>>64425414
I filled a large plastic container with scalding hot water, submerged my Wz.29 stock in it for about two minutes, and scrubbed with the rough side of a kitchen sponge. All surface dirt and grime instantly peeled off, no damage to the finish at all. Wiped down with a fresh towel and air-dried for a week.
Alternatively, I sprayed a spare K98k stock set with a fuck ton of Ballistol and gave it the same sponge treatment. Cleaned just as well, but took a while. I've heard of people getting similar results using simple green.