Goop off works very well and is less harsh than paint stripper. I just completed a refinish on an old badly water damaged Marlin model 80 and the goop off worked like a charm.

Once you do start sanding, if the wood is in good shape, start with a grit like 120 and finish with a 220 grit. If the wood is in really bad shape you can start with 80 grit. You can use more aggressive sandpaper but be very careful.

Biggest thing to remember is to ALWAYS sand with the grain. I have also found it easier to use a soft foam sanding block with my paper.