My approach is to work on one section at a time regulating the air pressure
to push the surface out. Then, using mostly my plastic hammer, I knock back
the high points, thereby bringing up the low spots. Success varies.
Several times I thought I had this all figured out only to learn that I don’t.
Tubs.
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.