Very sorry to hear (read?) that about your trouble Raggie.🤔
But pleased to hear that you are on the mend and getting back into our (sometimes🙄) therapeutic hobby👍
I can thoroughly endorse Dave's comments, especially re Tamiya masking tape. Normal masking tape ALWAYS has crinkles which lets paint creep underneath, or you spend ages trying to flatten the edges with the back end of a paintbrush handle or similar.
E.g. ACME Tools Professional Masking Tape De-crinkler, Special offer: TODAY ONLY JUST 15quid! 😁😂 Even the fine artists masking tape has fine crinkles I've found😭
Tamiya tape is superbly flat and easy to tape round curves and multi-plane surfaces, like most boat / ship hulls🤔
To 'straighten' (rotten pun intended😁) out your problem-
If there are any raised edges along your wanted waterline; flat them back with something like 1200 to 2000 wet&dry (Again Tamiya😉 I like to use their sanding sponges for this).
Mask off one colour along the waterline (e.g lower hull) with Tamiya tape. The rest of the lower hull with the usual newspaper held down with normal masking tape onto the Tamiya tape.
Spray on your upper hull colour. When dust dry slowly pull the Tamiya tape away from the waterline at an acute angle.
Don't wait for the paint to fully harden or you may find that it cracks and lifts at the edge, especially if it is a bit thick.
Leave to fully harden overnight and repeat the process to paint the lower hull.
Sorry if this sounds pedantic (takes much longer to describe than to do🙄) but it works for me - all tips learned the HARD way 😭
Bon chance mon ami🤞
Cheers, Doug 😎