Preparing Wood for a FinishPosted on August 2nd, 2010 By Bob Flexner No comments
The reason you have to sand wood before applying a finish is to remove machine marks. All machine tools leave cuts or impressions in wood that are highlighted by stains and finishes, especially by stains. Before machine tools appeared in the mid-nineteenth century no sanding was needed. Indeed, there was no sandpaper. Wood was smoothed with hand planes and scrapers.
You can still use hand planes and scrapers to smooth wood; you don’t have to use sandpaper. You can hand plane or scrape the wood straight from the saw, or you can begin the smoothing with a jointer and planer and then finish off with a hand plane or scraper. You can also use molding planes and scratch stocks to shape wood rather than routers and shapers.
But few woodworkers choose this route because machine tools are much faster and easier to learn to use than hand tools. The price, then, for using machine tools alone for smoothing and shaping wood is that you have to finish off with sandpaper to take out the machine marks.
This is an important psychological point. It makes sanding less burdensome when you remind yourself that you don’t have to do it. It’s just the price you pay for the increased efficiency of substituting machine tools for hand tools.
How to Sand
The trick to efficient sanding is to begin with a sandpaper grit that cuts through machine marks and other problems in the wood with the least amount of effort and without creating larger-than-necessary scratches that then have to be sanded out. This holds true whether you are sanding by hand or using a sanding machine.
When you have removed all the machine marks with the coarsest grit you choose, sand out the scratches left by this sandpaper using increasingly finer-grit sandpapers until you reach the sanding grit that produces the size scratches you want.
As an aid to determining when you have sanded out all the machine marks, draw some marks on the wood using a no. 2 pencil and sand until no evidence of these marks remain. To be extra sure you have sanded enough, do it again.
In practice, the best grit to start with is usually #80 or #100 grit. But, if the problems in the wood are so severe that #80 grit doesn’t remove them quickly, drop back to a grit that does.
On the other hand, if the problems can be removed with a finer-grit sandpaper, such as #120 grit or #150 grit, you are wasting time and energy if you begin sanding with a coarser-grit sandpaper. For example, factory pre-sanded veneered plywood or MDF is almost ready to use. Sometimes, beginning with #150 grit is as coarse as you need. Rarely do you need to begin sanding with coarser than #120 grit. Also if you’re refinishing, you can usually sand with just #180 grit to be sure you’ve removed all the previous finish. The wood was sanded originally, after all.
The best grit to end with is usually #180 or #220 grit, though it can be helpful to sand to #320 or #400 grit to reduce grain raising when using water-based finishes. Woodworkers disagree about which grit to sand to. I rarely sand beyond #180 grit.
The primary goal of sanding is to produce a surface that doesn’t show either machine marks or sanding scratches after you apply a stain or finish. If the scratch pattern can be made even, you may achieve satisfactory results sanding only to #150 grit. Stationary sanding machines are best for doing this, though hand sanding will also work, especially if you aren’t applying a stain.
If you could sand just the right amount with each sandpaper grit, it would be most efficient to go through each consecutive grit—#80, #100, #120, #150, #180—and so on. But most of us sand more than necessary with each grit, so you may actually spend less effort skipping grits. This is especially the case when using machine sanding tools.
But sanding is very personal. We apply different pressures, use sandpapers to different degrees of wear, and sand for varying lengths of time. The only way to know for sure that you have sanded enough is to apply a stain and see if any machine marks or sanding scratches show. It is therefore wise to practice on some scrap wood until you get a feel for what works best for you.
If you are sanding a flat surface such as a tabletop, and you want to keep it flat, always back your sandpaper with a flat sanding block. Ideally, the block should be soft enough to serve the dual purpose of absorbing coarser-grit dirt that may get between the block and the sandpaper, or between the sandpaper and the wood. There are good sanding blocks of all sizes and shapes on the market, including here. You can also make your own sanding block by gluing gasket cork to the bottom of a wood block.
Sanding with your fingers backing the sandpaper will cut away softer spring-growth wood faster than harder summer-growth wood. This will leave ridges in coarse-grained woods such as oak that you may not notice until after you have applied the finish.
Always sand in the direction of the wood grain when possible. Sanding across the grain or diagonal to it tears the wood fibers, leaving scratches deeper and more obvious than the scratches left with the grain.
Most woodworkers use random-orbit sanders because they are efficient, easy to use, and they leave a less-visible scratch pattern than vibrator sanders due to the randomness of their movement. For both of these sanders, however, there are two critical rules to follow.
First, don’t press down on the sander when sanding. Let the sander’s weight do the work. Pressing leaves deeper and more obvious swirl marks, or “squigglies,” that then have to be sanded out. Simply move the sander slowly over the surface of the wood in some pattern that covers all areas approximately equally.
Second, it’s always the best policy to sand out the squigglies by hand after you have progressed to your final sanding grit (for example, #180 or #220), especially if you are applying a stain. Use a flat block to back the sandpaper if you are sanding a flat surface. It’s most efficient to use the same grit sandpaper you used for your last machine sanding, but you can use one grit finer if you sand a little longer.
You can also use a belt sander, but it will be difficult to control. The slightest rocking motion will leave gouges in the wood that will take a lot of work to sand out.
Removing Sanding Dust
No matter whether you sand by hand or with a machine, always remove the sanding dust before advancing to the next-finer grit sandpaper. The best tool to use is a vacuum because it is the cleanest. A brush kicks the dust up in the air to dirty your shop and possibly land back on your work during finishing.
Tack rags load up too quickly with the large amount of dust created at the wood level. These sticky rags should be reserved for removing the small amounts of dust after sanding between coats of finish—except with water-based finishes, when you should use a water-damp cloth instead.
Compressed air works well for removing sanding dust if you have a good exhaust system, such as a spray booth, to suck the dust out of your shop.
It’s not necessary to be overly compulsive about getting all the dust out of the pores. Vacuums, brushes and compressed air all do an adequate job. You won’t see remnants of dust deep in the pores under a finish, or under a stain and finish. Just get the wood clean enough so you don’t feel or pick up any dust when wiping over the surface with your hand.
In fact, just before applying a finish, whether directly to the wood or over a previous coat, wipe your hand over the surface to check that it is clean and to remove any small amount of dust that may have settled. Clean your hand by wiping it on your pants leg.