Things You'll Need
1 gallon boiled linseed oil
Orbital electric hand sander
To properly finish a wooden truck bed you need to use penetrating oil. Truck beds can take a real beating from moisture. Traditional wood coatings like varnish and shellac do not penetrate the wood and can scrape off in rough use. In addition, surface coatings get scratched off and lose their protection with exposure to the sun. Penetrating oil such as linseed oil is best for truck beds. It penetrates deep and another application of linseed oil can be applied at any time with minimal effort.
Sand the truck bed using an orbital sander with 100-grit sandpaper. Sand with the grain up and down the bed until the surface is free of splinters and rough spots. Put the 180-grit sandpaper on the sander and continue sanding until the surface of the wood has a dull sheen to it.
Wipe the bed down with a soft cloth to remove any residual dust. Alternatively, you can use an air compressor or leaf blower to get rid of sanding dust.
Pour a small amount of linseed oil directly onto the truck bed. Let it spread out to about the size of a baseball. Begin rubbing the linseed oil into the wood with the soft cloth. When the wood has absorbed the oil, pour on some more oil and continue rubbing the oil into the wood. Work your way the length of the bed until the wood has a dark color from absorbing the oil. Wait one hour.
Pour more linseed oil onto the bed. Wipe it down. Continue the length of the bed until the bed looks wet. If the bed still does not look wet, keep applying the linseed oil and wiping it in until the wood will not absorb any more oil. Wait one week.
Pour more linseed oil onto the truck bed. Wipe it in the length of the bed. If it continues to soak into the wood, apply more linseed oil until the bed looks wet again. Wait two weeks to one month and apply another coat of oil. After that apply linseed oil every six months to one year or whenever you want to get a deeper, richer finish.
Don't be concerned with any metal parts in the truck bed. Linseed oil will not harm them. The linseed oil will finish them just like it does the wood. After the initial application of linseed oil, its okay to use your truck in the meantime.
Even though linseed oil is nontoxic, because you're working with large volumes of the oil it's a good idea to wear breathing protection when using it.