Spot etches or surface deterioration are spots in plaster. The water removes Calcium Hydroxide from the plaster. This happens when the pool water is not properly balanced. Spot etches are weakened areas of the plaster, and if you scratched one of these spots with a key from your pocket you would be able to dig out some of the material. Try that same test on a smooth, un-affected, area of the plaster and all you be would be able to do is make a scratch on the surface.
Many companies will acid wash your pool surface prior to installing the bond coat they are using. However, acid that companies use to try and etch the surface, will not etch a spot etch. The acid gets “sucked” into the spot etch and fills in the void inside the weakened plaster spot. It does not completely come out when the pool is rinsed down. That means acid remains inside the plaster spot etches and winds up being trapped under the bond coat. The acid will eventually leach out to the surface to the back of the bond coat. The acid will be directly on the bond coat that was installed. Acid will break the bond that may have existed between the bond coat and the plaster that it was used on. When this happens your plaster investment will be at risk.
The only way to make sure your pool replaster investment does not become compromised in any way, is to remove all of the spot etching. This can be done with heavy sandblasting and then using a proven bond coat, an acid wash is not sufficient. Only a proven manufactured bond coat should be used. Homebrew bond coats should never be used in any case. Another way, the best way, is to Water Jet the surface with 40,000 PSI UHP water pressure. G&B uses a 40,000 UHP Water Jetting Machine in our Hydro Demo Replaster Prep process.