craftgrrl | to seal or not to seal?

Hi all,

I have been asked to do a bit of decorative painting on this wooden VHS movie storage thing. It's about 5 feet tall and has a birdhouse kind of thing on top, and the woman wants me to paint it with vines and pretty things. Nothing overpowering, and she doesn't want a basecoat. She just wants the vines and things painted right on the wood, which looks like it's been stained but not varnished, and I'm a little scared about this. I'm not sure how well the acrylic paint will go on the wood without sanding it first, but she doesn't want it sanded. And if the acrylic goes on without a problem, which it might, what do I do about sealing it? I can't just seal the painted areas and not the rest of the piece, I think that would look bad. But I cannot think of the best way to go about sealing the whole piece. Maybe brushing on a coat of polyurethane? I've never used that over acrylics before, do you guys think that would be a reasonable way to go? Erg. Thanks for letting me ramble in my overcaffeinated paranoid way.