Mast base

My boat is stored at the beach, and we are off to Oman this weekend. I had started sanding the inside of the mast (corrosion) and the outside of the cap, but with 200 grit sandpaper (all I had). I'm going to dremel the outside edge of the cap at home, and try the tappity-tap method. I tried that, but the cap was just too big. Thanks again all for the helpful input. What a great community!
Even after sanding, the new cap did not fit. I found a used cap in the club supply shed that fit my 1978 mast perfectly. Just an observation, but the o.d. of the new mast cap was slightly larger than the old mast cap.
I ordered the cap from Sunfish Direct...looks very similar, but diameter was slightly off. I donated the new cap to the club shed in exchange for the old one. I will try to measure it the next time I go to the beach.