If you sanded with something as rough as 80-grit, I would make sure you remove all the big scratches. Notch-failure is the most type of failure in tubing and starts at a surface scratch and expands from there. The highest load areas are under the downtube about 2-3" away from the head-tube. And on the seatstays about 2-3" behind the bottom-bracket. As long as those areas are clean and smooth, you should be OK.

You can get sandpaper on cloth rolls, like tape. I prefer the 1" wide ones for bike tubing. Cut off 12" strips and wrap it around the tubing at about 45-degree angle. This gives you complete 360-degree coverage of the tubing. Then floss it back and forth and move it up and down the tubing. This ensures you remove a uniform layer of paint (and metal if any). Much better than using something like belt-sander which takes material off in flat strips. Use 220-grit, 320-grit, 400-grit and 600-grit before you prime for paint. Then wet-sand between coats with 800-grit wet-n-dry paper. Then 1000-grit before final clear-coat. After clear-coat has had 2-weeks to fully dry and harden, wet-sand with 2000-grit before polishing.