Secrets Of a Floor Refinisher - continued

The Art of Sanding

When I sand a floor, I’m mainly trying to remove old finishes and stains. I don’t want to remove any more wood than I have to. Most things in construction you can fix — you can touch up paint, and you can patch drywall. But once a wood floor is sanded, you can’t put the sawdust back. That’s gone forever.

Most floors have a life of two or possibly three refinishings. After that, there’s nothing there. With 3/4-inch tongue-and-groove boards, when you wear down almost through the top lip of the groove, the lip starts to lift and people start to get big splinters. It’s over.

You’ll sand a floor several times each time you refinish it. It starts with a diagonal cut, using rough grit, to level up the boards. There’s usually a second diagonal cut at right angles to the first. Then it takes one or two straight cuts (parallel to the boards) to smooth the surface, using progressively finer grits. Finally, you screen the entire floor with the finest grit to create a consistent fine scratch pattern.

Nitty gritty. You always step from coarse grit up to fine grit, but the particular grits are different for different cases. You have to get a feel for it — going too coarse burns up too much wood, but going too fine burnishes the wood and can make it reject the stain.

The lowest grit we have is a 12. Twelve-grit means there are 12 pieces of grit per square inch of paper, so your grit is far apart and pretty thick. A 120-grit paper has 120 pieces per square inch; that’s a fine grit for floor sanding.

A 12-grit paper is like gravel. I wouldn’t hit wood with 12-grit — it’ll shred it. But I would use it to tear off linoleum and tar and paint. Linoleum dust is nasty, but you can tear off linoleum in big chunks using a 12-grit belt. Old linoleum is often sealed down with tar, but if you move fast with a coarse grit, that will come up in chunks with the tile, and you can sweep it up.

If you start with a 12-grit, you’ll step up to a 36 or so next. Why just a 36? Well, you’re trying to match what came before. You can’t jump from a super-low-grit paper up to a fine grit in one step. A 100-grit paper would just clip the ridgetops off whatever the 12-grit left behind and leave the grooves there. You want to use a grit coarse enough to cut a lot off the high points, without digging any new low points.

For a bare wood floor, or one with nothing but old finish on it, I’d start out higher than 12 to begin with. Take a brand new oak floor, just installed. A good first cut would be a 40-grit on a diagonal; then I’d do a 60-grit straight cut, an 80-grit straight cut, and finally screen it with a 100-grit screen — or even just another 80. But if you’re working with a sappy old fir floor with varnish on it, you might go down to a 36- or even a 24-grit for the diagonal cut. Then you’d step up to maybe 40 and 60 for your straight cuts, and screen it with — at most — an 80-grit. Use fine grit on a fir floor, and your belt will instantly load up with sap.

Diagonal cuts. Your first diagonal cuts bring the board edges even. Boards are often at slightly different heights, so if you ran parallel to the boards, it would put the drum off kilter and keep you from sanding smooth. So I make a diagonal cut first, and get down to bare white wood.

In some old houses, you can encounter pine floors with boards more than 22 inches wide. Often the boards are cupped — some concave and some convex. In a true restoration you don’t level those out — you hand-scrape them all.

On a newer house with a wide 1x6 or 1x8 pine floor that had some cupping, I might need two diagonal cuts to get it level. If the boards were convex, the center of the board would get cut first; if they were concave, just the edges would get cut. All along each board you would be able to see parts that were still untouched (Figure 3), so I’d make a second diagonal crosswise to the first one, to get the boards down level for a flat surface. (By the way, if it’s the edges that are high, you may want to hand-scrape the floor or leave it alone. If you grind down just the edges on your early passes, you might end up exposing the tongues before you get the boards flat.)

Figure 3.A first rough cut with the drum sander using coarse grit can pull off linoleum and its adhesive but may not expose the wood in low spots (left). The drum sander also leaves an unsanded area near the walls (right), which has to be sanded with the edger.

All around the perimeter, the first cut leaves a margin that’s still the original color, because the drum sander can’t reach all the way to the wall. We have to hit that with the edger next.

Once the whole floor’s down to fresh wood, it gets harder to see where you’ve sanded and where you haven’t on subsequent cuts. The trick is to get a carpenter’s pencil and go around the whole perimeter of the room and scribble like crazy over about an 18-inch margin. Anywhere you don’t hit on your straight cuts, you’ll see pencil and you’ll know you have to edge there.

Straight cuts. After the diagonal cuts and the first edging, it’s time for a straight cut (Figure 4). I will hit the wood with two grits, but I don’t like to start out any coarser than I have to. I always experiment first — take an old belt out of my truck, say a 50-grit, and put it on my drum. If it cuts the floor perfectly white, that’s the lowest I’ll go; then I’ll step up to an 80.

I never go above 80-grit. I won’t use a 100-grit belt — all it does is burnish the wood, preventing it from taking the stain or finish properly.

Figure 4.Straight cuts parallel to the boards using grits as fine as 80 create a flat, smooth surface that will accept stain. It’s important to wear hearing protection and to keep the power cord out of the way of the machine.

Feathering. When you start each pass with the floor sander, you feather it gently down onto the floor as you move forward. The belt pulls the machine forward, and as you approach the wall you ease the machine back up, like taking a boat out of the water. Then you ease it down again and pull it all the way back in the same track. Now move it over and repeat the process, overlapping each track just a little bit.

Most sanders cut an 8-inch path. With 1x3 strip flooring, which has about a 2 3/4-inch reveal, I like to line up and take two boards at a time. On each successive course, I overlap about two thirds of a board. It keeps me straight and blends everything together.

At the end of each stroke, the machine will get within 6 or 8 inches of the wall. But you have to start each stroke 4 feet from the wall, because you’re standing behind the machine. To hit that 4-foot section, you have to turn the machine around and go the other way. Instead of starting the same distance from the wall each time, however, I like to stagger my cuts. I’ll start one stroke 4 feet out, the next 6 feet out, the next 5 feet out, and so on. Or I’ll just start in the middle of the floor, and stagger randomly. When I turn around, I’ll feather into each cut where I started the last one. This breaks up the pattern. Otherwise, the transition between the two sections that you sanded going in opposite directions will be clearly visible in the finished floor.

That visual pattern is caused by the way the sanding belt pulls on the wood fibers. It’s called a corduroy effect because you can see the same thing on a pair of corduroy pants. Run your hand over the nap one way, and it will look one color; rub it the other way, and it will look darker or lighter. Just like fibers in cloth, wood fibers will lie one way or the other depending on which way the belt pulls them. You can’t see it in the raw wood, but when the finish goes on, it jumps out and you can’t miss it.

Edging

Where the big machine can’t get to the wall, you have to edge. You don’t just stab the edger in and out of the wall; you move it along the wall in circles, like waxing a car.

You have to stoop over to operate the edger (Figure 5). If you do it hour after hour, it’s pretty hard on your back. I like to keep my legs spread as far as I can and brace my elbows on my knees. That takes some of the strain off the back.

Figure 5.When operating the edger, the author rests his elbows on his knees to reduce the strain on his back. The edger is moved in a circular motion.

The whole 7-inch disk doesn’t contact the floor — just the front inch and a half does. An edger doesn’t sit flat on the floor; it’s tipped up. The wheel height is adjustable to vary the angle of the disk. It’s crucial to learn how to adjust the wheels, because if those wheels are too high, the front of the disk will gouge the floor; if they’re too low, the disk will clog up and won’t even cut.

As I work the edger in and around, in and around, I’m cutting my pencil marks, so I know that the margins are now level with the center of the floor. Then I pencil it again, and do my final straight cut. After sanding and edging one last time with a 60-grit or an 80-grit at most, I’m ready to vacuum again and screen.

Buffing

The buffer’s job is to create a uniformly scratched surface over the whole floor, blending the different patterns created by the other equipment (Figure 6). If the last grit on the drum sander was a 60, I’d buff with an 80-grit screen; if the last grit on the drum sander was an 80-grit screen, I’d screen with a 100-grit. You should keep the buffer as flat as possible so as to buff evenly; if the buffer leaves swirl marks, you’re holding it at too much of an angle.

Figure 6.The buffer sits on a synthetic fiber pad and a sanding screen and sands with a circular motion. The whole screen should contact the floor as evenly as possible to create a uniform scratch pattern with no visible swirls. An 80-grit or, at most, 100-grit screen should be used to leave the wood rough enough to absorb stain and finish.

It’s really important to vacuum before and after each sanding or screening. Otherwise, the screen or paper just rides around on sawdust and doesn’t scratch the floor. By the same token, there’s no point using too fine a screen. The finest sander belt I’d use is 80-grit; the finest buffer screen would be one step higher: 100-grit. Anything finer just burnishes the wood, fills the fine grooves, and prevents the wood from accepting stain or finish.

Once the wood is clean and smooth, it’s time for a stain and a finish. In a future article, I’ll discuss the finer points of coating a wood floor.