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designing and building with wood channels my creativity and challenges my mind.
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Monday, July 25, 2011

longer lasting sand paper & abrasive

my favorite power hand sander
Sanding is a big part of almost any furniture or home improvement project. In fact, it's one of the few steps everybody has to take, whether you're painting your bedroom, or crafting an 18th-century reproduction sideboard. If you're a weekend warrior doing your own home improvements, then you know how tedious and time consuming sanding can be, and how a new package of sand paper can disappear faster than you can say "my arm is cramping up." You've also probably noticed that in spite of all your work sanding, once the finish (or paint) dries, you can see dozens - or hundreds - of tiny scratches that were invisible before.

If all this sounds familiar, be sure to read this post before you sand again!

Furniture makers have more than a few tricks to minimize sanding, while ensuring that the finished piece is totally free of those "ghost scratches". Here's how to sand like a furniture maker:

Before you start
You'll need some tools to ensure you spend as little time as possible on getting a perfectly smooth finish:

sand paper eraser
First:  get yourself a sand paper eraser.  These things are absolutely necessary.  Sand paper has a tendency to load up with sawdust, and for most people once the paper is loaded it can't cut any more, and has to be tossed in the trash.     Sand paper erasers pull the sawdust out of the paper, returning it to almost-new condition.  Remember, while sand paper DOES get dull, it usually stops cutting because it's loaded with dust.

Second: get good sand paper made for your purposes.  Some people use aluminum oxide paper for the low grits (80-180) and garnet paper for 220 and 320 grits.  Aluminum oxide stays sharp as it gets used up (the little crystals break off in shards), whereas garnet crystals simply get dulled and rounded over.  Garnet, therefore, can provide a smoother final finish than aluminum oxide, for a given grit.  For grits 400 and above, I use a brand called Micromesh, which I use because the sheets last forever, and are easy to clean with an eraser.  When sanding with a block, I use Klingspor abrasive rolls for the Sand Flee.  This brand of paper is backed with Stick-Fix material for sticking on the Velcro bases of my sanding blocks, and it comes in 30-foot rolls of  2.5"-wide paper.
klingspor rolls

Third: get a good power sander and make a good sanding block.  Power sanders are great for working fast with coarse grits.  Orbital sanders are liked for their "random" scratch pattern, but in fact orbitals often leave "swirls" - pesky curlicue-shaped scratches that are hard to see and even harder to sand away.  The sanding block is essential for the finer grits, where you want the tiny scratches to blend in with the grain of the wood.

And now for the sanding: 

micromesh sheets
Step 1: plane the wood smooth. Furniture makers try and make their workpieces as smooth and flat as possible, before they sand. If you can put your work through a thickness planer, or flatten it with very sharp hand planes, the resulting surface will be very smooth and ready for 180 or 200-grit abrasive. Wood from the lumber yard or from Home Depot hasn't been planed like this, no matter how smooth it looks.

Tip: the first sanding step (sanding with the coarsest grit) is for removing scratches and blemishes from the raw workpiece. Every other (finer) grit is for removing scratches from the previous grit.

Step 2: Start with a low-enough grit. Many people, knowing that you have to "work up through the grits" in order to achieve a smooth surface with no scratches, want to skip the coarse grits, hoping that a finer grit will remove the deep scratches on un-sanded wood. This doesn't usually work, and you spend much more time sanding with the finer grit to eventually reach the deepest scratches. Instead, if you didn't plane the work flat, start with 80 grit, and sand until the work is uniformly scratched with 80 grit scratches.

Tip: Use an orbital sander or a rotary sander such as Festool's Rotex for coarse sanding up to 180 grit. Hand-held belt sanders can hog off enormous amounts of wood in seconds, so be careful and pay close attention if you must use a belt sander.

Step 3: Proceed up through the grits without skipping grits, and without stepping up too soon. Many people want to move on to the next finer grit, even though they can still see scratches from the previous grit. they think that they'll "get that one deep scratch on the next grit", as if every finer grit cuts deeper and deeper into the surface. It DOES cut deeper, but not deep enough to reach a deep scratch from a long-ago grit. Don't move to the next finer grit until all the scratches from the previous grit are gone.

Step 4: after sanding with 180 grit and a power sander, switch to hand-sanding along the grain, with a sanding block (see previous post on sanding blocks), and sand at 180 again, until all you see are straight scratches.  Next, do the same hand-sanding with 220-grit paper.

Step 5: you are ready to seal the work!  Sealing is often necessary to prepare the wood surface for the main finish, or topcoat.  Some topcoats are also good sealers, but many are not.  Consider carefully what your "topcoat" will be, and then choose a compatible sealer.  When I use General Finishes' Arm-R-Seal Oil & Urethane Topcoat, I don't use a separate sealer because the oil in the Arm-R-Seal acts as a sealer.  If I'm going to be using a water-borne topcoat, I often seal with de-waxed shellac because water-borne sealers don't bring out the wood as well as shellac or oil-based finishes.  De-waxed shellac is a good general sealer as it's compatible with both oil-based and water-borne topcoats.  I don't choose shellac if I plan to buff-on a wax finish, because the buffing action will rub off the shellac.  

Step 6: Sanding AGAIN?!?!  What's that?  You thought you were done sanding?  No such luck!  You need to sand the work again after the sealer has dried completely.  The "reason" for sanding after sealing depends on which sealer you used.  Water and alcohol-based sealers tend to make wood fibers stand on end, and once the sealer has cured the workpiece can feel rough like sand paper.  If the fibers are raised, knock them down with 320 grit sand paper, but be sure not to cut through the sealer coat.  Some oil-based sealers dry smooth, and need to be scratched to provide a purchase for the next coat of finish.  Again, scuff-sand lightly with 320-grit, and don't cut through the sealer.

Step 7: topcoat!  Apply the topcoat according to the directions on the can.  Follow their directions for whether to sand between coats, and how long to let each coat cure.

Step 8: Once you've applied enough topcoat (in several coats) to provide a base,  and it has cured for double the amount of time indicated on the can, you can start rubbing out the finish.  This is where you make the piece totally smooth.  Start with 400 grit paper and sand the topcoat until it is uniformly hazy, with no shiny spots.  Shiny spots are low spots in the finish that you haven't reached with your 400 grit paper.  Keep going until the shiny spots are gone.  Then, move up to 600 grit, then 800, using a little oil or soapy water to lube the paper.  Then rub with steel wool, then pumice and oil, then rottenstone and oil.

Wipe it all off, and boom!  you're done!

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