I'm a woodworker but I did a lot of research on finishes for the oak bed when I rebuilt my 53 Chyevy 1/2 ton a few months ago. Don't use linseed oil and then try to varnish over it. Linseed oil by itself applied in several coats, will protect and finish the wood, but it is a dull finish and will evaporate over time. If your oak wood is new and has never been refinished, first sand it with 120, then 220 and then 320. Use a tack rag to wipe off all the dust from the final sanding. A tack rag can be made up but they also sell them cheap in Home Depot or Lowes. Get a half dozen for use on this job. After using the tack rag, brush on Minwax oil stain. I think you'll like the Minwax Golden Oak color. DO NOT USE THE Minwax combination stain and varnish in one can. IMHO it sucks. One of those sponge rubber brushes works fine for the stain. Let dry for 10-15 minutes and then wipe off excess. If you want it darker, put on a second coat of stain. Try it on the underside of the board to make sure you have the color right....then do the rest. After the stain, use a high quality Marine Spar Varnish with the best UV protection you can find. This is an oil-based varnish. Don't use a water-based urethane. I know it is tempting for the easy cleanup, but you won't be happy a few months later. After my research, I settled on Z-Spar Flagship varnish. Get it at a marine supply house, or run an Internet search for the nearest retail place. It will be expensive; probably 2 quarts at 28 dollars per qt., but you can't beat it. For the first coat, make up enough to do one side of the boards. Use 25% varnish and 75% varnish thinner. Brush it on and let dry thoroughly. The sponge rubber brushes are ok here too. Buy several and use for the varnish and toss after each coat, or buy one very good natural bristle brush and clean up with mineral spirits. Do the edges and ends of the boards and inside the boltholes. Otherwise moisture will get in and swell the board. Smooth each stroke on with the grain and try to eliminate bubbles. Make sure you catch drips and runs on the lower sides and edges. Otherwise, you'll be sanding these spots down later. Varnishing needs to be done on a low humidity day, or indoors, and for sure, away from dust, pollen, etc. Let this first coat dry well. Sand lightly with 320 sandpaper over a block of wood. Use the tack rag to remove dust. I suggest you do all sides of each board before you put on a second coat. Mix up some more varnish. This time, and for all succeeding coats, use 75% varnish and 25% thinner. Just make up enough for each coat and don't pour excess back into the varnish can. Apply the second coat to each piece, sand, tack rag, recoat, etc. and apply a second coat. I suggest you give the boards at least six coats, letting dry completely between coats. Each coat will look better than the last as the varnish fills in and flattens. Don't sand the last coat. You're going to love this finish. It will hold up very well and it will last a long time. No varnish, not even marine spar, will last forever, and this will need recoating in about 4-5 years if exposed to constant sun, rain and snow.
