Official Luthiers Forum!

I am new to this forum and pretty new to lutherie too. I've made a Boucher design classical and now a Martin OM steel string with spruce top and flamed maple back and ribs. I used KTM-9 on both - all over on the OM but just the back and ribs on the classical (I gave this a lovely French Polish). Unfortunately on the steel string I've got a finish that's good but not GOOD. I can see lengthwise streaky sheens when I hold it up edgewise to the light. It's as if you can see the edges of different coats running from neck to tail.

I have used KTM-9 on the OM top, back and ribs (with a a water-based tint on the first 3 coats of the back and ribs). I brushed it on with a squirrel hair brush really thinly making sure I didn't get runs (happened on the classical!) and fairly quickly to ensure I always worked to a wet edge. I waited 1 hour between coats and usually got 3-4 coats per day. I cut back with 400 grit paper every morning - heavier cuts to level it with the early coats then really light with the later ones.

After I'd got about 20 coats (!!!) I left it a couple of days and sanded with 400 then thru the grades to 1200 (all dry). Then micromesh (again dry) all the way to 12000 grit but it still didn't have a high gloss.

I don't have a buffing machine, so I thought I'd use T-cut to get rid of the remaining scratches - all done with elbow grease. I got a good gloss but then these streaks appeared.

From reading other posts, I can see I didn't leave the finish to cure so it was too soft. But what else did I do wrong and can I recover it without starting all over again?! I'd appreciate whatever advice you guys can give on what are the really critical things.

A pictures would help -- but sounds like witness lines caused by failure to burn-in either brushed rows or layer to layer. This product never made it beyond the test stage at our shop due to the burn in issue, we observed layer witness lines even during the leveling stages sprayed and brushed. Perhaps we were doing something wrong?

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Ken Cierp

http://www.kennethmichaelguitars.com/

I've achieved some darn good finishes with KTM-9, but it isn't without its issues. Primarily, it can hold on to scratches really well and it is very difficult to get them out. Witness lines do come out when you have a good buffer and really apply the needed compounds and pressure.
As Ken mentioned, we would need pictures to see what has happened.

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"I want to know what kind of pickups Vince Gill uses in his Tele, because if I had those, as good of a player as I am, I'm sure I could make it sound like that.
Only badly."

Thanks Ken & Don for the quick replies. I've attached a pdf photo of the lower bout. It's really difficult to capture on a photo but if you zoom into the area just below the reflection of the light from the doors you can see the ghosts of these streaks. I'm starting to think it's witness marks from brush strokes that haven't burned in to the layers above and below, but see what you think.

I have to ask, what are you using for a block when sanding? The surface itself looks like you left a lot of high spots in streaks. I'm not seeing any "witness lines" in the photo, but it's not very clear.

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"I want to know what kind of pickups Vince Gill uses in his Tele, because if I had those, as good of a player as I am, I'm sure I could make it sound like that.
Only badly."

I used a cork block in the early stages when I was levelling the surface, then just folded wetordry paper between the later coats and when I went to finer grits. Sorry this photo was the best I could do. The finish looks perfectly transparent from most angles. It's just low down like this against the light that you see the problem.

I have done plenty of guitars with KTM 9 but have never brushed it on so I am guessing.

It looks to me like the slightly cloudy areas are caused by the coats being to heavy or not dry enough before the next coat. This could be caused by appling in less than disable conditions (to cool or to humid, or both)

Level sanding may also be a problem from the photo.

Also 20 coats could be to many.

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Joe Beaver
Maker of Sawdust

Thanks again for the comments. I guess my main question now is - is it recoverable? I have a friend who has a Stewmac buffing machine and I might be able to sweet talk him into letting me use it. Will these marks buff out or do I need to go back to bare wood again?!

I would first do some more level sanding with s harder block behind the paper, just to make sure it us dead flat. Then try buffing. Use wet sand paper with some liquid soap as a lubricant.

Did you by any chance, take a measurement of the film's thickness?

If after buffing the marks are still there, I would strip it and re-apply, but spray rather than brush it if there is any way possible for you to do so.

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"I want to know what kind of pickups Vince Gill uses in his Tele, because if I had those, as good of a player as I am, I'm sure I could make it sound like that.
Only badly."

Thanks, I will do exactly as you suggest. I have a rubber 3M block that is harder and flatter than the cork one. I'm also coming to the view that spraying is a surer way to get a more even coating. It's probably more economical on the coating too.

I didn't measure the film thickness but I can deduce it. I masked off the bridge gluing area with 2 layers of Scotch tape and when I peeled that off the coating had gone over it a bit. My calcs say that means a film about 0.1 mm thick. Judging by the amount of KTM-9 left in the can (1 pint), I will have laid down a film about twice that, so to get to 0.1 mm I must've sanded off half of it! Sounds about right to me.

Thanks, I will do exactly as you suggest. I have a rubber 3M block that is harder and flatter than the cork one. I'm also coming to the view that spraying is a surer way to get a more even coating. It's probably more economical on the coating too.

I didn't measure the film thickness but I can deduce it. I masked off the bridge gluing area with 2 layers of Scotch tape and when I peeled that off the coating had gone over it a bit. My calcs say that means a film about 0.1 mm thick. Judging by the amount of KTM-9 left in the can (1 pint), I will have laid down a film about twice that, so to get to 0.1 mm I must've sanded off half of it! Sounds about right to me.

I make that 0.1 mm thickness about 4 thou/mils if that means more to you by the way. Cheers...

4 mil thickness sounds good. A little thin for me since I live in fear of sand thru. If you didn't have any of those then I would say it is just right.

I have always sprayed and never have had burn in problems with KTM 9, so I'm beginning to think your troubles started with the brush.

KTM 9 usually does a great job when sprayed, even for a hack like me. (of course humidity, temperature and drying time play a big role)

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Joe Beaver
Maker of Sawdust

Thanks Joe. Looks like I'm a maker of finish dust!

If you say 4 mil is on the thin side for you, I'm going to have to be really careful doing one more wet sand before buffing. I'll maybe go for it tomorrow if I feel confident in the morning!

Oh, and I'm never using a brush again, squirrel hair or any other kind!

Latest Update: I couldn't get quick access to a buffing machine so I did some more investigation of the streaks by polishing with Meguiar's Ultimate Polish. This brought the surface to a fantastic high sheen. And what stands out now it's really shiny is that I can see the remains of brush strokes as predicted by Ken, Don & Joe above.

So my basic mistake was not getting the finish really flat. The back and ribs were already completely flat so they have come out absolutely beautifully with the polish.

Thanks for all the guidance!

Wonderful! Looks like you're on your way. Keep us posted on how it turns out, and by all means if you can find a way to spray, do so. And if you really want to use a waterborne, try their KTM-SV. It is a much better finish. Or give the Royal Lac a try. It is SO Easy to use...

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"I want to know what kind of pickups Vince Gill uses in his Tele, because if I had those, as good of a player as I am, I'm sure I could make it sound like that.
Only badly."

Thanks for the recommendations on alternative finishes. The SV looks very good, and it's amber too which was what I needed on this project.

On spraying, there's a bewildering range of kit out there. On this side of the Pond you can pay anything from £50 to over £1000 - unbelievable! With my budget I'm aiming towards the lower range needless to say... The Apollo Spraymate looks a reasonable compromise but is this too crude for musical instrument finishing?

I can't get parts for my old Binks touch up gun anymore,
so I bought a Graco/Sharp Finex FX1000 touch up gun for about $150.
Works nicely on git boxes.
You need a compressor as well though.
I've been spraying shellac, and like it.
The water base finishes are nice to work with,
but some of the darker woods look better with lac or oil based finishes, to me. Alan

Hey, I don't know how it would work on an acoustic,
but I used Generals wipe on poly on an electric solid body,
and I really like it!
A