A smooth, uniform surface is one of the most important components required to achieve a beautiful finish. A poorly prepared surface can be the true culprit in undermining the results of a finish. As with many woodworking tasks, there is more than one technique that will deliver a well-prepared piece of wood ready for a finish to be applied.
The three most common processes for smoothing wood are sanding, scraping and cleaning up the surface with a hand plane. (In this article we are primarily talking about smoothing solid wood.) Is one of these methods superior in delivering a suitable surface for finishing? The answer is no; all of them can make a smooth, finishable surface. So, the question becomes: is one of the methods faster or easier or more forgiving than the others? Here the answer is less cut-and-dried — and the qualified response is that it depends on the task at hand.
Sandpaper and Sanding
Certainly the most common practice for smoothing wood is sanding. It can be done by hand, but most of us use a power sander of some kind. Sandpaper, in all its many varieties, is basically a substrate of some sort of paper — the quality of that paper varies — with rocks glued to it. (OK, “rocks” is really not an accurate way to describe today’s modern abrasive materials, but you get the idea.) With sandpaper, you remove and smooth wood fibers by abrading them. After you have sanded off the machining marks and other surface flaws with a coarse-grit paper, the primary goal of each successively finer size of grit is to remove the scratch marks that the previous sandpaper put into the wood. It is an effective system when done properly, and the results are predictable and can be made to be uniform. The biggest problem with sanding is that many woodworkers sand ineffectively and therefore get inconsistent results. For example, random or bit sanders and vibrating pad sanders are designed to be moved across the wood at a rate of about 1 inch per second. Run your finger from the top to the bottom of this page and count slowly to 11 — that will give you a good idea of what speed to move a sander.
Sanding is effective when done right, and it’s easily mastered. The materials are easy to come by and reasonably priced. It is also the most practical method for smoothing curved and shaped forms. For that reason, it is likely the best overall method of surface preparation available to woodworkers.
• Use good quality sandpaper.
• Don’t skip grits. A rule of thumb is to start with 80-grit and move successively up to 180-grit.
• Plan a consistent sanding pattern for your project, so you don’t skip over an area.
• After each pass, make sure you have removed the scratch marks from the previous pass.
• If you use a power sander, it’s OK to sand across the grain, but the last pass should be done by hand in the direction of the grain
Scrapers and Planes
Scrapers and planes differ from sandpaper in a couple of significant ways. First, they cut the fibers of the wood rather than abrade it. Second, while surface preparation is something these tools do, it is not their only task: planes and scrapers also shape wood. The most significant point to be made about smoothing wood with planes and scrapers is that they perform this task best on flat aspects — curves and other shapes are not easily dealt with. If you are smoothing squared-up rectilinear stock, a properly set up plane or a sharp cabinet scraper are superior tools for the task. They produce a smooth, ready-to-finish surface much more quickly than sandpaper. In fact, if you watch someone who is proficient with a bench plane smooth a piece of wood, it is truly amazing. But that is one aspect of these tools — they require some skill, both in sharpening and setup as well as in use. While not difficult to master, there is a learning curve to overcome.
Mix and Match?
As I’ve said, sandpaper abrades, and planes and scrapers cut. All prepare acceptable surfaces to apply stain and finish. But one thing to avoid is putting a planed or scraped panel next to a sanded piece; i.e., a planed flat panel surrounded by stiles and rails that have been sanded. If you apply stain to these pieces, it will likely be absorbed differently, and their appearance will be markedly off. If you must join pieces of wood that have been variously prepared, I recommend a final hand sanding to ensure a uniform surface.