Tuesday, March 30, 2010

Preparing for the Spray Booth

In spite of the cold weather, I've not been completely inactive regarding Clark's guitar. There's actually a lot of work that needs to be done to get it ready for the finish to be applied.

But before that there was some other work to be done. See, when something goes out with your name on it, you get pickier about what you can live with. There were two things that happened that I wasn't happy with. The B/W/B purfling under the rosewood binding at the horn of the cutaway wasn't completely straight--it was kind of wavy. This purfling is being made to bend in a way it really doesn't want to; think of trying to bend a board, not along its width but along its edge, and that's what is happening. To help this bend the purfling strip is dipped in water, but on this tight edge the water caused the purfling to ripple some. So I chiseled out about an inch of this area and patched it with another piece. You now have to look hard to see the repair.

So don't look hard.

The big deal, however, was with the binding adjacent to the top cutaway. In sanding this area smooth and flush--well, I just went too far, and there was hardly any binding left in one small section. It didn't look that bad, but the more I though about it, the more I just couldn't live with it.

And I was thinking about it in the darkness of early morning; that's when you really know you can't live with it.

So I got up, ordered some more binding, then went out and removed the binding/purfling on that side. Used an iron to loosen the glue and a chisel to (carefully!) remove the rosewood binding.

When the binding came in it had to be bent, using the old blowtorch and pipe method. Fortunately the new binding and purfling went on fine, and I was much more careful in sanding it flush this time.

Next the whole guitar gets sanded. I started with 100 grit, then moved to 150, finishing with 220 for the back and sides, 320 for the top. For the neck and headstock I'm going to use an oil finish, which needs a finer sanding, so I continue to 400 and then 600 grit paper, which is practically polishing the wood.

There are invariably going to be little gaps between the binding and the top, back and sides, and these need to be filled, otherwise the finish will simply sink into these areas and a smooth, level surface will be impossible. To fill these gaps I use some medium viscosity cyanoacrylate (super glue), which fills the gaps, and a razor blade to level the glue. A quick squirt of accelerator hardens the glue and it can immediately be leveled flush to the surface, first with the razor, and then a little sanding. The gaps practically disappear. I probably treated about 10-12 of these type of spots.

The final step before spraying the finish is to fill the pores and seal the wood. Some woods, like rosewood and the zebrawood we are using, have large pores that need to be filled for the same reason as the gaps needed filling, so that the finish doesn't just sink down into the holes. Then the wood needs to be sealed so that the finish isn't just absorbed into the wood but will sit on top of it. This is usually done with a thin coat of shellac.

I'm using a product called Z-Poxy, which is a slow-drying epoxy originally designed for auto bodies.

The reason I like Z-Poxy is because it both fills the pores and seals the wood, combining two steps into one. I like to use it on the top because, although spruce is a closed-pore wood, it still needs sealing and the Z-Poxy gives the wood a slight amber tone that really pops the grain--especially the bear claw figure.

First I have to mask off the neck and headstock:

Then measure to find the exact bridge location and mask that off as well. The bridge has to be glued on, and you want bare wood for a good bond. The area I mask off is a little undersized since I don't want any bare wood peeking out from under the bridge.

Now I'm ready for the epoxy. Wearing latex gloves because you don't want this stuff on your skin, I mix up a just little, because it goes a long way. I start on the back and use a flexible spatula to spread it across the grain, working it into the pores.

And it comes out looking like this, which is a pretty good preview of what it's going to look like under a finish:
Holding it by the neck I spread it on the sides and end at the top. Now it gets hung up for an overnight dry:
The next day I have to sand the epoxy level. Since this first coat is relatively thick, I hit the high spots with 220 grit sandpaper before moving to 320 and finishing at 400. I'm looking to achieve a uniform dullness; since the epoxy dries shiny, any low spots appear shiny, and that's how I know I still have work to do.

On the top there's one shiny spot that just won't go away; on closer inspection it is clear that the spruce has gotten dinged and this is actually a fairly deep depression. I sand the epoxy level everywhere but here and hope that the next coat will fill this depression.

So, yeah, the entire guitar gets another coat of Z-Poxy, and hung overnight.

When I come back, the pores are filled nicely, and just working with 400 grit paper I quickly achieve a uniform dullness.

Except for that darn ding on the top. So I switch to 220 and sand down to bare wood. Before sanding more I try an old trick. I heat up a soldering iron, place a wet and folded paper towel on the spot and touch the soldering iron to the paper towel. The resulting steam will swell the wood fibers. A lot of scratches and dings will just disappear with this method, but not this one.

So I have no choice but to sand the whole top down to bare wood and then sand the ding away. Since the top only needs to be sealed, one coat will do. So I do that, and sand it level the next day. Finally, I mix some more Z-Poxy and thin it with denatured alcohol, and use a paper towel to apply this to the entire guitar. When the alcohol dries it leaves a final very thin layer of epoxy which is very smooth.
I've got a call in to my buddy Mike, who owns Mike's Auto Body in Thurmont, to reserve some time in his spray booth. In the mean time I'll go ahead and work on the bridge.

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