Guitar Finishing (9 steps)
Step 1: THE SPACE
Spraying finish requires a dedicated space that is well-lighted and ventilated by something other than the same system that heats and cools your home. The shed pictured below is 12'x16' and was built for spraying lacquer. It cost less than a spray booth. It also has work benches and tools for miscellaneous luthier tasks and teaching when not spraying lacquer.
Woody tries to keep the shed organized and somewhat clean and free of dust. Because there is no temperature or humidity control, the shed is used for spraying lacquer only when the temperature is above 50 F and the relative humidity (RH) is below 50%. That translates to a few hours in the afternoon on about half of our Virginia summer days.
The shed is ideal when the temperature is right. Guitar finishing is tedious work at times and a dedicated work space has helped significantly.
Hope to see you next month...
Step 2:THE EQUIPMENT
Spraying lacquer requires:
A compressor: Pictured here is a good, but relatively inexpensive, 4 hp; 15-amp compressor that can deliver 8.8 cfm @ 40 PSI; with a 12-gallon tank. This compressor is about as small as one can get by with for spraying lacquer and/or sealer.
The 15-amp rating is the in-rush current requirement (when it starts up). A dedicated circuit is a good idea.
The Gun: Pictured here is an Eastwood Concours PRO HVLP Paint Gun 1.3mm needle/nozzle
This gun it totally adequate to spray instrument lacquer and sealer. It comes with a smallish nozzle (1.3 mm) which is all that you need for lacquer. Heavier paints require larger nozzles.
Step 3: MATERIALS (nitro-cellulose lacquer)
A quality vinyl sealer; musical instrument lacquer; lacquer thinner; and retarder are essential. Different brands for different folks. Mohawk/Behlen have always worked for Woody.
In Richmond, VA the relative humidity (RH) is one of the biggest challenges for the home-shop luthier. On humid days (55%+ RH), Woody adds about 2 oz. of retarder to a 24 oz. cup of lacquer. Retarder slows down the cure so that moisture entrained in the spray can escape from the lacquer as it cures on the guitar surface.
Moderate coats are best when trying to keep the lacquer from blushing (i.e., fogging the finish due to water vapor captured in the spray). The worst mistake one can make is to apply too much lacquer. When a heavy, double coat of lacquer is applied, even retarder can’t hold the lacquer open (volatile) long enough for the moisture to escape. The final cure will appear to have a misty cloud in spots.
From my observations, when the RH is below 50% and falling, that’s the best time to spray. For the home-shop luthier, that might be only a few hours of the afternoon between 1:00 PM and 4:00 PM in summer.
Apply lacquer to the top and ribs of the guitar in similar fashion as described above, holding the guitar by its neck. The sides of the guitar are sprayed crossways with a vertical spray pattern_ back and forth across, not up and down. Keeping the gun the same distance from the work surface is a challenge when spraying the guitar sides. The surface of the sides of the upper bout, the waist and the lower bout are all in different planes. Again, holding the guitar by the neck, Woody sprays the ribs while changing the position of the guitar with one hand and squaring up the spray gun with the guitar surface with the other.
If one is a novice, this might sound tricky. Actually, while one hand keeps a firm grip on the guitar neck, not allowing the guitar to touch you (or anything else), the other hand is moving the spray gun and overlapping the spray pattern as necessary.
When one becomes good at this, it becomes more Art than Craft. Learning to lay down an even coat takes lots of practice. Woody tolerates spraying and polishing lacquer finishes as a necessary evil. But that's no reason not to do your best.
ALTERNATIVE: …or… One can hang the guitar with a Woody Strings Hanger Gizmo* and spray the whole guitar while it hangs vertically in front of you.
Woody's summer of 2019 lacquer finishing is presented here in photos and explanations.
Woody's winter of 2019 was spent building and assembling 5 guitars. Four are dreadnoughts and one is a mini with a cutaway. This summer all five guitars have received a lacquer finish in the Woody Strings finishing shop which is detached from the wJs main shop in Woody's home.
The objective here will be to explain guitar finishing for the home-shop luthier. There is a lot to explain.
Step 8: Buffing
Let me introduce you to the Stew-Mac ¾ hp buffing machine. To one wheel apply a medium buffing compound and to the other apply a fine buffing compound. This machine is not cheap. Woody bought this a couple of years ago for about $600 (I think) and that was without the ¾ hp motor which I already had.
This is a great machine and has saved old Woody many hours of rubbing guitars by hand to make them shine. Pictures are truly worth a thousand words.
Didn't get into yoga but made some great sanding pads.
Step 4: SAND AND FLATTEN (aka cutting back)
After 2 or 3 coats of sealer have been sprayed and allowed to cure for a couple of days, the guitar is ready to be sanded flat to ensure that the pores are filled. Woody uses the sealer as a filler because 1) it is clear and 2) wood bindings pick up stain from wood fillers. Woody, as one might expect, likes the look and feel of wood bindings (usually maple, as seen below). No wood stains are used on Woody guitars (unless it is a rare sunburst).
After the sealer has been sanded flat there are usually some spots where the 220-paper sanded though to bare wood. That’s OK. One final coat of sealer sprayed on the entire guitar will set you up for the application of the lacquer… usually 5 or 6 wet coats applied over 2 or 3 days. Some apply more coats, some less... luthier's choice.
Step 9: PLAY AND LISTEN TO MUSIC EVERY DAY
(otherwise, what's the point?)
Step 6: CLEANING THE GUN
One must keep the spray gun clean in order for it to spray efficiently and in a predictable manner. It’s OK to leave the gun hanging with a cup of lacquer for a few hours. But it’s not a good idea to leave the gun loaded with lacquer overnight.
After the final spray of the day, clean the gun with acetone (needle, nozzle, spray cap, cup and orifices) . It takes 5 minutes. Blow out the orifices with an air nozzle while there is still pressurized air in the tank.
Now, reassemble the gun and put a couple of ounces of acetone in the cup. Spray the acetone through the gun until the cup is empty.
Finally, open the drain cock on the bottom of the compressor tank. Allow the air and water to blow out. Otherwise the condensate will begin to rust the inside of the tank (not good). Leave the stop cock open until next time.
Step 5: SPRAYING LACQUER
You are wearing the 3_M spray mask; the compressor tank is pressurized (regulator set to ~30 PSI); the spray gun cup is full of lacquer; the spray pattern has been tested on scrap cardboard. You are ready to spray. The objective is to lay down an even layer of lacquer over all parts of the guitar that are not masked.
The guitar body is sprayed first. Holding the gun by the neck with the left hand, (as shown below) will put the body directly in front of you. It is important to maintain the same distance between the spray gun and the guitar while spraying.
Trigger the gun when it is not aimed at the guitar. As one moves the spray pattern across the back of the guitar, be sure to keep the travel of the gun at a robot-like steady pace. The objective is to lay down an even coat of lacquer from side to side.
Overlap the spray pattern as you reverse course across the back of the guitar. When the entire back has been coated with horizontal strokes, some like to cross again with vertical strokes. If one feels it necessary to do this, be careful not to apply too much lacquer. Applying too much lacquer is the worst thing that you can do.
Note: When you change from a horizontal to vertical spray path, be sure to change the spray pattern from vertical to horizontal by rotating the spray cap 90 degrees. The spray pattern should be across the spray path to avoid runs and drips.
*Woody Stings Hanger Gizmo
This little home-shop gizmo works great for hanging and spraying a guitar. When the entire guitar has been sprayed, the wire loop and dowels also give you a place to lift the guitar and carefully carry it to a place that is out of the way.
Step 7: CURE FOR A WEEK
After the guitar has 5-6 coats of lacquer applied, allow it to cure for a few days…or a week, before sanding with 320, 400 and 600. At this point, if one wants a matt (satin) finish you could be done.
If a glossy finish is the objective, there is still another step… buffing.
An effort was made in December, 2018 to post a comprehensive overview of learning to do inlay work with a CNC. There is much to learn but the results are turning out to be well worth the effort.
See Luthier's Journal, December 2018... "Intro to CNC Inlay".