Linseed oil forms a film much like oil based paint. It cures by absorbing oxygen from the air which starts a polymerization reaction. Boiled linseed oil will cure much faster than raw linseed oil. I doubt you really need 50% thinner, just enough to make it easy to brush on. In fact it will work without the thinner but may be slower drying due to the thicker film. I haven't had much occasion to use it on metal parts but frequently use it on wood tool handles.
