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designing and building with wood channels my creativity and challenges my mind.
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Monday, December 27, 2010

Finshing Techniques: Oil ON TOP OF urethane?

A project that began last week is almost finished.  The four-drawer chest, made of bamboo plywood, was commissioned by a friend.  As of this writing - the night before the chest is due to be picked up from my Allston studio - the chest is curing after being brushed one last time with super-fine, 00000 steel wool.

After constructing the chest using 3/4", 3-ply "horizontal grain" bamboo plywood & MDF (per my client's spec), I fit 3/4" bamboo plywood faces to each MDF drawer.  After a bit of adjusting to ensure all the faces were parallel, straight, and flush, I decided to finish the whole piece in oil...with a twist.

Why I chose an oil finish:
Oil is one of the easiest of all finishes to apply: you rub it in, rub it out, and almost never worry about dust.  In addition, while it's not very good at protecting wood from scratches, it is wonderful at bringing out character in wood (bamboo included), and unlike many other finishes it improves with age and human contact.

The Twist...
Having heard of a famous woodworker whose favorite finish was oil urethane with a Danish oil topcoat, and being familiar with oil urethane, I decided to use this combination on the chest.  First, I took off the drawers, and sanded the whole piece (drawer faces included) to 320 grit with my Festool Rotex orbit sander (grits 80-180) and a sanding block (grits 220 and 320).

Then, I rubbed in a liberal amount of Danish oil, followed barely an hour later with a thin coat of urethane, which I rubbed almost completely off after 30 minutes.  A few hours later, I applied another coat of oil urethane, again rubbing most of it off after half an hour.

The next day, I applied another coat of Danish oil, then another six hours later.  Two days later (with Xmas in between), I rubbed out the oil finish with medium-fine (FF) pumice, using the Rotex with a hard felt pad attached, and mineral oil for lubrication.  Once each surface had been buffed with the pumice, I wiped off the residue with walnut oil and a squeegee, and finished with a rag to pick up any last bits of stone.

Then I applied a coat of Teak oil.  I used Teak oil at this stage because, unlike Danish oil, it doesn't sweat out of the wood over the course of days or weeks.  

Six hours later, after digging our car out from a 15-inch snow storm, I returned to buff the whole piece again with super-fine steel wool.  By that time, the Teak oil had cured enough for steel wool, and the end result was a super-smooth surface across the whole piece, and a warm satin, amber glow. 

Why put oil over urethane?
I'm not sure if the technique I used here is "proper", or why the furniture-maker from whom I learned it loves it so much.  All I know is that, contrary to what I believed before I tried it, oil does soak through urethane, provided - I suspect - the urethane doesn't completely seal the pores of the wood.  It's possible (comments are welcome here...) that oil soaks through urethane in any case; I'm not sure.  All I do know it that my finish came out quite nicely, and future coats of oil will be absorbed by the wood, instead of sitting on top of the existing urethane. 

Part of the reason I chose to do things this way is that an oil urethane finish is a pain to do in my shop.  Unlike straight oil, urethane IS sensitive to dust in the air, and also to the manner and skill of the application.  Brush marks are a common unwanted side-effect of urethane finishes.  By putting Danish oil both above and below the urethane coat, I hoped to gain some of urethane's protective properties, while still having an easy, fool-proof finish.

So far, it looks like this unusual technique (oil on top of urethane) works as it's supposed to.  :)

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