Monday, February 2, 2009

Finishing, lessons from my first guitar

(Click on any picture for a larger version)

To help expand my knowledge of instrument building, I decided to build a guitar (as I've mentioned in a previous post here).   The guitar is almost finished and I'll now go into some details on my finishing schedule.  The type of finish is up to the maker and I wanted the "so glossy it looks like it's wet" look.  Many builders feel that whoever decided that guitars should have that high gloss finish should be dug up from his grave and shot....again.  This is because that glossy look is really time consuming to achieve.  Taylor guitars have done a great job of re-popularizing the matt finish in high end guitars.  They did that because it's way less labor intense to do a matt finish vs. a gloss finish.
Finishing starts with a good preparation of the surface.  I generally sand the bare wood to 320grit.  Next because the back and sides are made of a porous wood (East Indian Rosewood) we need to do a pore fill otherwise there will be tons of little pits in the finish under each pore.  I used Z-poxy finishing resin. The  Z-poxy is a bit messy but does a really good job of filling all the little pores.  It takes almost nothing, much less than one ounce to put a coat on the guitar.  It's applied by pouring a few drops on the guitar and spreading it in with a credit card. Here's a good tutorial on how to do it.  

I used two coats of Z-poxy, sanding back to bare wood after the fist coat and trying to leave a thin coat after the second session.  The Z-poxy really "pops" the grain of the EIR.  The one rub is that when sanding the second coat on the neck, there were a couple spots where I sanded back to bare wood. They are visible as faint light spots on the heel of the neck.  Normally one would re-coat the neck but it wasn't too bad and I wanted to finish the guitar so I could get back to bandura building!  After a light coat of shellac, it was time to spray on the final finish.

The finish I'm using now is the Stew Mac branded Target Oxford USL. They're both the same thing I just ordered from Stew Mac because I had a couple other things to order from them as well.  Nitrocellulose lacquer is the "gold standard" of guitar finishes but it's also quite toxic and quite explosive (nitrocellulose is also gun cotton) and I'm just not interested in spraying it in my basement.  It can be done but you need some pretty elaborate venting systems which I don't have.  Polyester finishes are becoming quite popular with builders now too because they are very durable and because they're catalyzed, you can spray one day and buff out the next.  These finishes also have a very steep learning curve and if you screw up, you might end up having the finish harden in the spray gun which doesn't interest me either.  The waterborne finish I'm using is relatively benign, it doesn't stink up the house, and is safe to spray at home so long as you use a respirator.

For my spray gun, I just recently picked up this gun to replace my old school high pressure gun.  The HVLP systems have much lower over spray which means less dust floating around the shop.

Finally, even thought Stew-mac recommends building coats using their sanding sealer, I chose to use all USL this time around so I wouldn't have to buy two products.  Now to the finishing schedule itself.

The settings I used on the gun were the following:
1.2mm tip, 27psi at the handle
Air Screw - 1.25 turns out
Fan Screw - 1.5 turns out
Fluid Screw - 2 turns out (tried 1.75 but didn't like it.

Applied 9 coats over two days and did a quick level sand
Applied 4 more coats and measured the film thickness by peeling off some masking tape. Thickeness was .005" (13 coats so far).
Applied 5 more coats and measured, .007" on tape and .009" in the soundhole cover.

The guitar now had to hang for a month to allow the finish to fully shrink and harden.   After all this work, the finish looks like the photo to the left.  A far cry from the glossy look we're after.  To get from there to glossy is where the work really starts.  Level sanding and buffing.

Level sanding is where you use sandpaper and sanding blocks to knock down the orange peel to make a perfectly smooth surface. I started with 400g which is a bit risky: it levels fast but there is a risk of sanding too much and the 400g scratches are really large.  I had a hell of a time getting the surface scratch free and think that next time I'll start with 600g.  It's also essential to use good sandpaper.  I prefer the 3M free cut gold because they can be used dry and don't load up.  I do brush off any sanded finish and the paper after about 10 to 15 strokes.  For a sanding block I use either a small rubber eraser or a black board eraser for curved surfaces.  I went from 400g to 600g to 800g dry, then switched to an Abralon 2000g pad on my random orbit sander (my thumbs were hurting pretty bad at this point from holding the little sanding blocks).  I then took it to the buffer (you can see it in the background on the picture on top).  The results were disappointing.

Apparently I didn't spend enough time with the higher grits after the 400 because there were still a lot of scratches that could be seen at certain angles.  I had to go back to 600g, 800g then the 2000g.  I had to go back and re-do areas of the guitar several times because of this problem. I also used some 1000g wet paper in some areas to get rid of 800g scratches. I think this could be avoided with the following schedule:

Start with 600g, it might take longer to level but there will almost certainly be no deep scratches.
800g dry
1000g abralon pad wet (I didn't have one this time but I found some on line)
2000g abralon pad
Buffer.

By the way I also have a 4000g abralon pad but found that you can go from 2000g to the buffer and not have any scratches.  (on the other hand, a buddy, Mark Swanson, says that on nitro he can go from either 400g or 600g, don't remember, straight to the buffer with no scratches!!! what the hell am I doing wrong?).

Once all the sanding was done, it was off to the buffer for about a two hours on the medium wheel followed by about a half our on the fine wheel.  I suspect had I done a better job level sanding I could have spent less time on the buffer.  

At the end of the day (about 7 hours total level sanding and buffing), I ended up with the finish in the shot to the left.  It came out pretty nice I think.  I suspect that as I get better at this I'll be able to get it done in less than half that time.

After finishing the neck the same way I attached the neck to the body.  All I have left is to glue on the Bridge, make the nut and saddle and final setup.   

Phew! What a long post.  Hopefully someone will find it useful.