What It Is
Mastic is a special sealant that can expand and contract with deck movement and prevent moisture penetration. It is waterproof and is a type of putty that is placed between your coping and concrete sections. It is important to use mastic to keep water out of your pool decking.
Mastic is elastic and protective, but it will need replacing at one point or another. When you see your mastic begin to peel and crack, it is time for repairs. You might see mastic peel, crack, or shrink away from the coping or decking. These are signs that you should repair your mastic or that some of your coping might be loose and unstable. Any change noticed in the concrete around your pool, your first action should be to check your mastic.
Repairing pool mastic generally entails removing the entirety of your previous mastic, and applying a new one. Because it is a large waterproof putty adhesive, it is done similar to caulking. This is a very important part of the pool that is over-looked. If water gets under the coping and freezes, it will force the coping to pop-up and come loose from the beam. This will be a safety hazard and can lead to waterline tile popping off as well.
Although it is a small part in the overall look of your pool, mastic protects your pool inside and out by providing an expansive, moving sealant.