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designing and building with wood channels my creativity and challenges my mind.
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Friday, August 19, 2011

Oil-Based Beauty with General Water Borne Topcoats

This post shows how to bring out the deep figure in wood furniture using water-borne finishes, which are notorious for leaving wood dead and flat.  

Water borne topcoat finishes - like Minwax Polycrylic and General Finishes Enduro-Var and Pre-Cat Urethane - are very popular among carpenters.  There are many reasons to choose water-borne.  For one, water borne finishes dry in about an hour, as opposed to traditional organic finishes (polyurethane, tung oil), which require 24 hours to cure.  Water-borne finishes don't smell as bad as traditional finishes, the worst-smelling of which (lacquer) is very poisonous and explosive.  Water-bornes aren't flammable, and they clean up with water instead of noxious organic solvents.  Finally water-borne finishes have excellent resistance to heat, water, alcohol and other solvents, making them appropriate for high-wear applications. 

But water-borne finishes are much less popular with fine furniture makers, for the simple reason that they just don't look as good as their oil or lacquer counterparts.  Water-borne finishes don't bring out the depth and figure in wood at all: what you see before you lay down a water-borne is pretty much exactly what you'll see once the finish is dry (some brands have yellow dye to imitate the look of oil-based polyurethane, which changes the tint of finished wood).  This is very different from solvent-based finishes like polyurethane, shellac, oil, and lacquer, all of which make wood "pop" with chatoyance, and show off the holographic shimmer inherent in many wood species.

Less expensive water borne finishes have poor clarity.  When water-borne finishes first hit the market (before my time, in the 1980's), they all had a bluish cast to them...very unnatural!  Nowadays, cheaper brands (like Minwax Polycrylic) have a yellow dye to combat the blue, but the cloudy, plastic look remains.  More expensive (read: high-tech) new water-borne formulations are less cloudy.  But even the very best water-bornes look flat and dull compared to traditional topcoats.  

The best of both worlds:
But there is a way to get most of the benefits of traditional finishes, while avoiding the majority of the hassles of organic solvents.  This method is the only water-borne one I'll use on my projects, and I happen to be using this method right now, on a table commission for a new pub opening soon in Davis Square, Cambridge.  
The secret is shellac.  Shellac is known to bring out the beauty in wood better than any other finish besides lacquer (the undisputed king of finishes, and an explosive carcinogenic killer of brain cells).  Shellac is unique among wood topcoats for being alcohol-based.  It's moderately noxious - like water-borne finishes - and dries very fast.  A thick coat of shellac is ready to sand in about an hour.  For all intents and purposes, shellac is the best of all worlds when it comes to finishing wood.  The only problem with shellac is that it's not very resistant to anything.  Heat causes shellac to blush, as does water, and alcohol will easily dissolve a shellac topcoat. 

But you'll find 100% de-waxed shellac in almost every furniture project of mine.  I use it as a sealer coat (the first coat) to prep the surface for polyurethane or pore-filling, or as a barrier coat between finishes with different solvents (for example, between a water-borne pore filler and an oil-based topcoat).  I also use 100% de-waxed shellac to bring out the figure in wood before applying a water-borne topcoat.

Before I show the method, let me credit the book that taught me almost everything I know about finishing.  It's called "The New Wood Finishing Book" by Michael Dresdner.  I've read other books on finishing, but you shouldn't bother.  This book has it all, and unlike most other writing on the subject, it clearly organizes the arcane and complex world of finishing.  I don't think Mr. Dresdner actually describes this method in his book, but it was easy enough to conceive once I had read up on the properties of shellac.

The Method:
  1. Start by sanding the raw wood to 220 grit.  You can do all the sanding with a power tool except the last grit (220), which should be done with a sanding block (sand paper wrapped around a flat piece of wood), by hand, and sanding along the grain.  Flatness and complete sanding are absolutely key at this stage.
  2. apply a coat of 100% de-waxed shellac to the sanded surface with a rag or brush.  If using a rag, fold it so the surface is smooth and without creases.  Don't over-rub or over-brush, as this will cause the shellac to roll up and get bumpy.  As before, flatness is key.  As you go along with this process, maintaining flatness is of the utmost importance, since every coat of finish has to be sanded back to knock down brush marks and knock off dust particles, but can't be sanded through to the coat underneath. 
  3. After an hour, gently sand with 320 grit paper, to knock down the wood fibers, flatten the surface, and clean up dust particles.  
  4. Apply another coat of 100% de-waxed shellac.  Does it have to be de-waxed?  Yes, 100%.  If it's not completely de-waxed, the water-borne topcoat won't stick to the shellac, and eventually the finish will peel away.  Since I guarantee my work forever, such peeling is, shall we say, undesirable.
  5. After another hour, gently sand the shellac to flatten it.  You'll know it's flat when the whole surface is uniformly milky in appearance, with no shiny spots.  Use 320 grit paper, and don't rub to hard or too fast. Not only must you be careful to not sand through the shellac and into the wood, but heat from friction can melt shellac very easily.  
  6. Now your surface is ready for the water-borne topcoats.  Using a spray gun or paint brush, apply a thin coat of your choice of water borne finish.  If brushing, take care to not over-brush.  Water-bornes dry so quickly that the finish can ball-up under the brush in less than a minute.  It's best to brush only as much as you need to thoroughly wet the surface, and don't try to flatten the coat with the brush.  Let the finish flow out on its own...it will end up flatter that way.  Work small areas at a time; it's not necessary to brush with the grain, just be careful not to work over partially-dried finish too vigorously or it will ball-up.
  7. After an hour, sand the surface with 400 grit paper.  Sand until the surface is uniformly milky, but don't sand through the topcoat!  The water-borne layer will only be a few thousandth's of an inch thick.  Remember this at step 1, when you're trying to decide whether your surface is flat enough to start applying finish.
  8. Apply another coat of water-borne finish, wait an hour, then repeat the sanding with 400 grit.  As you can see, dust is only a problem for those who don't sand. 
  9. Repeat the application/sanding sequence until you've built up a suitable thickness of finish on the piece: enough to protect the wood and shellac underneath from scratches, dings, and life's other offenses.  If, during sanding, you happen to sand through the top layer of finish, you'll see a white line where the top layer gives way to the layer underneath.  This "witness line" can form with any finish that doesn't melt the previous coat as it's going on.  Once a witness line has formed, you can't erase it, not even with more coats of finish.  Remember this at step one and ask yourself: "is my surface flat enough?"
  10.  Now that you've applied your last coat of water-borne finish, it's time to "rub it out".  Carpenter's tend not to rub out finishes, and frankly too many furniture makers don't rub out.  Most people who work with wood don't even know that most finishes are meant to be rubbed out.  They therefore worry a great deal about dust and brush marks.  It's true that if your last finishing step is applying the last coat of finish, dust and brush marks are a big deal.  If you rub out the top coat, however, you won't worry about these annoyances, and your finish will look as the product's manufacturer intended.  Rubbing out starts with flattening, then moves to polishing.  You can use any method, really, including methods and products used for rubbing out automotive finishes.  The idea is to make finer and finer scratch patterns in the surface until the scratches are small enough for your taste.  Here's one method I use:
    1. sand with 400 grit until the surface is uniformly milky.  
    2. sand with progressively higher grits: 600, 800, 1200, all in the direction of the grain
    3. rub the surface with super-fine steel wool, lubricated with mineral oil or soap and water.  
    4. Or, using a buffing pad on a rotary power sander or polisher, use an automotive polishing compound, jeweler's rouge, or a furniture polishing compound like Behlen Deluxing Compound, to give a final polish to the surface.  
A note on "satin" and "gloss": 
you'll see each can of finish labelled as "Satin", "Semi-Gloss", or "Gloss".  These labels refer to the way the product reflects light.  Satin finishes contain additives that scatter light, similar to the way fine scratches scatter light.  A well-done satin finish is reflective, but it reflects an out-of-focus image.  If you rub out a gloss finish, but stop at the steel wool step in the method outlined above, you'll have a satin finish.  

If everyone rubbed out their finishes, the only finish needed would be "gloss", and the fine-ness of the polish would determine the level of satin or gloss at the end.  But since most people don't rub out their finishes, manufacturers have devised the additives for "satin" and "semi-gloss", to give a rubbed-out look to un-rubbed surfaces.  

Well, there you have it!  This finish gives you most of the look of a traditional finish, bringing out the figure in woods like cherry, curly maple, mahogany, and walnut, with the speed, ease, and durability of water-borne.  Here's a pic of a table that's getting this treatment at this very moment:

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