Most importantly is that the doors need to be sealed soon upon arrival; protect them from changes in temperature and moisture. Also they should be stored in a manner that is perfectly vertical or flat so as to not be subjected to warping. If you do not want to do final coats until later, that is fine, but an early seal coat is important. Do not expose unsealed doors to humidity, temperature fluctuations, unconditioned environment, paint or drywall curing, or sun. Also store perfectly flat and supported or vertical; do not store for long periods where they could sag or warp.
Here are some options to do finishing. First lay the door flat on sawhorses. Make sure it is dusted and sanded the way you want it. Check for any cross grain scratches on rails and soften corners to your satisfaction.
The most commonly used finish combination for interior doors is an oil based stain, followed by sanding sealer, and oil based lacquer. We prefer a dull rub five degree lacquer. The advantages of lacquer are nice build, fast dry, easy sanding, commonly used and available. The disadvantage is the odor.
Our favorite combination to bring out the beauty in the wood and have a more user friendly product is the following detailed description. Start with one coat of Waterlox Original Sealer/Finish. This coat gives the rich, warm, depth, and amber color. The later coats would look dull and plain without this coat. If you choose to stain or darken your wood you can either tint this product or substitute a stain for it. This finish will go bad once you open the can. Retailers sell a product called Bloxygen that is an inert gas and will keep unused Waterlox from turning bad. It is important to let this dry a long time- longer than recommended before moving to the next coats. All you have to do with this coat is get the wood evenly wet. It will soak in with varying degrees from board to board but you can ignore that. Wait about 10 minutes for the product to soak in after coating it initially and with clean cotton rag wipe off any excess and polish very minimally. Then after a couple hours or when it is tack free with help flip the door over and do the other side. Do not wait too long before finishing both sides as it will cup panels if you leave the door unevenly sealed and make sure to do top and bottoms. This product has some VOC odor, so you need good ventilation and/or respirator. Temperature needs to be over 55 degrees. The advantage of using Waterlox under water based finishes is that it is not water based so it will not raise the grain causing more sanding; it will also enhance the colors of the wood. Second, move on to one coat of EF Sanding Seal by General Finishes. Before starting this wait as long as possible, preferably four plus days for a cure. The Waterlox is oil based and these other products are water based. When applying these coats make sure it is a dust free environment and no wind. Do not touch these coats until they are completely dry. Let this sealer coat cure about five plus hours, and when it is hard you can sand it. It is best to hand sand only with a fine sanding sponge and after it is not gummy at all. Just sand it well enough to get rid of hairs, dimples, rag fibers, and so forth. Do not sand through finish. Do all sides of the door. Third use Poly/Acrylic Matte. To make this product tougher you can add General Finishes’ Cross Linker (especially if you are doing table tops). If you are really worried about durability substitute the High Performance water based for the poly/acrylic. Do two plus coats of this. It is up to you if you want to sand between these coats. If you are going to do this yourself and use a brush, then make sure it is in good shape; an HVLP gun is the best applicator. We prefer not too much of a build that looks caked on, but we like to do a little bit heavier build of finish in rough texture areas to make the product feels smooth and protects against slivers. More coats will just make it feel smoother.
If you have the time to wait for it to cure, you can use just three plus coats of Waterlox Original satin sealer for a complete finished door. Make sure to wipe of excess oil (about 20-40 minutes after applying) before it turns gummy between coats. To get a smooth finish buff lightly between coats with a sanding sponge after previous coat completely hard and dried. The advantages of this finishing process are that it is one product, easy to repair scratches, and can be brushed or wiped on. The disadvantages of this are it takes much longer to cure and has odor until fully cured. You should consult Waterlox's website for further information. Take special note to read the Dry time of Waterlox Original Tung oil finishes.
The above recommendations are for interior doors only. For exteriors we prefer not to use typical urethane based products because they tend to yellow with age. We have not done much testing in exterior applications. Waterlox makes exterior glossy oil that can be used on its own. You could also use the above technique and substitute General Finishes High Performance Polyurethane Satin or Exterior 450 Satin for the final coats. For the most common type of application on exterior doors by professional painters we recommend the following: M.L. Campbell's Euro X, Prothane from Rudd, or Prolane from Sherwin Williams as a final top coat for a quick drying and durable finish; these finishes tend to have less problems with yellowing over time; these three finishes should be applied with a spray gun. Whatever finish you use it needs to be reapplied more often than the manufacture states that it will last. It is not unusual to have to do touch up coats on exterior wood doors every two to three years. Sometimes on new wood it needs to be redone after the first year. Sun is the greatest enemy to wood finishes. Wood doors need complete 100% protection from sun and water; do not depend on your finish to do this work on its own.
Note of caution- Do not use water based finishes (even if applied over oil based sealer coat) on pickle wood or any wood that may have salts, brine, or oil in it. Although we like water based finishes for the user friendly safe aspect and quick drying, oil based finishes as a whole and lacquer seem to have more predictable and perfect results when used by a professional. Since reclaimed lumber's previous life was sometimes unknown, one may get more finish failures and undesirable results with water based finishes.
Here are some common mistakes with finishing reclaimed lumber:
ALWAYS TEST your finish before doing the whole door or floor.Softwoods and rougher textures will take and soak up more finish than hardwoods and smoother sanded textures. If the wood is taking the finish, you need to put as much on as it will accept (especially on the first coat). This will make the finish harder and more durable. Don't be surprised if your coverage is nowhere near as much as the manufacturer states it should be. For example, our skip sanded softwood floor may take five coats and our smooth sanded oak might be done in three coats. This also varies with the type of finish. Waterlox and tung oil products take more coats. Heavy polyurethane floor finishes may take less coats.Buff and sand lightly between coats. This along with enough coats of finish will make the product more user friendly and less likely to snag or have splinters.With Waterlox and other "soaking, rubbing oil" type finishes it is more difficult to get a build that covers rough wood fibers and prevents slivers. It will take more coats. Make sure to wipe off excess finish in areas that it is not soaking in after it has been allowed to set for 30 minutes or so; this will prevent the "gummy" effect that happens to tung oil based products where they do not want to dry.Make sure to budget extra time to cure between coats. Since you may be putting on heavier coats, it may take longer to cure.Always follow finish manufacturer’s directions.Your finishing results are going to be less predictable with reclaimed lumber, too.
We do not warranty any of these recommendations. We are not liable for any of the undesirable results from finishing results or following these options. It is recommended you always experiment with a sample before applying any finish to the product. Personal preferences vary greatly and there are an infinite number of finish combinations to achieve different results. It is up to the customer to make sure the door and flooring is properly sanded and prepared to receive finish, too.