When we still had the shareboat the roof was repainted due to having a large rust patch forward of the rear hatch, round the mushroom vents and chimney, and on the leading cabin edge above the welldeck. I was not privy to the fine details of the job, but i imagine it was just rubbed down flat and then painted and was done at the cheapest/quickest quote from the Marina. Afterwards the roof looked great.
Within a year shadows of the former rust patches could be seen through the paint, corrosion was starting to show again by some of the mushrooms, and the lightest scratch would reveal undercoat. Cost £2.5k and took a week back in 2019.
Our own boat is booked in with a proper boat painter (booking made two years ago) to go at the end of next month for above the water repaint, it will be shotblasted back to clean metal with everything removed, then properly painted. This will take 6-8 weeks. Quote was £12-13k when booked, but i'm sure this will have increased, at the time they guesstimated 5% more but who knows now with the way things have been going up. Having seen the results of a quick turnaround paint job at a Marina we are loathe to repeat that mistake.