Some paints contain LEAD, which is toxic.
I'd never consider using paint on a harp slide.
Or, any making other home-made alterations,
of fabric or liquid, to the slide.
If the slide leaks air, tighten the mouthpiece
a little, or play quieter.
Use of a CX-12 type harmonica will make the
sound more air-tight.
Your results may vary from mine.
Best Regards, Stay Healthy
JB