You could tr4y oxyalic acid which is in powder form. You mix it with hot water the hotter the better and brush it on, or there is a two part wood bleach that you mix up. I used one of the two part mixes once to do an old mil surp stock. It reacted to the grease on the stock and started seriously started to foam up and get very hot. So as I was outside I dumped it and used the garden hose to clean it up. Don't remember the name but also do not know if todays mixes would still react. So some caution would apply here. Frank
