In order to create a quality, grain filled timber, you need to pick timber with deep grooves. Clean it up and open the groves/grain by wire wheeling the timber. Dust regularly as you rake out the soft, pulpy sections of the timber. Once you're happy with how much you've removed, sand back the surface with 80 grit sandpaper, again, dusting off regularly to prevent the sandpaper from gumming up.
Get the black bog filler and mix a small amount of the 2 pack with a flexible, flat blade on a pallet. Push the bog out diagonally across the timber. This fills the grain; going directly along the grain will pull the bog back out, especially if it's begun to set. Make sure you cover the entirety of the piece, working the bog in to each section as best you can.
Once the bog has cured, sand it back with 80 grit sandpaper, dusting regularly. Sand it back so that only the grain has black bog left in it. Repeat the previous step, but after sanding with the 80 grit sandpaper, finish it with either 120 or 320 grit sandpaper, depending on the character of the timber.
If the grain is not sufficiently filled, you can repeat the above, but with a thin film of bog. Sand off with 120 or 320 grit sandpaper, remembering to dust thoroughly along the way.
We recommend sealing the first coat with Gloss Polyurethane, as it gives the timber a richer appearance than most other finishes, and gives it a better protective layer. Be sure to seal the end grain of the timber.
Once cured, it should be lightly sanded back with 320 grit sandpaper, thoroughly dusted off, and sealed with 2 coats of the desired topcoat, such as Satin Polyurethane, Eggshell Oil Varnish, Danish Oil, or another appropriate top coat.