Should I prime a painted wall before spray texture?

The headboard of my antique bed smells funky and i'd like to seal in the smell with something that's odorless. It's just the unfinished back of the headboard that smells. One hardware store recommended Varathane No-Odor Polyurathane, and another said to use Polycrylic by Minwax, which is water-based. Any recommendations? I assume i need something waterproof? Thank you

I'm getting ready to have my kitchen repainted, and was thinking about trying to smooth out the texture of the walls first. They have a very rough texture, I don't mind some texture, but this is kind of over the top (maybe called halk and trowel texture?). I've looked into having it professionally done, but it seems prohibitively expensive. Parts of it look relatively smooth, with ridges like they went back and forth with a trowel, but then there are these big lumps here and there. Anyone know how hard it would be to smooth out the texture? Is there an easy way to do this? If I sand down the bigger bumps, would I need to go over the wall with some kind of joint compound? I don't need it to be completely smooth, just less "hilly."