Touching up two toilets' ceiling was pretty tiring. I used quite a bit of milk power to scratch clean the chipped paint with a putty knife.
Instead of just repaint directly with Ceiling White, I applied Nippon Vinilex 5170 solvent-based primer sealer twice beforehand.
1st coat of primer- wait for 20 mins to dry
2nd coat of primer - wait for 30 mins to dry thoroughly
Then 2 layers of Nippon odourless white.
Surprisingly it was easy and smooth to paint over the base primer.
Still, doing digital retouching is much easier. No need to climb up and down the ladder, and no need to clean up the mess.
Hopefully with such extra works, it will last even longer.