re: Bathroom sink

I don’t know about availability en France but here in N. America I would strongly recommend S1 Sealer from System Three. I used LOTS of this stuff when I was building yachts and it is the only epoxy that I know of that has the ability to penetrate any distance at all before cross-linking. Three or four coats (recoat as soon as it is tacky) would make the sink very waterproof and your colours will pop too.
You should also do the outside of course as it will absolutely seal and can cause uneven stresses if only the inside were coated.