KTM-SV - Sanding and Buffing

There are a multitude of guitar finish products available, and it's generally acknowledged that each of them have their advantages as well as their shortcomings. The dilemma for the guitar builder, then, is in deciding which particular compromise they can live with, and what product best suits their particular circumstances.

My current choice is Grafted Coatings' KTM-SV, a water-based, oil-modified urethane that a growing number of musical instrument makers are using with great success. I'm in the throes of completing the guitar pictured - my third using KTM-SV. Although I've been pleased with this product so far, if a little of its flexibility could be sacrificed for the sake of hardness, I'd be truly ecstatic. I'm keeping an eye out for the release of the manufacturer's new product - which I believe will be marketed as KTM-10 - in the hope that it makes up for what KTM-SV lacks in terms of hardness. Early reports sound very encouraging.

My technique with the spray gun tends to improve over the course of finishing a guitar, and by the time I've applied the last of the top coats, I've generally sorted myself and the gun out to the extent that the guitar's finish is level and reasonably smooth. After the finish has cured for a few days, the process of sanding through the grades of sandpaper can proceed. With the paper wrapped around a cork block, I begin sanding with 600 or 800 grit paper, depending on the quality of my last coat. I'm pretty cautious when sanding the edges of the body and the headstock; I skip the coarser grades and begin my sanding of those areas with 1200 grit paper, dispensing with the sanding block. I use soapy water as a lubricant throughout this process, and constantly check my progress by squeegeeing off the water and holding the body or neck up the the light to check for rogue sanding scratches.

Inevitably, I'll sand through the topmost finish coat to one or more of the underlying coats; witness lines are evidence of that, even after a thorough sanding with the finest of the papers. It's at this point, of course, that my attention turns to the buffing wheel, but I've learned that this finish needs a couple of weeks to harden enough that buffing will completely remove these witness lines. I've wasted a lot of time and effort and become quite frustrated when attempting to achieve a mirror-like gloss before the finish has had time to harden - it's just not going to happen! Not unexpectedly, it's also apparent that the finish shrinks ever so slightly over the first few weeks. On future guitars, I plan to delay sanding with 1500 and 2000 grit papers - if not all of them - until a couple of weeks has elapsed and I'm about to buff. Perhaps that's not so much of a consideration where the soundboard is concerned as ridges in the finish corresponding to grain lines seem to be accepted, given their inevitability when any of the more traditional nitrocellulose lacquers are used.

Links to Grafted Coatings' web site:
KTM-SV
KTM-10

To view previous posts dealing with KTM-SV, click here.

Cheers
Pete

The Fingerstyle Guitar - Latest Photos

I seem bogged down in the details at present, and it feels I'm making little headway. Nevertheless, here are some more progress shots.

Cheers
Pete

The Fingerstyle Guitar - Slow Progress

Despite the fact that I'm devoting most of my available spare time to completing a claro walnut and spruce instrument for a friend's daughter, there has at least been some movement with this redwood and claro walnut guitar lately. I'm content to proceed patiently, and I'm currently gluing purflings and bindings one piece at a time, through the week after I arrive home from work.

Acknowledging that the tuners and bridge are yet to be attached, the guitar is very light and, knowing how much I strived to brace the soundboard as lightly as I dared, I have great hopes that I'll achieve my goal of producing a very responsive instrument well suited to a fingerstyle player.

Cheers
Pete

Mitre Madness!

With the aid of lutherie's most underappreciated item of machinery, the disk sander, I've been preparing binding mitres and have almost finished these fingerboards and headstocks. 

Should I ever find myself building guitars for profit, bound fretboards and headstocks will definitely be an optional extra, priced accordingly. The rewards are great, but there's a lot of additional work involved.

Cheers
Pete

Claro Walnut - Not So Subtle!

With the luxury a few days off over the Christmas/New Year period, I'm proceeding steadily with the pore-filling process while I wait for delivery of the KTM-SV top-coat material. I'm perpetually amazed at the figure in this guitar's Claro Walnut back and sides, hence the repeated attempt at a photograph that will do this wood justice. Thanks Allied Lutherie!

Cheers
Pete

Pore Filling With Epoxy

After installing purflings and bindings, it's very rewarding to scrape them level, carefully round over the edges, then sand the top, back and sides in readiness for the first steps in the finishing process. It feels like the end of a journey while at the same time signifying the beginning of another equally challenging one.

With final sanding on this guitar complete, I've taken the first steps towards pore-filling the back and sides with epoxy, a process that finally reveals the true beauty of the wood. As a hobby builder, many months have inevitably elapsed before I get to this stage, but the rewards make the effort and the wait worthwhile. Along with stringing a completed guitar up for the first time, seeing the figure and colour of the wood magically appear as the first coat of epoxy is applied is surely one of the highlights of the building process.

On a cautionary note, a problem with some epoxies is that of amine blush, a waxy residue that forms as the epoxy cures that can interfere with adhesion and curing of subsequent top coats. From what I've read on the topic, spending a little more on a good quality epoxy is worth considering as they're less likely to be susceptible to blushing. Whichever product you choose, once the epoxy has fully cured a thorough rub down with a damp cloth - or, better still, one dampened with a 50/50 mixture of water and denatured alcohol - is a simple precaution that eliminates potential problems. There's wisdom too in avoiding the application of epoxy when the ambient temperature is falling and relative humidity is therefore likely to be rising - conditions said to exacerbate blushing.

Hats off to Todd Stock for putting together these very informative tutorials:
Epoxy Pore Filling Part 1: Application of First Coat & Sealing a Rosette
Epoxy Pore Filling Part 2: Prep & Application of Second Coat
Epoxy Pore Filling Part 3: Prep & Application of Third Coat and Prep for Finishing

Cheers
Pete

The Secret's Out!

A friend contacted me yesterday seeking advice about a suitable guitar upgrade for her daughter. As I've secretly had plans all along to make a gift to her of one of the guitars I currently have under construction, it seemed like a good time to reveal my intentions. As a result, I've decided to prioritise the completion of this guitar with a view to presenting it to her as soon as I can.

As luck would have it, I finalised the construction stage only a week ago - the task of finishing therefore begins in earnest. The first step is to fill the pores with epoxy after which two coats of Ilva TF23 sealer will be applied immediately prior to the first of the KTM-SV top coats.

I'll document the process here for the recipient's benefit and in the hope that it's of some interest to others as well.

Cheers

Pete