pool plaster. Using pool patch (found at any pool supply store) or some white Portland cement, mix in some white sand and an acrylic cement bonding agent. The consistency will be thick like peanut butter. Use equal parts of each substance.
Because the surface of the mixture may harden quickly, stir it often and, if necessary, moisten the surface. With a putty knife, apply the moist plaster to the crack . Smooth and even out the surface, and if needed, take a sponge and tap the surface to match the rest of the consistency of the pool's surface.
Repeat the process for all cracks. However, if the pool won't be filled for a few hours or longer, be sure to keep each freshly fixed crack covered with a wet towel until it is covered with water in order to prevent it from cracking again. Fill the pool back up with water as soon as possible.
Fixing hollow spots
After draining the water out of the pool, look for any areas where the plaster is visibly loose and not bonded to the pool. Gently chisel away the loose plaster.
To find any hollow spots that are not visible, use a hammer and gently tap along the pool's surface. You should be able to hear any hollow spots. Go ahead and take your hammer and chisel and chip them up.
Next, you're going to prepare the area for an acid rinse. This will strip away a small layer of plaster and reveal a fresh layer of plaster beneath it. First, add one part acid to one part water (never the other way around). Wet the area with a hose and keep the hose running. Slowly pour a small amount of the acid mixture on the area. Don't let the acid sit on the area for longer than 15 to 20 seconds. Rinse it thoroughly. If you don't rinse thoroughly, the acid will continue to eat at the plaster.
To neutralize the acid mixture before pumping out, add soda ash to the acid mixture and stir with a brush. Be cautious in where you pump the acid mixture out to because it can kill vegetation as well as fish, frogs and other animals. After pumping out, rinse the area again.
Next, mix your plaster mixture the same as in Section 1 Step 4 above. After moistening the mixture, use a putty knife or pool trowel to apply the plaster to the area. Smooth it out to ensure there are no air bubbles in mixture. Wait a few minutes and then smooth it out again; you may need to add a little water to the edges.
Be sure to keep the freshly fixed area wet until the pool is filled. Fill the pool as soon as possible.