Almost all my BB ply is behind glass, so I don't use a finish. On the one's that I do finish, after sanding up to 300 grit, I use spray lacquer. It dries reasonably fast so dust doesn't have time to build up. The downside is the odor. I only use lacquer in the garage or outdoors. If I have to stain a project, I use a wood conditioner first then stain before I cut. I've never tried to get stain into the frets. I think it gives the piece a little more depth if the inner plies are a little lighter than the top and backer.