Decide if you like the look of roughly finished wood where any gouges or scrapes from its previous use show through. This is the most distinct mark of salvaged barn wood. If not, additional processing easily removes these marks.
Look for tighter grain patterns, which are inherent in old forest wood. This wood is less likely to warp during the salvage process and construction phases.
Use a wire brush to scrape off any mud or residual debris left on the wood from its time outside. Pull nails and any other metal that might catch in machinery and possibly damage it.
Pressure wash the wood with water. If any old paint is on the wood, remove it with the pressure washer or scrape it off if necessary. If any paint is to remain on the finished product, seal it with polyurethane sealant thoroughly.
Allow the wood to dry out for several days in the hot sun, flipping it often to avoid mildew on the underside of the wood. Kiln drying, if available, makes this process much faster with no risk of mildew.