I tried Formby's with a recent M70. Ugh, what a mess. Wound up using mostly laquer thinner. Depending on the finish applied, methyl alcohol works, as well as acetone, like
@8x68s suggests.
Remember to use gloves that will withstand the solvent you are using, as well as a solvent rated respirator.
I painstakingly recut a lot of the hand checkering in the stock because it was completely gunked up with the Formby's garbage. What a PITA. In a moment of impatience, I used a steel brush (clean, unused one) that I have a few of for cleaning up welds. Wow, it worked great!
Here's info on my refinish:
You guys ever use it? I have done some refinishing without using it to this point, but I was wondering if you all use it or not. I know I could look it up, but 600 grit in between coats or steel wool. Thanks
www.northwestfirearms.com
Edit to add:
some stains are just surface. Those you strip and sand.
Others, absorbed into the wood, may require more diligent approaches. Some methods:
Bleach with Oxalic acid
bleach with dilute chlorine solution then follow up with ammonia wash
Try different solvents. One of the tricks we did in wood shop when a stain was absorbed preferentially in an area, leaving a dark blotch, was to saturate a rag with thinner (turp or mineral spirits) and "rub it out". Think of it like washing a dog poo out of the rug.