Jaycee,
I use 400 grit to sand, but you may well have a very open pored piece of wood.
You could seal the wood using sanding sealer or spar varnish.Which is what I would do if time is of the essence.
You could also sand using oil and whiting (plaster of paris) but beware of using any fillers, they can cause more problems than they cure.Once you have achieved your closed grain proceed upto 600 or 800 grit wet & dry (silicon carbide paper).