Surface preparation
Most woodworkers will agree: the fun is in the making, not the finishing.
We all seem to switch to “groan and moan” mode when it comes to putting the “finishing” touches to our projects. The truth is, putting a decent finish on your project is not as difficult as it may seem. It all begins with care and attention to the way we prepare the wood surface before we apply our chosen finish.
So, to start off this series on wood finishing, we will be looking at wood surface preparation. In upcoming articles we’ll be looking at applying various finishes, including shellac, oil, varnishes and lacquers.
If you want to improve your finishing skills, a couple of things will help: access to reliable information the appropriate tools and finishing supplies, and lots of practice.
To properly prepare wood surfaces for finishing you need a few basic tools, which many readers may already have at hand. You’ve correctly guessed that the main “tool” is sandpaper. I recommend that you buy premium sandpaper. It lasts longer and gives better performance.
The two brands that I particularly like are Norton’s No-Fil Adalox Aluminum Oxide “Champagne Magnum” paper and the Mirka Royal brand. Both of these stearated papers are flexible yet tough, with excellent edge wear and superior load resistance.
For power sanding I recommend a variable speed random orbital sander (R0S). Use one that comes with a dust bag or that can be connected to a shop vac. ROSs have an offset drive bearing that causes the sanding pad to move in an elliptical orbit, which reduces scratching against the grain. You can move the sander any direction on the wood: with the grain, diagonal to the grain, and even against the grain. I’ve been using the Bosch 3727DVS for the past two years and am very satisfied with it.
You’ll still need to do some hand sanding. For this, a sanding block is the way to go. You can make your own, buy a rubber sanding block from a hardware store for $3-4, or get a top of the line wood block from Lee Valley (#05Z14.01, $29.95), that takes 1/6 of a sheet of sandpaper.
Finally, you may want to try using a scraper (Lee Valley cabinet scraper, 05P32.05, $49.50, hand scraper, 05K30.01, $15.95.) Cabinet scrapers, in particular, are quite easy to use, and raise a lot less dust than sanding. If you have glued up boards that need to be leveled, a cabinet scraper will do a less nerve-racking job than a belt sander. See Flexner’s book (below) for easy instructions on using and maintaining scrapers.
Flat and Smooth
The goal in preparing a wood surface is to make the surface both flat and smooth. This is important because applying a finish to wood not only brings out the natural grain and beauty of the wood, it also magnifies any defects in the wood. In particular, fine scratches and thin slivers of dried glue that may be barely visible prior to finishing will stand out like a sore thumb after the finish is applied.
If you’ve glued up several boards you will likely end up with ridges along the glue lines, which will need to be removed. Planers and jointers leave mill marks on the wood – a series of small washboard like marks, where the knives have taken shallow cuts out of the wood. You can see these marks by holding your work surface at about a 30° angle to a bright light. These mill marks must be removed as well.
Taking Care of Business
You could begin smoothing your wood surface with a hand plane. However, proficiency with a hand plane takes a fair amount of skill and experience to acquire. Most of us will want to get out our power sanders.
Some sandpapers (abrasives) are designed for sanding raw wood, and others for levelling finishes like lacquer and varnish. The most common types of paper are: garnet, an orange coloured sandpaper made of a natural abrasive: aluminum oxide, a man-made sandpaper that is more durable than garnet, and is probably the most widely used sandpaper in woodworking, and silicone carbide, another man-made sandpaper generally used for sanding finishes, as it can be used with a lubricant.
Both aluminum oxide and silicone carbide also come as “stearated” paper, in which a metallic lubricant soap is added. Stearated paper doesn’t clog up as much as unstearated paper. Sandpaper comes in a series of “grits” from 60 (coarsest) to 2000 (finest), although you will really only need 100, 120, 150, 180, 220, and 240 grits for preparing wood surfaces.
There are Five Basic Rules in Sanding
1. Sand progressively. Use a progression of grits. Start your power sanding with a 100 grit paper until you’ve removed the milling marks. Then move up to the next grit (120). Sand until you’ve removed the scratches left by the 100 grit. Then continue on up through the 150 and 180 grits. If you plan to use an oil finish you can also sand with 220 grit.
2. Don’t skip sanding with each successive grit size, or you’ll have difficulty getting rid of the scratches left by the coarser grit.
3. Dust off. Clean the surface before moving to the next grit (use a cloth, brush, vac, or blast of air).
4. Renew: Change your sandpaper often, as sandpaper wears out quickly and looses its effectiveness. If you look at your paper you’ll be able to see when its worn down.
5. Lastly, sand by hand. Even random orbital sanders leave swirls, so do a final hand sand using the next highest grit. Remember to use a sanding block and sand with the grain.
So, there you have it. It’s not too complicated, really, but good sanding requires a consistent and diligent approach.
In the next article, Carl continues his series of articles on wood finishing, with pore filling.
DUST ALERT
The US National Institute of Environmental Health Sciences (www.niehs.nih.gov) has recently classified wood dust as a carcinogen. According to NIEHS, unprotected exposure to wood dust can increase the risk of cancers of the nasal cavities and sinuses.
As well, breathing dust and fumes isn’t a terribly good thing for your lungs. Do them a favour and, minimally, wear a dust mask. Better yet, wear a respirator when you do a lot of sanding. The 3M model 7500, half face piece respirator ($35), is an excellent choice. It’s made of soft silicone and has a cool flow exhalation valve, easy to adjust, light as a feather, and super comfortable. It’s easily used in conjunction with glasses and earmuffs.
Replaceable filters run about $12 a pair. To find a dealer contact 3M at: www.3m.com or 1-888-364-3577.
There are dozens of books about finishing. For my money, the best is “Understanding Wood Finishing” by Bob Flexner (1999, Readers Digest, ISBN: 0762101911, 310 pgs, $26.95 CDN).
If you really want to understand how to select and apply the right finish, then get this book. It provides a comprehensive technical reference that is very well organized, yet easy to read.
The book is filled with useful charts, tips, and excellent photos of sample wood stains. Flexner covers all the essential topics, including oil finishes, staining, glazing, pore filling, shellac, lacquer, varnishing, water-based finishes, rubbing out, repairing finishes, and stripping finishes. He even includes some step-by step finishing schedules for common woods, including pine, maple, mahogany, cherry, and oak.
Don’t be put off by the fact that there is a lot of technical information in the book. Flexner is able to demystify the finishing process and conveys information in a way that is easy to digest. I’ll be referring to this book regularly in the upcoming articles in the “Wood Finishing” column. Flexner is also the editor of “Finishing and Restoration” magazine, the information voice for the professional finishing and restoration industry.
Carl Duguay - [email protected]
Carl is a Victoria-based furniture maker and the web editor at Canadian Woodworking & Home Improvement.
