I fill and finish my gunstocks with a combination of Permalin ( Laurel Mountain forge through brownells) and a version of Mr. Paranee's method. You are past the permalin stage, but you can always apply the last step.
I mix rottenstone with linseed oil to the consistency of tooth paste. Dabb a little on with you finger and hand rub it to build heat. Don't put on coat after coat, just do it once, set it aside for the day where it can get natural light. Linseed oil only hardens with UV light. Repeat for as long as it takes to fill the grain, probably about 5 coats, so in a week it will be ready.
The rottenstone will polish the wood as you fill the grain.