paint remover

Hello, I'm putting new tabbing on some parts of my boat as I have noticed some area have broken slightly. I'm also reinforcing around in prevention of future issues. But before that I have to go through the paint inside. I have sanded and sanded... and sanded. I found this stripper that is...

I have a 1972 Flying Scot with very hard, badly flaking white deck paint of unknown origin. This white "paint" is on top of the original beige gelcoat, and it is very hard. I can break off bits of flake, and they are very hard and inflexible. I'm not sure if this is a second layer of gelcoat...