When restoring furniture, the open grain in some wood species such as oak may require the application of a grain filler. If your looking to fill the grain in a piece of furniture, prior to finishing why not try a home made recipe like the Victorian restorers used:
Put some plaster of paris in a bowl and a 50/50 mix of water and meths in another.
Dip a ball of cotton cloth into the water/meths and then into the plaster powder. Rub this into the grain of the wood and wipe away the slurry across the grain not with it. Make sure the surface does not get to wet or it will raise the grain.
Once the filler has completely dried, flat back with 180 - 240 grit abrasive and finish with 600 wet and dry.
Tack rag off; at this point your furniture will look like Torvill and Dean have been practicing for dancing on ice, completely normal.
Mix some boiled linseed oil and meths to a milk like consistency and gently rub with another clean rag into the prepared surface: Viola! The plaster of paris grain filler will become invisible.
Clean off any excess oil and leave to dry for approx 3 hours. You will then be able to apply whatever finish you choose?