Official Luthiers Forum!

Well I’m buffing a guitar body and ran into a little problem. The finish is Nitro which was sprayed. As I was buffing I noticed little tiny (we are talking small) bumps in the finish. It seems that these tiny bumps are harder than the surrounding finish so that when I buff, the finish around the hard spots is removed and the bumps remain. I took a look with my hand held microscope to see if I could see anything. I can see some tiny specks of something but the microscope is not powerful enough for me to see what they are. My thinking is that there must be tiny dust particles in the finish. The interesting thing is that the problem is really only showing up in the back of the guitar. It showed up in the top, but after a light sand and rebuffing it went away. I tried sanding the back and rebuffing but each time I repeat this process more bumps show up. I know I can’t do it many more times or I will go through the finish, it is not helping anyway.

I spray a few extra coats on the back and sides of the guitars. My thinking is that dust must have gotten into the few extra coats I sprayed on the back. I am thinking that I will sand the finish on the back a bit more and spray a few new coats over it. The neck for the guitar turned out great.

I sprayed the guitar in a booth that I made out of shower curtains. It was air tight enough that I was able to pressurize it with a bathroom fan blowing air in the bottom. I ran a vent from the top of the booth out my shop window.

Any thoughts from you more experienced finish guys would be useful. Overall I am really happy with the quality of the finish. I’ll take Nitro over KTM any day of the week.

Josh

_________________
Josh House

Canadian Luthier Supply
http://www.canadianluthiersupply.com
https://www.facebook.com/canadianluthiersupply?ref=hl
House Guitars - Custom Built Acoustic Instruments.
http://www.houseguitars.com

Josh
   Did you do the level sanding and wet sanding? When you spray you have (orange peel) this is the surface not drying perfectly flat. though it is normal you have to sand this flat.
    I like to use 600 -800 - 1200- then wet sand up to 2500 and then buff and polish. Use a block not your hands on the coaser grits
   good luck
john hall

John

The finish came out really flat off the gun.

I do wet sand (with a block) 500-2000 then it goes to the buffing wheel. The finish was perfectly flat before it hit the wheel.

This is about the 10th guitar I have finished. The first with Nitro.

Josh

_________________
Josh House

Canadian Luthier Supply
http://www.canadianluthiersupply.com
https://www.facebook.com/canadianluthiersupply?ref=hl
House Guitars - Custom Built Acoustic Instruments.
http://www.houseguitars.com

Sounds to me like loose grits stuck in the finish. What brand and grit of
sandpaper did you use between coats?David Collins38735.0775115741

_________________
Eschew obfuscation, espouse elucidation.

Not sure what the bumps are but, final prep is very important in the finishing. Always use a tack rag just before spraying and I always use the spray gun (just air) to blow the guitar off just before spraying to remove any small particles or dust.

David
I hadn't thought of that. I use 3M Gold finshing paper. The grit was 600 or maybe it was 400. I have not had any problems with this paper before.

Jeff
I follow those practices as well.

Josh

_________________
Josh House

Canadian Luthier Supply
http://www.canadianluthiersupply.com
https://www.facebook.com/canadianluthiersupply?ref=hl
House Guitars - Custom Built Acoustic Instruments.
http://www.houseguitars.com

Josh,
you said the bumps weren't there after you finished leveling, but they
appeared during/after buffing. I find that I have to use a very light touch
on the buffer or I risk causing small ripples or bumps in the finish. Heat
from the buffer can soften the finish enough to cause it to flow slightly.
Just a thought.
Craig

Not on a guitar but on a keepsake box I built several years ago I bought some cheap sand paper from my local Ace hardware to prep the box, Long story cut short, I had a similar issue. The best I could tell was that small particles of aluminum oxide got trapped. I replaced my paper with some good Norton papers, sanded the finish off and re-applied. This is probably not what you wanted to here. By the way, I do not use tack cloths as some contain wax. Typically I blow off, clean twice with naphtha.

Craig

Thanks for the tips. I do use a fairly light touch on the wheel

Michael
The 3M paper I am using is not cheap, but I guess the paper could be the cause. It is a no load paper that works great for finishing. I have used it in the finishing process on the last few guitar I have done. I should have clarified that I do not wipe with a tack cloth either, as you said some contain wax. I do wipe the guitar and then blow it off before finishing.

The thing that is getting me is that this is only showing up on the back. So I'm still thinking that I got something in those extra coats that I sprayed on the back of the guitar. I will be more careful in the future. I plan to refinish the back in the next day or two as I think that is the only way I am going to get an acceptable finish.

Thanks guys

Josh

_________________
Josh House

Canadian Luthier Supply
http://www.canadianluthiersupply.com
https://www.facebook.com/canadianluthiersupply?ref=hl
House Guitars - Custom Built Acoustic Instruments.
http://www.houseguitars.com

That 3M paper is quite good and usually doesn't crumble like hardware store paper, but who knows. Whatever it is, it's harder than the finish and has to go. I'd say you're looking at having to dig it out and drop fill the areas (I hope you didn't do any tinting), or just refinish it as you said.

_________________
Eschew obfuscation, espouse elucidation.

    There could be a few other causes. Without seeing this it is difficult at best. How long after spraying did you let it set?
     I don't touch them for at least 3 weeks. Also did you wipe the finish ? I use
1st mineral spirits
2nd naphtha
3rd alcohol
   this takes off all the major finish contaminates. If you get silicon on lacquer that can cause alot of problems. Also if you get on the finish before it is cured you can just plague yourself.
    Good luck
john hall

David
There are to many to dig out. I think refinishing is the only way to go.

John
I should have gave more info on my finishing in my first post. This guitar cured for a good 3 1/2 weeks. Can you tell me mkore about when you wipe the finish. Is this what you do after you sand between coats? Do you use all three of those products (MS, nap, alcoh,) one after the other? I don't think silicon is the issue as I wasn't using it for anything when I was finishing (or weeks before and after for that matter). Thanks for you help and more info on when to wipe the guitar would be appreciated.

Josh

_________________
Josh House

Canadian Luthier Supply
http://www.canadianluthiersupply.com
https://www.facebook.com/canadianluthiersupply?ref=hl
House Guitars - Custom Built Acoustic Instruments.
http://www.houseguitars.com

Josh,
I used Kelly Moore lacquer on my last guitar and had the same problem. Everything was perfect until I hit it on the buffing wheel. I think that the finish melted in a few spots because it just got too hot, and caused these little ripples/white specs that I couldn't remove. I had to mow them down, and refinish that area to get rid of them. I think it makes a difference what type of Nitro you use. Good luck!!
Tracy

Hey Josh,

Not sure exactly what your spray scedule is, but here is what I do:

Shoot about 8-10 coats on over two days, no sanding at all in between. let this sit a day or two, then 400 grit klingspor paper with soapy water until its flat, or at least so close I know the next 4-5 coats will fill. let sit a day, wipe with a paper towel any of the remaining residue, the white stuff usually on the guitar, then a light tack cloth. Shoot another 4-5 coats.

Unless something got stuck in it, like sometimes my gun drops and shoots a little piece of brass or a big goober of lac thats needs to be dug out, stop. If you need to dig something out, then drop fill, let sit a day, light sand and then reshoot 1 or 2 coats os it all melts in.

Now its 3-4 weeks cure, then wet sand with 400 600 1000 and 1500, then menzerna on the wheels, fine and glansw ach, then hand polish with number 10 Meguiars. Finish up with some Meguiars 26 wax, and ship it !!!

_________________
Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

    I won't touch lacquer for at least a week to let it cure properly. Nitro under great conditions needs a minimum of 3 days. If you get on it too soon it will look great at first then the sinks start to show.
     A week is such a small amount of time to wait as compared to a refin
john hall