You've determined a repair patch is necessary in the plaster pool, because you see a spot where loose plaster is no longer bonded to the surface. How fortunate you are: not only will you save yourself a lot of money doing it yourself, but compared to replastering an entire pool, it's a cinch to simply patch a plaster pool.
While it's somewhat tricky to make the transition between the new plaster and the old plaster evenly smooth, and get an exact color match between them, yet even if you tend to repair things as if you have two left hands, this repair job only requires some stick-to-itevness to get it right.
Prepare the Repair Patch
Lower the water level in the pool to at least two feet below the repair patch, if you aren't using electrical tools. Empty the entire pool, if the plaster repair patch is near, or on, the bottom of the pool-or if you're using electrical tools.
Use your hammer and chisel to pry away all old, loose plaster at the repair patch. If necessary, use your hard putty knife or taping knife to get the loose, finish pool plaster away from the surface.
Outline and bevel around the repair patch 1/4 inch deep, using the angle grinder and disc. Hammer and chisel around the repair patch 1/4 inch deep, if not using an electrically-powered angle grinder. Remove finish pool plaster to 1/4 inch deep throughout the repair patch area.
Spread soda ash directly below or around the repair patch, so that after acid washing it will further neutralize the acid wash runoff. Acid wash the surface where the plaster was removed by spraying it with the pump sprayer and using a broom-handled acid brush to spread and work the acid wash into the repair patch surface.
Wait for about thirty seconds after acid washing and then rinse down the repair patch area. Make sure all of the acid wash runoff is thoroughly diluted with water and soda ash, if the pool was emptied to patch the pool plaster.
Plaster Pool Patch
Mix finish pool plaster in 5-gallon bucket to a color that's at least as light, if not lighter, as the color of the rest of the pool plaster. Fill repair patch with finish pool plaster.
Trowel pool plaster over entire repair patch, including over the edges, using 1/2 by 1/2 inch notched trowel. Finish smoothing pool plaster patch with stainless steel smooth-finish trowel.
Let plaster patch set and then use your angler grinder and disc and/or diamond polishing pad to further smooth the pool plaster patch to at least same smoothness as rest of finish pool plaster. Remove soda ash and acid wash runoff and refill swimming pool.