I have been working on a very rustic bookcase made from 100+ year old fence pickets, and I decided to use some red oak for the actual shelves themselves. I have already begun the staining process for two of them, at least two or three hours ago. Every time I go to check on them, there are a bunch of little droplets of tacky stain seeping up from under the wood. My final sanding on these was with 320 grit sandpaper. Would this be enough to keep the wood from absorbing the stain (it is oil based)? Also, I should point out that I used mineral spirits to clean the boards about 20 minutes before I began staining. I am basically a total beginner, so if the answer is extremely obvious, try to be patient with me. Thanks for the help.
320 is fine to sand to if using an oil-based finish.
For staining though 320 grit is a bit fine, it isn't the source of this effect it's just that the smoother you sand the surface the less the wood will take the colour.
Oaks are coarse-grained or open-grained species and some oil will generally exude from the pores and grain structure for a time after application. This generally occurs mostly within the first hour, sometimes over a few hours and more rarely it continues overnight.
American red oak in particular has a pronounced pore structure which tends to make this occur even more than in other oak subspecies.