This seems to be a difficult problem.

I printed a plant pot in PLA which leaks. I have tried spray painting it inside, which didn't really do anything. Have now tried flame polishing and painting again.

Problems with paint: adhesion, and differential thermal expansion.

Flame polishing / sintering is difficult to do with a blowtorch well enough to work, and to not set the thing on fire. It can also discolour the print.

Epoxy coating doesn't work. A colleague has experienced it cracking off a PLA printed rudder when it heated in the sun. Maybe works if you make it thick enough to have it's own strength, or in this case it might need to be fibreglassed.

Things I've read on the internet that might help:

• Use thicker layers. Reduces the number of layer gaps, so opportunities for problems. Still a lot of gaps though…
• Overextrude.
• Print hot.

I did two prints of the same pot, one with thicker layers, and it was watertight, despite having a worse surface finish (more defects / burrs).