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PostPosted: Sun Aug 07, 2011 8:17 am 
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Koa
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Location: Grover NC
First name: Woodrow
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Guitar finishing, and wood working in general is dangerous. Educate yourself on the proper safety equipment, and necessary precautions.



First some basics, and equipment I use. Reguardless of type of finish, if you're spraying you need a steady supply of clean, dry air. I don't brush finishes, but Varnish is a good finish to brush. It flows out well. For brushing you can use a quality natural bristle brush for the best results, but good results can be had with a disposiable foam brush. When brusing use even strokes, with a resonably wet, but not soaking brush. You don't want to pile varnish on, just apply a thin coat.
A huge compressor isn't necessary to spray something small like a guitar. A guitar is small, so you don't use alot of air, over a long period of time. I've got a big compressor, but rarely use it. The compressor I use for guitar finishing is a 2HP with a 20 gallon tank. I've got 3 filters. An inexpensive water trap at the compressor, another one in my booth, and a disposable filter at my gun.
Attachment:
Filter.jpg


I also use a small regulator at the gun. I keep the pressure in my air line up pretty high, and regulate it at the gun. Prefered PSI varies depending on the gun, and personal preference. I'm usually ~20 PSI at the gun. Experiment to get the pattern and flow you need.
I use a Walcom EGO spray gun, with a 1.2 MM tip. A lower cost gun that also works good is a "Kobalt Small Gravity Feed Gun" from Lowes. It doesn't atomize the finish as good as the Walcom, but it costs alot less. For occasional finishers I believe it's a good alternative. (I use one for spraying polyester, in case the polyester sets up in the gun.)
This picture is my Walcom gun , but the basic controls are the same on most guns.
Attachment:
gun controls.jpg


The bottom control, next to the air inlet is an air control. I do major adjustments with the regulator and make fine adjustments with this control. The next control, not at top, right below the top is the fluid control. This limits how far fluid needle is allowed to open. Some finishers like to set this all the way open, and just pull the trigger however far is needed. It's easier to set this for the maximum flow you want though. More on fluid control later. The top control set the spray pattern, or fan size. I usually have this all the way open on the top and back, but close it down a little for the sides. Use whatever size pattern you're comfortable with.

When spraying you want to lay down nice even coats. Fluid control, finish viscosity, and the distance and speed you move the gun will control this. If the fluid control is set to high, you move the gun too slow, or the finish is thinned too much you'll get runs and sags. If the fluid control is not high enough (or you don't pull the trigger far enough), you move the gun too fast, or the finish isn't thinned enough the finish won't flow out good enough. You'll either get orange peel, or pebblely dry spray. Each coat should be wet and smooth. Practice on scrap to get the right setting for fluid, air pressure, and technique. Start spraying off the edge of the guitar, and continue spraying until you're off the other edge. Keep the gun's distance from the guitar constant, usually 6" to 8". It's also important to keep the gun at a 90 degree angle to the surface. This takes a little practice on the sides, but isn't hard. Here's a short video of some basic spraying.
http://www.brackettinstruments.com/Spraying.wmv

More in next post


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These users thanked the author woody b for the post: Hibdon Hardwood (Tue May 13, 2014 1:38 pm)
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PostPosted: Sun Aug 07, 2011 8:45 am 
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Koa
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Location: Grover NC
First name: Woodrow
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Country: USA
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Now, on to actually applying the Varnish. I use Sherwin Williams Fast Dry Oil Varnish.
Attachment:
Varnish.jpg


There's alot of similar choices, but they may require a different schedule. A dust free enviroment is needed. Varnish dries slow, so any dust specs in the air will end up in the finish. Mask any areas that need masking. After years of all kinds of soundhole plugs I finally started using a sponge, with a paper plate to block the soundhole. (thanks Todd, and Laurent) I blow the guitar off, in an area away from my finishing area. I wipe it down with Naphtha, then ammonia to insure a clean surface. After the naphtha and ammonia are dry wipe it down with a tack cloth to remove any remaining dust. I thin 8 ounces of Varnish with 6 ounces of naphtha in a seperate sealable container. Stir it well. You'll be able to tell when it's mixed good. I also check the thickness of the top through a specific place at the soundhole. I'll check during the finishing process to see how much finish I'm getting on the guitar.
Attachment:
Mixing.jpg


You may want to thin it slightly more or less to suit your technique. For brushing you may want to thin with Mineral sprits instead of naphtha. Mineral sprits evaporates a little slower, and will allow it to flow out better. Spray the first coat. A nice wet coat is good, but don't get carried away. Spraying it on really thick is just asking for problems.
Attachment:
1st coat.jpg


Return the varnish from the gun to the container you mixed it in. Clean your gun, and add bloxygen to the container to keep if from curing. Clean the gun with Naphtha or mineral sprits. I don't disassemble the gun every coat. I just run naphtha through it until it runs clear. Don't clean your gun with acetone or lacquer thinner. It will make a gummed up mess instead of cleaning the gun.

Let the guitar sit overnight. More in next post.


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PostPosted: Sun Aug 07, 2011 9:22 am 
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Koa
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Location: Grover NC
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Country: USA
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After sitting overnight I scuff it, and apply more finish. On some woods, and certain weather situations the first coats may need to cure a little longer. I normally use 400 grit 3M fre cut gold for this with a sanding block. Some people use scotch brite type pads for this. You're not trying to level sand at all, just lightly scuff, for adhesion. All your doing it killing the gloss. At this point it would be really easy to sand though. Like any time sanding, don't use any pressure. Let the abrasive do the work. Blow off of vacuum the dust, and wipe down with a tack cloth. The varnish isn't fully cured at this point, so you don't want to wipe it with Naphtha like you did before the first coat.
Attachment:
after scuffing.jpg


I used to add a small amount of acetone (5%)to the mix after the first coat, but no longer do. It didn't cause any problems, but I think it flows out a little better without it.
The second coat, and every coat is pretty much the same as the first. Let it sit overnight
Attachment:
2nd coat.jpg


The finish should be starting to build some now. You can block sand a little more with 400 grit, but don't get carried away. Use no pressure, and be consistant with your sanding. Don't sand alot in one area. At this point you won't get it perfectly level, but you'll be starting to get it level. Blow off or vacuum dust, and wipe with a tack cloth.

Apply 2 coats, with 3 to 4 hours between coats. You can do this with this varnish, without scuffing as long as your within the 3 to 4 hour window. If you have dust or other problems after the first coat or have exceeded the 4 hour window lightly scuff before the next coat. Good lighting is really important, especially during the second coat of the day. It's hard to see the wet varnish on top of the glossy varnish without good lighting. Let it sit overnight or longer.

At this point you should have enough Varnish to do a pretty good level sand. I use a RO sander with 600 grit for the top and back, but still use 400 grit by hand on the sides. If I try to use a power sander on the sides I get in trouble. YMMV
Be consistant with your sanding. Blow off or vacuum the dust, and wipe down with a tack cloth. If you can't get it level in areas spray 2 more coats and try again. The sanded finish should be smooth and level.
Attachment:
Level sanded.jpg


Now you should be ready to apply the last couple coats. Apply 2 coats with the 3 to 4 hours window between coats. Be sure to apply them even, and keep your spray area clean to prevent dust problems. On the last coat I sometimes add a little varnish to the mix to thicken it up some. It should be shiny and smooth after the final coat. I'll let it sit a few days before buffing.
Attachment:
Final coat.jpg


Part 3 will be buffing


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PostPosted: Wed Aug 24, 2011 6:49 am 
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Mahogany
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Great stuff Woody. Looking forward to Part 3. Thanks for sharing your process.

Erik


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PostPosted: Wed Aug 24, 2011 9:34 am 
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woody b wrote:
......Now you should be ready to apply the last couple coats. Apply 2 coats with the 3 to 4 hours window between coats. Be sure to apply them even, and keep your spray area clean to prevent dust problems. On the last coat I sometimes add a little varnish to the mix to thicken it up some. It should be shiny and smooth after the final coat. I'll let it sit a few days before buffing.


Woody...just a few days??? Not a few weeks?

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PostPosted: Sat Aug 27, 2011 11:13 am 
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Brazilian Rosewood
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Yo woody I have never sprayed varnish before(brushed) I bought some behlens rockhard varnish and was wondering if it compares favorably to the S W fast dry .There is a local SWdealer nearbye, wheras woodcraft where I got the behlens is 27 mi each way thanks


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PostPosted: Sun Aug 28, 2011 11:01 am 
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Koa
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Location: Grover NC
First name: Woodrow
Last Name: Brackett
City: Grover
State: NC
Zip/Postal Code: 28073
Country: USA
Focus: Build
Rockhard is darker in color than the SW. I haven't used Rockhard so I'm not sure of other differences, or similarities.

It takes the SW several weeks to cure to full hardness, but all the shrinkage is done really quickly. It's easier to sand after 3 to 5 days than it is after a month, as long as you don't get too agressive and ball up the finish. I usually sand with 600-800-1200. For the best shine I'll let it sit another 5 days before hitting it with the buffer, but I have buffed it sooner.

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