As Laquer I used „Cramolin Urethan klar“ which is just urethan laquer. The other one is an off-brand low cost Alkyd laquer. I just sprayed the inside of my housing twice with a thick coat and did let it dry for days. That got it leakproof as far as I can tell. My standard PLA print soaked the laquer almost like a sponge. I did not sand much, only wiped my print with denatured alcahol. Maybe I degreased it with acetone, but I don‘t remember exactly as I opimised my print settngs instead.
I do however recommmend to tune your printing parameters first. Look in my old post. Use many perimeter lines (>=4) and maybe some overextrusion and a higher printing temp. You can use pressurized air to check for leaks. Usual prints for me leaked like a sieve. With optimized settings it should hold air well. You can use laquer aferwards to bulletproof it.