I have a pinhole leak in my Hayward sand filter, at the seem near the valve assembly. The filter is 20 years old. Anyone know if this can be repaired with some sort of patch from inside or outside? Or would I be better off buying a new filter?
Welcome to the forum!
Filters are pressure vessels. They cannot be repaired. So replacement is necessary. Catastrophic failure can injure people and damage property. A filter that is leaking from a compromised vessel should be taken out of service immediately.
I suggest you read ABC's of Pool Water Chemistry.