How many coats should I apply?
For underwater applications we require 20mils dry. So if you are covering 40sf it will take one gallon to get 20mil dry. The Dura-Rubber is 60% solids so if you apply 3ea 10mil coats you will get 18 mil dry close enough. The easiest way to do it. If you purchased 4 fives that is enough to cover 800sf at 20mil so if your pond has 800sf apply until it is gone. 10mil coats would be about the max per coat you can apply. To give you an idea, latex paint for walls goes on at about 1-2 mill per coats about as thick as  copy paper.