Like Dennis, I put a drop of japan drier in my final coats. Don't add to your entire stock, leave that unmolested. A good grain filler is Easy Sand 20 (or 45) quick drying drywall compound. Available at any building supply, hardware store. It has a nice fine texture and is not abrasive. I just wet sand it in. If you want to experiment, I find it a quick patch for drywall repairs.