A first coat of oil. Linseed oil is interesting stuff - it's called a drying oil, but it doesn't dry as such. On contact with air it polymerises, eventually forming a plasticky substance which seals the pores in the wood and hardens the surface a little as it solidifies. It takes lots of coats and it darkens the wood considerably.
The polymerisation process is exothermic - gives off heat. One has to be careful with linseed oil soaked rags, because they can spontaneously combust if left wadded up. I saw it happen to a nieghbour once - fortunately he caught it before any damage was done (well the rubbish bin was a write-off), but he had a garage full of smoke.