Finishing wood involves process. You can get outstanding results by taking the time to understand the processes involved in finishing, and by taking your time to perform each process with care.
Great finishing depends on both mechanical and chemical processes. Mechanical processes are all about sanding and scraping wood, applying finishes with rags, brushes or spray equipment, and other techniques in finishing. Chemical processes have to do with the products you choose to use: Stains, dyes, oils, lacquers, varnishes, solvents, etc. Each finishing product has its own unique set of properties, which give you different effects or results, and the rules vary guiding the use of these different chemicals.
The wood finisher needs to know what is important and what is not so critical about all these processes. Begin with preparing your woodwork for finishing. This will cover sandpapers, scrapers, sanding machines, wood putties and body fillers, and tips on getting prep work done quickly.
Finish means end result, you need to think about the end result as you approach each step of the finishing process. How do you want the piece to look? Rustic, satin sheen, glossy, open-grained, mirror-smooth?
The prep work for a rustic piece will vary from that of a fully-filled-grain, mirror-like finish. For the rustic look, you need not repair many physical defects in the surface; in fact, you may want to add some (finishers call this "distressing the surface"). If, on the other hand, you are aiming for a mirror-like finish you'll want every little defect to be repaired. Small defects look terrible in an otherwise smooth and shiny surface. So it is important that you begin with a clear idea of what you want to see in the end.
Your Goal in Surface Preparation
Preparing the surface, for most finishers, primarily means sanding the wood. We scrape or sand the wood:
To get a smooth, level surface for finishing
To get some "tooth" to the surface to promote a mechanical bond with certain finishes,
To get a clean surface.
For most finishes, it is important to have a smooth, level surface. There may be times when a rustic look is your object, and a rough surface with dents and dings may be desirable then, but if your aim is a smooth, level finish, then you need to apply your finish to a smooth, level surface.
At the same time, most of the film-forming finishes need some "tooth" to the wood in order to help the initial coats adhere well. For this reason, it is usually unwise to "over-sand" the wood, meaning, sanding it up in too fine a grit. Sanding raw wood to a very fine grit is usually a waste of time, a waste of sandpaper, and for the extra labor and expense you get less adhesion at the raw wood to seal coat level.
Another important factor in promoting adhesion of your seal coats is having the surface clean. Again, proper sanding goes a long way in getting a clean surface for finishing.
To re-cap, your goal in prep work is to get a smooth, level surface, while providing some "tooth" for the seal coats, and a clean surface.
Repairing Defects in Raw Wood
There are two basic approaches that finishers make in taking care of defects during the finishing process. The first is to repair defects during the prep stage, making repairs directly to the raw wood. The second is to make repairs sometime after the wood has been sealed, when it becomes easier to fine-tune color matching. We will be covering the prep stage repair here, but anyone trying to master finishes should understand both approaches.
Often when you begin sanding a piece, it becomes obvious that there are low spots (dings, dents, scratches, gouges) that can be easily seen and repaired right away. You may also find splintering, chipping, veneer lifting or bubbling at this time also. Following are some procedures and products that you may find helpful in repairing these defects.
Steaming out Dents
Using steam to repair wooden surfaces works really well due to the sponge-like cell structure of wood. Steaming often helps whether you're dealing with a veneered surface or solid wood. The procedure is to apply steam to the dented area then let dry. The steam will raise the grain of the wood, and raise the dent out of the wood simultaneously. You can repeat the process a few times if necessary. Once the area is dry, you sand it back smooth - the dent, if not already invisible, will sand out now.
Steaming wood works best on dents where the wood fibers are not actually torn or broken. Depressions in the wood which have not torn or broken the wood fibers will become totally invisible after you've steamed and sanded. If the wood fibers have cracked around a dent, it may still steam out, but the small crack where the fibers were broken may appear as a dark line when finish is applied.
Visible lines due to cracks through the cell structure generally run cross-grain due to the nature of wood. They sometimes become less noticeable when the piece is stained.
There are numerous methods of steaming out dents in wood. In a furniture repair shop, the technician often has access to commercial steamers used in the upholstery shop. These steamers do a fantastic job, but the expense is usually not justified unless you do alot of upholstery work along with your woodwork.
A more economical purchase would be a "travel steamer" like one by Samsonite. You can find these and similar models in discount stores or department stores. Personally, I like "free" the best, so I use a old clothing iron. Although it is a steam iron, I do not use the steam function of the iron because it is difficult to control. Instead, I use wet or damp rags placed over the defects, and I apply the iron over the rag to produce steam. The iron is set on a fairly hot "cotton" setting. A bonus is that I can use this iron for applying edge-banding to my projects as well.
When using a steamer, simply pass the steamer over the area several times, let it dry, and inspect to see if the defect has "popped out." Sometimes this will take several treatments. Once you are satisfied that it has gotten as good as it's likely to get, let the area dry out and sand it flat with 120 grit cabinet paper using a sanding block. You will soon find that steaming out dents has a very high success rate.