There's no absolute answer to that, in terms of process or method.
In fact, the only valid answer is, "Pick the contractor who has a demonstrated history, going back 5 years or more, of repairing pools with his preferred method".
Many methods can work, in the hands of someone skilled and conscientious at that method. All can fail in the hands of someone who is neither. A few will fail regardless of who applies them.
Ben
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