For a couple of years now, some builders of premium guitars have been extolling the virtues of Grafted Coatings’ KTM-9, but everyone has overlooked KTM-SV, a waterborne spar varnish. KTM-9 is certainly an excellent finish for some applications, but for mandolin work, I had concerns about the possible need for an epoxy base under the finish and some complaints about adhesion over shellac. Also, the word “varnish” is extremely valuable in the mandolin world and can command a premium price compared to lacquer finishes. As I got to know KTM-SV, I found a modern finish appropriate for all stringed instruments.
I had spent most of 2007 researching waterborne finishes after experiencing some contamination problems with the waterborne varnish I had been using, and I was looking for a harder, slicker finish that did not have the bluish cast (seen in the picture) common to waterborne finishes containing acrylic.
I found that most waterborne finishes that could be used on musical instruments unfortunately contain acrylic. And many do not have the self-leveling property that makes spray finishing enjoyable. Some finishes foamed excessively when brushed sometimes leaving bubbles in the film. Out of all that research, the only new candidate was the overlooked KTM-SV. KTM-SV was developed for garage doors, but on the Grafted Coatings website, it is also suggested for use on musical instruments. A call to Grafted Coatings revealed that no one knew why it was listed for musical instruments, and they had no experience using it on musical instruments, but it worked great for garage doors. I didn’t ask how the garage doors sounded but did get some helpful application information. I also studied the published and posted information on the application of KTM-9.
Working with KTM-SV
When I worked out the application procedure, I used KTM-SV on two mandolins in mid-2007 and sent them out into the world for real-life use. They did fine. Several more mandolins were finished with it, followed by using it on two very top-end mandolins and on several since then. I found that KTM-SV applies easily, forms a hard but flexible finish, is wonderfully clear, buffs quite well, shrinks almost not at all after curing, and should not be overlooked by instrument builders. The main drawback is that, being a harder finish than I had been using, it takes longer to sand. It also takes getting used to when spraying as it is easy to spray a coat that is too dry, making a pebbly surface that does not sand well. And it does not repair as easily as some other waterborne varnishes. But when you learn all the tricks of it, KTM-SV is an excellent modern finish for musical instruments.
Applying the finish
Applying KTM-SV involves the usual stages of preparation. Since both the shellac I use (Target’s Oxford Ultraseal) and the KTM-SV are waterborne, it is important to minimize grain raising by dampening the wood between sanding grits. I rough-sand with 120 then go to 150, 180, and 220---the last three with garnet paper which produces softer sanding scratches. To be avoided are sandpapers with stearates or silicon and tack cloths.
The Spray Gun Setup
The woods I use for mandolins don’t require pore filling, so after the 220, I’m ready to stain. I use the TransTint dye stain mixed with water because I like to see the figure of the wood even in the darkest stained areas, and I use an Iwata Eclipse air brush at 45 psi to shade. If I am covering a large area with stain, I’ll use a Walcom STM hvlp spray gun. This is the same spray gun used for the KTM-SV. Jeff Jewitt at Homestead Finishing recommended the Walcom gun for instrument spraying, and I have since bought a second gun both to have a backup and to have one gun each for staining and finish spraying. The dyes used in water will not sufficiently darken some areas and will leave a mottled appearance on the top in the recurve area. So, after the dye stains have dried I use the airbrush with a shellac mixture to blend or darken any area that needs it. KTM-SV does not mix easily with TransTint dyes. With some vigorous stirring, it is possible to mix a few drops of amber tint into KTM-SV, but that’s all. To get my dark shading mix, I mix the concentrated TransTint dyes into the shellac; it works very well and I can blend, darken, and even hide problem spots. Once that dries---about an hour---I remove any tape that was protecting areas I didn’t want colored. I use a scuff pad to make sure the headstock overlay is clean, and clean up edge and binding areas with a Micro-Mark knife with a curved blade, or an Exacto-type #25 blade, and with Gesswein’s scratch brush pen with a fiberglass tip on wood parts. Then I spray everything with two coats of shellac an hour apart.
My spraying compressor is a 3hp oil-free 28 gallon Porter Cable, which runs the Walcom just fine. The most expensive part of the air system is a membrane filter which stops every possible contaminant. That filter is just off the compressor, and then in the spray room is a secondary filter with pressure regulation for the sprayers (80 pounds for the Walcoms; 45 pounds for the Iwata airbrush). I don’t recommend a turbine system for waterbornes because it heats the air and too often dries the finish between the gun and the instrument. The pipes that run from the compressor to the spray room are 1/2 inch, and I use 3/8” ID hose to the gun with Japanese high volume fittings---all for maximum air delivery. The air pressure measured by the gauge that comes on the gun is set at 1.65 bar, which seems to give the best results. I keep the humidity in my shop at 40-45%, but the temperature in the room where I spray and the room where I hang the mandolins while the finish is curing is kept about 80. An explosion-proof fan exhausts the spray room air through a furnace filter into another part of the shop so I don’t lose my heat in the winter.
KTM-SV, according to the manufacturer, adheres well to almost everything providing the base is fully cured. I let the shellac cure at least 12 hours before spraying over it and have had no adhesion problems. To avoid introducing air bubbles into the system, the KTM-SV should be stirred gently but thoroughly. If you are adding a dye tint, it is best to do that several hours before using and stirring quite thoroughly to allow full dispersion; I add two drops of amber to a quart. I also add 1 ounce of distilled water to each quart of KTM-SV for smoother flow-out. Always strain the finish when pouring it into the gun cup; a medium strainer works best for waterbornes. Again, according to the manufacturer, the temperature for spraying should be 65 degrees or higher for proper film formation; I never spray at less than 75 degrees. It is important to keep the finish from drying before it reaches the instrument surface, so the proper distance between the spray gun and the instrument is 4 to 6 inches.
Under normal conditions, the finish is dry to touch in 20 minutes, dried through in 40 minutes, and dried hard in 60 minutes. So it can be recoated every hour! I plan my finishing schedule to get six coats on in one day, level sand the next day, then two more coats. One reason waterborne finishes have not jumped into popularity is the learning curve in spraying the finish. For mandolins, I like a spray pattern just over 2 inches high; for guitars, I would go to 3-4 inches with a 50% overlap of passes with the spray gun. The pattern must be wet. How wet? This is the hard part of learning. Too wet, and you get runs; too dry, and you get a pebbly surface that doesn’t flow. My only suggestion is to practice, practice, practice. It is helpful to have a brass brush, like Gesswein’s 118-3400, on hand to scrub the build-up off the spray gun nozzle. A fiber brush won’t touch it, and a steel brush will eventually mess up the spray cap. Waterbornes have the bad habit of creating a build-up on the tip of the nozzle which narrows your spray pattern. It will do this several times while spraying a coat. I have a place to hang the gun so I can grab the brush and scrub the nozzle while still holding the instrument I am spraying. Also think through your spraying sequence so you minimize overspray. Oh, and do wear a spray mask because even waterbornes contain chemicals that should not be breathed.
After each coat, clean the spray gun well. Waterborne finishes do not contain solvents that “clean out” the leavings of previous coats, so you need to scrub the air cap with the brass brush, spray water through the gun, take apart the cap and nozzle and blow the finish out of all the passages, reassemble and then run air through the gun again to make sure no finish remains in the gun.
In between coats, you are not sitting around eating Bon Bons or napping. A half an hour after spraying you can safely handle the finish, and that’s the time to check for embedded nits to sand off, places to drop fill, or places that the spray doesn’t reach well like underneath the fingerboard extension on a mandolin. I use 800 grit 3M 216U for any sanding or a scuff pad to knock off nits. Then I brush KTM-SV smoothly on these areas.; it will adhere nicely to the still-hardening finish already there. I generally lightly scuff the entire instrument after the first coat of KTM-SV, do the nit check after every coat, and do the brushing after the 3rd and 5th coats.
After overnight curing of the six coats of KTM-SV, I level-sand the finish using 800 grit 3M 216U. After level-sanding, I spray one coat and drop-fill or touch up any areas that need additional finish, and then after an hour spray the final coat. If I have sanded through any spots with the level-sanding, I touch up the color with a pen or colored pencil and brush some additional finish over the area before spraying the last two coats. If it has been more than twelve hours since the previous coats, I sand and then mist-spray the previous finish with alcohol, which briefly activates the dried finish.
After the last coat, I usually let the finish cure two days before level-sanding again. The surface looks really flat and shiny, but it needs another leveling. I use the same sandpaper as the first level-sand, but I’ve been unhappy with random deep scratches. The grit is supposed to be a P grade, which means very consistent granules, but it doesn’t act like it. I am still testing some other papers but have not found anything that sands as quickly and nicely. Most closed-grain papers load up too quickly, but some people have had good results with 3M’s 366L 15 microns and Mirka Q-Silver 1000 grit. This level-sanding is done to perfection.
Final Sanding and Buffing
A day after that, I sand with the Carborundum sponge pad in 1000-800 grit to take out the scratches from the level-sand. Another day later, I follow up with the Carborundum 1500-1200 sponge pad and sometimes Abralon 4000 a day after that. It is likely there will be some “witness lines” where you have sanded through one layer of finish into the next, but these will buff out. Some people go to buffing without doing all the fine sanding, but the previous procedure works best for me. Buffing should not be done before five days and is best after seven. Almost all the shrinking and curing is done by then, but the curing goes on for a month according to the factory. I honestly can’t notice any difference in the film after five days. I power buff with coarse compound followed by medium compound. With guitars or some other instruments, further buffing with finer compounds is required. With this finish, you do not try to heat the finish when you buff. Some users of nitro lacquer like to heat the finish when buffing and “move it around,” but this should not be done with waterbornes. By the way, heat will cure this finish faster. The factory recommends not exceeding 120 degrees, but I never go over the 80-82 degrees of my “warm” room.
Repairing the finish can be tricky. KTM-SV remains flexible, as a good musical instrument finish should, so a ding will dent the finish rather than crack or chip it. But that leaves a ding to be repaired. KTM-SV does not contain acrylic which repairs well with cyanoacrylate, so the super-glues are not much help. The main problem is that layers varnish never truly melt into each other as do the solvent-based finishes, and new finish applied over the old never invisibly melds together. This is true of all varnishes, not just waterborne. Alcohol will somewhat reactivate cured KTM-SV, but while it will not totally prepare the old finish to meld with new, it helps. So I dab the area I want to fill or repair with alcohol then drop fill with KTM-SV. After a day or more, I go through the sanding, polishing, buffing steps beginning with 800 grit. Beyond that, you can only encourage the instrument’s owner to cover the spot with political stickers. That’s life with varnishes. But still, this is one finish I would not overlook.
Scratch brush pen 115-2314 (for cleaning wood binding)
Brass brush 118-3400 (for cleaning spray gun nozzle)
Grafted Coatings, Inc. www.graftedcoatings.com
KTM-SV waterborne spar varnish
Grizzly Indusstrial, Inc. www.grizzly.com
Carborundum sanding sponges H8919 and H8920 (also scuff pads)
Homestead Finishing www.homesteadfinishing.com
Spray gun – Walcom STM with 1.2mm nozzle
Waterborne shellac – Target Oxford Ultraseal
Dye stains – TransTints
Sandpaper – 3M 216U Fre-Cut Gold
Sanding sponges - Abralon
Knife/scraper – Micro knife handle 81067 and surgical blades
(the chisel blades are great for working on binding slots)
#25 Exacto-type blades for scraping plastic binding
TCP Global www.tcpglobal.com
Scuffing pads – Scotch-Brite scuffing pads, grey
Masking tape – 3M Scotch 233+ (green tape, premium in 1/8” and ¼”)
Airbrush – Iwata Eclipse HP-BCS
Strainers for finish
Other sandpapers and abrasives