Sunday, October 31, 2010

Joining the Back and Top Plates

Preparing the back and soundboard halves for gluing is a relatively easy but critical step in guitar construction. Many luthiers use a traditional shooting board similar to the one pictured below. The back or soundboard halves are placed flat, one on top of the other, with the edges to be joined overhanging the ledge slightly; the plane is pushed along the length of the overhanging plate edges.


My own take on this has been to build a shooting board in such a way that the plates to be joined are held vertically between two cork-lined jaws as the picture below shows. The top or back plates are clamped lightly and accurately aligned before the jaws are fully tightened. Truing the edges then takes place with the plane guided by the vertical fence, with pressure applied from above and the plane held in the familiar and more controllable upright position. The advantage of this configuration for me is that, once they're firmly clamped, the plates are immobilised and I feel I'm able to apply more even pressure than a more typical shooting board would permit. To ensure an accurate cut, my trusty old wooden plane has been fed through the drum sander to true up the side which bears against the fence.


A few judicious strokes with a sharp plane blade are generally all that's required to achieve the perfect joint, after which, the halves are glued together using the jig shown below. As you can see, wedges are tapped into place to apply the necessary clamping pressure. The underside of the blocks is faced with 80-grit sandpaper which reduces slippage once the wing nuts are tightened and the wedges are tapped home.




Cheers
Pete

Saturday, October 23, 2010

Some Final Observations on KTM-SV

I realise I've banged on about Grafted Coatings' KTM-SV water-based finish a bit over recent months but, as a final word on the subject, it's probably worth reviewing what I've read or found out for myself for the benefit of others who find themselves facing the same challenges I did with the finishing process.


From memory, I was alerted to this finish by Rolfe Gerhard’s article “KTM-SV – An Overlooked Finish”. The way Rolfe described it, Grafted Coatings’ KTM-SV sounded like the safe, environmentally-friendly finish material I’d been vainly searching for to that point, possessing most of the positive attributes of the other commonly used water-based products, with some important differences. Grafted Coatings’ web site describes it as “a waterborne, oil-modified, self-crosslinking urethane”, which immediately separates it from the better known waterborne acrylics.

On the plus side, it’s reportedly harder than the likes of Target Coatings’ EM6000 or Grafted Coatings’ other instrument finish, KTM9, although how that’s been determined with certainty I’m not quite sure. Also, not having an acrylic base as they do, it doesn’t display any of the blue cast some report with those products. For me, however, the most telling statement in Rolfe’s article is that he’s had instruments in the hands of players for two or three years and no problems have arisen in that time. For my own peace of mind I wish it were longer, but I’m comforted by that information nevertheless. For me, this is the acid test in terms of any new or newly adopted product’s suitability as an instrument finish. How many stories are there of instruments finished with KTM9 developing bubbling or sticky necks over time for instance? KTM9 still has its fans, but I think it’s fair to say the enthusiasm for that particular product has diminished over the past few years, due in part perhaps since prominent advocates such as Mike Doolin and Charles Fox have reportedly jumped ship and turned to polyester and nitro respectively. The disappointments with U-Beaut’s original Hard Shellac are well documented and, although it’s been reformulated, I’m still unconvinced of its suitability given my recent experience with it as a sprayed finish – French polishers (and Australian ones for that matter!) may have more luck. In any case, I don’t think it’s proven itself over a long enough period to be considered seriously, particularly if you’re as cautious as I am and share my view that warranty work is bad advertising, even if it’s expertly carried out.

Because I avoid mineral spirits (turpentine) whenever possible, another plus for me is that unlike other waterborne products, KTM-SV can apparently be wet-sanded using water as the lubricant. In his article, Rolfe says that witness lines buff out invisibly, which I’m able to confirm with greater conviction since I steeled myself and ordered a buffing arbor from LMI in the U.S. Used together with two grades of Menzerna buffing compound, buffing is now an easy task, and the gloss I’m able to achieve effortlessly is pretty impressive. My wallet may be lighter, but my finishes have improved out of sight as compensation.

Of course, with any finish material there are trade-offs and compromises and KTM-SV is perhaps no different in that regard. One of KTM-SV’s supposed disadvantages is that, unlike Target’s EM6000, once it’s cured, subsequent coats don’t burn into the preceding ones meaning that invisible finish repairs are difficult to achieve. I’ve come across some conflicting advice, however, which says that rubbing the area to be repaired with acetone or lacquer thinner softens the finish enough to allow an invisible repair, but I have no first-hand experience where that’s concerned and can only repeat here what I’ve read elsewhere. Less significantly in my view, it’s said that it can’t be buffed to as high a gloss as nitro, although I’m pretty happy with the gloss level on the guitars pictured. The only other real negative - which I'm all too used to living in Australia - is that freight from the U.S. is more costly than the material itself.

The guitars pictured in my last two posts were finished with KTM-SV top coats over epoxy pore fill and three seal coats of de-waxed white shellac. I followed the finish schedule described by Randy Muth in a thread on the OLF, whose prescription is for six coats per day over two days, with the exception of the soundboard which receives two coats fewer for a total of ten, i.e., only four coats on the second day. Following Rolfe’s suggestion, I added 30ml of distilled water to my quart tin of KTM-SV which seemed to help with flow-out and leveling, although it’s also possible that the coats began to lay down more smoothly for me as my novice spraying technique improved; by the time I applied the last top coats on the second of these guitars, it was laying down beautifully, to the extent that I was able to begin my level-sanding with 800-grit paper.

Interestingly, Rolfe also remarks that there’s very little shrink back as the finish cures, however, I notice that minute ridges corresponding to the grain lines are evident on both tops; in fact, it looks a lot like a typical thin nitro finish which isn’t necessarily a bad thing given that for many, nitro is still the standard by which all other finishes are judged. As best as I could determine with dial calipers, my ten coats on the soundboard measured around 0.1mm, or 4 thou in thickness after leveling and buffing – a very crude guide to be sure, but nevertheless helpful in the absence of any more accurate means of measurement.

At some point I may try Target Coatings’ EM6000 (Stew-Mac’s Colortone Waterbase Lacquer) purely for the sake of comparison, but for the foreseeable future I’m a KTM-SV convert, and greatly relieved to have at last a found a finish product which allows amateurs like me to produce near professional results without the bother and expense of a spray booth, explosion-proof fan, etc. I know I could cross my fingers and shoot nitrocellulose in the back yard as many others do, but I’ve been there and done that and as I’ve grown older, a little wiser, and a lot more conscious of health and safety issues, I’ve decided not to repeat that experience and expose myself and others to the attendant risks. Like a reformed smoker, I’m still tempted at times to ignore the risks and revert to past practice, but I’m confident that good sense will prevail on both counts!

Some relevant links:
Official Luthier's Forum discussion

Cheers
Pete