Michael,
They've just darkened over the years. No debris, just an even yellowing. Someone once mentioned shaving cream but e-gads, can't imagine that would work. I suppose I should just count my blessings that they yellowed evenly.
I'm actually looking forward to sanding these down. I can see the bottoms as they lay in the back of my van and they're just beautiful. I think Dave went a little nuts with the BLO and there are globs of dried, sticky oil all over these. They have a real nice matching grain pattern with quite a few knots.
'76 Klipschorns (Mahogany), '80 Cornwalls (Zebrawood), 1979 Cornwalls (Raw Birch) - Components Listed In System Profile