A Very Fine Musical Instrument Finish

The search for the ideal mandolin finish has taken a new road for me. It is still the waterborne road, as modern instrument building demands whatever safe and healthy finishing process is possible. My latest experiments and tests have led to Target Coatings Oxford Hybrid Varnish #7100, an acrylic/tung oil combination. When I originally contacted Target Coating several years ago, I reviewed my projected luthier use with owner Jeff Weiss, and we chose the acrylic-based Premium Spray Lacquer because: 1) it works well over shellac sealer, 2) is designed to mix with TransTint dyes, and 3) is relatively easy to apply, very easy to sand, and decent to buff. The PSL finish was the first waterborne to meet my standards of appearance, acoustics, and ease of repair. We rejected the hybrid varnish at the time because Jeff considered it difficult to repair.

However, I have recently become aware of a high rate of failure of the older v2.0 PSL finish on the necks of my mandolins, due to friction and body chemistry, and a couple of failures for the same reasons on the body. These failures involve a total change in the chemistry of the finish---it gets soft enough to scrape off with a fingernail and stays that way. For me, this meant the finish must be completely removed in the effected areas and something else put on. The hybrid varnish, that was originally rejected, is far more resistant to body chemistry, friction, and scuffing, so I decided to try it for the repair. I sanded the finish off the effected areas with Norton's Norzon A855 Z320-A grit designed for waterborne finishes and feathered the sanding onto the areas of good finish. I then stained the bare wood and sprayed six coats of #7100 (over two days), feathering it over the sanded finish while laying down full coats on the bare wood. Surprisingly, the finishes blended when sanded out and polished, and the repair is not obvious. The owners of the repaired mandolins tested them for more than six months and have been delighted with no problems.

The Repair Problem Solved

It occurred to me at that point that if the #7100 blended into the PSL, might it be possible for PSL to be used for a repair to a #7100 finish---kind of like spirit varnish is used as a repair for oil varnish in the violin world. I experimented with spraying a finish on some clear Plexiglas pieces. I laid down the sealer coat of shellac, then followed my usual procedures with six more coats of #7100. I let it cure then dug out three almost-half-inch gouges and tried to repair each gouge with, respectively, shellac, #7100, and PSL. I let the repairs cure then sanded and buffed. The PSL was the most invisible repair and was very acceptable in appearance. I sent this sample to Jeff, and he agreed that it worked. It looked like PSL could be the repair agent for #7100 hybrid varnish.

A Severe Test

I tried a new experiment when I received three new cans of finish: PSL, Hybrid Varnish, and Conversion Varnish (a castor oil based product). There was a good layer of wet finish on the inside of the lid of each can, so I set the three lids aside to dry. The next day I attacked each layer of finish with my weapons-grade thumbnail. The PSL and the conversion varnish both chipped, and with moderate pressure both separated from the can lids. The hybrid varnish grooved but did not lift from the can lid. The second day, I attacked again---keep in mind that the chemical cure times for these finishes range from 100 to 200 hours, so I was still early in the cure cycle---and I again chipped and lifted the PSL and conversion varnish. But to my surprise, the groove in the hybrid varnish had "healed!" So I grooved it once more, again with no separation of the finish from the can lid. That "thumbnail" test continued for ten days until all finishes were chemically cured. The PSL and conversion varnish continued to chip, even more dramatically as the finish became more brittle, and they continued to separate from the can lid with moderate pressure. The hybrid varnish stopped "healing" after a couple of days, but with each further day of curing, my thumbnail made less of a mark, and at no point in the test did the finish lift off the can lid. When the hybrid varnish was fully cured, it was really tough with great adhesion, yet it kept a high degree of flexibility---all qualities that I had been looking for in a mandolin finish. I was now ready for a full instrument trial.

The owner of one of the v2.0 PSL-finished mandolins with a neck finish problem said that while the neck was the only problem area, he often played shirtless (sorry about that image), and he was worried about eventual finish breakdown on the body. Could he upgrade to a new mandolin completely finished with what I was going to use to repair the neck on the old mandolin? I did that, and I also used the #7100 on another mandolin being finished at the same time. The results were very, very encouraging.

Advantages and Disadvantages

That was two dozen mandolins ago, and I now use the Oxford Hybrid Varnish #7100 as the standard finish on all my mandolins. It has all the virtues of a waterborne finish. Safety, ease of clean-up, and all the virtues of Target products: clarity, lack of witness lines, repairability, and great factory assistance. The only down sides are: 1) that it took some tinkering and consultation with Jeff Jewitt at Homestead Finishing to get it spraying well. 2) The #7100 likes to be sprayed in thinner coats than PSL, which means that a couple extra coats are needed to get the proper build, and it requires the same three hours between coats, so the overall application time is extended by six hours. 3) The sanding of the finish takes almost twice as long as with PSL (1/2 hour for very light level sanding between coats three and four and 1 ½ hours for level sanding between coats six and seven, which I didn't use to do with PSL. And up to 1 3/4 hours vs. 1 ¼ hours for final sand-out after the last coat. Plus, an extra half hour for the cross-sanding following the level sanding), so the sanding time is up about three hours; 4) the chemical cure time of the finish is also almost twice as long as PSL (200 hours vs. 100 hours). That's not so bad when you consider that the actual work time is increased only about three and a half hours per instrument. Once I got into the new overall finishing schedule, the extra cure time really didn't matter; I'm doing other things in the meantime. Of course, I have now raised my prices to cover the extra time, and I keynote in my promotion the glamour and durability of a hybrid varnish finish.

Preparing for the Finishing Process

My finishing process begins with sanding the mandolin down to 220 grit, using garnet paper for the 150, 180, and 220 grit sandings, and lightly wetting the wood twice, once after 150 and again after 180 before using the 220. I then tape the fingerboard, stuff rags in the body, and tape any areas I want to protect from stain (i.e. bindings, headstock overlay edges). I color either by using Trans-Tints mixed with water as stains or by using Trans-Tints mixed with fresh shellac (2# cut). Recently, I have been using some excellent spirit varnish coloring for violins from International Violin, and mixing that with the shellac about fifty-fifty---or a bit more shellac than coloring, depending on the density of coloring I am seeking. After coloring, I wait a few hours (or overnight) and scrape the bindings and other problem areas clean. If I used the water stains, I seal everything with a coat or two of shellac. Either way, the shellac is my sealer before applying the finish. Shellac dramatizes the grain patterns. Some waterborne coatings do not do well with shellac, but Jeff Weiss assured me Target does, and he was right. I let the shellac dry overnight, but three hours will do with thin coats. In spite of wet-sanding, there still will be some raising of the grain with shellac so I use the gray (very fine) plastic wool to smooth the surfaces. Do not use the maroon plastic wool as the maroon dye can migrate into any spot of finish that is not 100% cured.

Since regular paint cans are awkward for me to use, I transfer the product into high density polyethylene wide-mouth bottles from Consolidated Plastics, the 1000ml size which has room to add a few ounces of material if you need to. But I don't reduce or retard the finish in any way. Your equipment may work better with it reduced (up to 5% distilled water) or retarded (a half ounce of Target's SA5 mixed a day ahead for full dispersion). Or both. I also like my finish to have a definite vintage tint, so I add 2 drops per quart of finish of Trans-tint (or Stewart MacDonald) vintage amber (5030). The tung-oil already in the #7100 may be enough tint for some builders.

Spraying the Finish

I have found that the hybrid varnish spraying is as good as your gun. I use a high quality conversion Walcom mini-HVLP detail gun (bought from Homestead Finishes); it has a 1.2 mm needle set. I had been running the gun at 2 bar pressure with PSL, but with #7100 there was a rapid build up of finish on the nozzle, which messed up the spray pattern. Jeff Jewitt at Homestead recommended reducing the pressure, and it runs well at 1.65 bar. I don't remember the conversion from bar to psi, but you can get the proportion. A 1.4 mm set would also have helped, but it was not available with that gun. I set the gun to give a 2 to 3 inch pattern sprayed from 4 to 6 inches away. Water-borne finishes harden prematurely if they travel too far through the air, so distance is important. You want the finish to be wet when it lands so it flows out. If you are spraying a larger instrument, you would want to widen the pattern, but don't back away more than 6 inches. I put on a barely wet coat, which turns out to be about one-half mil (.0005 inch) thick---measured after it dries. With practice, you will learn the sequence of areas to be sprayed and how fast to move the gun to get an even wet coat that flows well without developing runs. The hybrid varnish is a little slower to set than PSL and other waterborne finishes, so the thinner coats are important to avoid runs.

Further Coats

Target says to wait two hours for initial chemical cure between coats; I usually wait three or four. But I will do touch-up work after two hours. It feels tacky to hold, but it is okay to handle. Some of the active elements in the finish that help the coats "burn" or blend together also slightly activate the colors in or under the shellac, and those colors sometimes "bleed" on the binding. I use a small surgical knife to scrape the binding clean, but you have to do this carefully because at this early stage of cure, the hybrid varnish scrapings will love to adhere to some other spot on the finish. At the two-hour mark of the second or third coat, I also brush a coat under the fingerboard support block and around the neck joint and any other place that doesn't take spray well. Unless there is something massive in the finish, I don't sand between coats until I have three coats on. The first day I usually get three coats on. The next morning the finish has cured enough so that it feels slick but not tacky. I will then do a light level sanding with 800 grit 3M Open Coat Fre-Cut paper 216U. That means I will level bumps and smooth over-spray, but I won't go after every little shiny spot. There is usually less than 3 mils (.003") of finish on at that point, and I don't want to risk sanding through. I will shoot the fourth coat, wait two hours, and do a second brush coat to problem areas. If I have to wait more than twelve hours between any coats, then I will scuff-sand with the gray plastic wool to give a better mechanical grip. After six coats and an over-night cure, I do a more complete level sanding with the 800 paper. If any spots look like they need more finish to level, I will touch-up with a brush after I've finished level sanding but wait an hour before spraying. If I have sanded through in a colored area, I will either touch up with colored pens or pencils, or, in an extreme case, mix the appropriate color TransTint with the #7100 and air brush the repair. Then I spray the last coat.

A side note on brushing: many waterborne finishes, especially ones with acrylic compounds, they tend to foam if brushed. It sometimes helps brushing to add more retarder and water to make it all perform to brushing fashion. I still make too many bubbles when brushing. Slow even strokes with artist grade brushes seem to work best. When I get bubbles, I immediately brush them flat. This is probably not a finish that can be applied totally by brush, but one is being developed.

Sanding Out the Finish

The #7100 hybrid varnish has a chemical cure cycle of 200 hours---eight days and eight hours---and it cures best is a warm, dry place. I have a small room dedicated to finish and glue curing that stays pretty much at 80 degrees temperature and less than 40% humidity. There is a "window" during which it is easiest to do the initial sanding out of #7100; it is after two to three days of curing. Before the two day mark, the finish is still a bit on the gummy side; after four days it is on the tough side. Both take longer to sand out than in that "window." I do a full perfect sanding of out the mandolin with the 800 grit 3M Open Coat Fre-Cut paper 216U, but I still stay from edges that might have slightly thinner finish. I let it cure for another day after the initial sanding, and then I carefully cross-sand the whole mandolin with Woodworker Supply abrasive foam sponge (#960-058), which is 800 to 1200 grit to get out the scratches from the first sanding. You can also use Micro-mesh 2400. You might see an occasional "witness line" (an edge of layers of finish where you have sanded through one layer to the next), but the witness lines all disappear with buffing. In fact, they usually disappear by the next day as the finish seems to melt into itself for at least the first 100 hours. For the same reason, you don't want to let sanding dust stay on the finish for any length of time; it will start to melt in. Blow or wipe any sanding dust off the finish when you are done with each sanding. I let it cure another day and cross-sand again (now going parallel with the original scratches) with abrasive foam sponge (#960-065) 1500-2000 grit. Or you can use Micro-mesh 3200 and then a day later go to 3600 or 4000. The next cross-sanding is with Abralon 2000 and then Abralon 4000. If you power sand, you might go with Abralon in different grits all the way, but I haven't tried this. After the first "window" sanding, you don't have to follow my schedule of sanding through the grits; it doesn't make much difference.

Tape Trouble

I remove the tape masking the fingerboard before buffing, and this must be done very, very carefully. I discovered how carefully when I was refinishing just a neck and had taped off the body to keep over-spray off it. A couple of days after spraying the last coat, I began removing the tape from the body and patches of new finish came off with it. Jeff explained that until the cure is complete, the finish holds to itself better than to what it is sprayed on. So anywhere the finish overlaps tape, the edge must be cut or broken. Where I tape the fingerboard, I file, scrape, or sand through the finish. I file either parallel to the tape line or carefully file away from the finish and toward the tape to avoid pulling any finish off. Where the tape overlaps the string nut, I cut the finish. After I have removed the tape, I go back with a very sharp knife and cut and scrape any finish that is still on the fingerboard. This sounds time-consuming, but it might take ten minutes.

Buffing and Polishing

Any time after that eight-day and eight-hour cure has been reached, you can go to the buffer. All those grades of sanding may not be necessary; some do more with the buffer than I do. But I don't like to heat this, or any, finish with long or intense sessions of buffing. I buff with coarse compound, then hand polish areas the buffer can't reach using the same compound in paste form. Then I buff with medium compound and hand-polish where necessary with medium paste. I sometimes use Micro Mesh 4000, 6000, and 8000 to get out problem scratches. Some instruments might need finer compounds, but I am getting a beautiful finish with medium compounds. Use good light and work and rework if you see any scratch marks. I finish up with a light coat of natural wax.

This is a finish you can take out into the sunlight and not see any micro-bubbles or layers or "witness lines." It really is beautiful and incredibly durable. It reminds me of the best old-time finishes.

Rolfe Gerhardt
Phoenix Mandolins
159 St. George Road
South Thomaston, ME 04858