I'll just mention the virtues of the vacuum cleaner, for this purpose. I like to vacuum the balsa surfaces before ironing, and that includes the interior of built up surfaces. I use the brush thing and sort of scrub the surface or stringers, gently. I think it picks up pretty much everything of importance inside the built-up structure, while I'm at it. I bought a computer keyboard vacuuming set, but haven't tried it yet.

Good luck,
Dave Olson