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greenscoe
03-18-2016, 05:46 AM
I usually finish in True Oil, a finish that I very much like. I have virtually no experience with nitro cellulose.

Last Summer I used my DIY compressor and gun to spray a whole instrument in nitro cellulose. I built up the coats (thinly) and then after a few weeks used wet/dry etc to get the mirror finish I sought.

Over the Winter several cracks have appeared in the finish on the cedar soundboard (along the grain). What's the best way to rectify this? Can I simply spray over with a thin mix and expect the cracks to fuse together? Is this perhaps a sign that the finish was too thick and it's therefore necessary to sand back before applying further lacquer?

The rest of the instrument is fine, but is this a sign that further cracks may appear in the future?

BlackBearUkes
03-18-2016, 06:14 AM
What it means is that your instruments wood moved due to humidity changes over the winter. While wood moves, lacquer is not as forgiving and may crack. IMO is it best to let it be. If you can't stand the thought of living with it, you could sand the lacquer back and use a thinner mixture of lacquer to thinner ( about 50/50) and re-spray, amalgamating the finish. You can then add your standard mixture to add a few coats, let it settle and then wet/dry, etc.

The best course of action is to keep the instrument in a more stable environment during the winter months. I don't know how many coats you sprayed on the first time, but maybe it was too thick, or the mixture was not right?

Pete Howlett
03-18-2016, 06:34 AM
Or use instrument lacquer.

greenscoe
03-18-2016, 06:55 AM
Or use instrument lacquer.


It was Bolgers Nitro cellulose Guitar lacquer.

Beau Hannam Ukuleles
03-18-2016, 08:15 AM
Was the lack of humidity your fault?- ie did you have your house heated and not humidify your uke at around 45%???- if so, and you fix it, will you do that again?

If it was your fault, that would suggest you didn't humidify the wood during the build. Again, if so, and you fix it, will you do that again?

Also, exposure to cold (even if humidified) also cracks wood and finish.

IF its just yours to own, leave it and move on- you will learn more by just building and spraying a new uke.

If you want to experiment for the sake of learning finishing techniques, the you could:

1- Use retarder thinners to melt the cracks- normal nitro thinners usually flashes off to quick, but try normal thinners first- you never know. This will need some pretty heavy leaveling as the retarder thinners will melt into it all and create shallow trenches.

2- Get actetone/thinners on a rag and rub the nitro into itself until the cracks are gone- you will very probably go back to wood doing this.

3- Remove finish (Wipe off with acetone/thinners is quickest) and re spray.

Michael Smith
03-18-2016, 08:37 AM
Certain lacquers require sealer to prevent cracking. Not sure if the one you used. The idea being the sealer is just a little flexible and acts as a buffer between the moving wood and the harder top coat.

greenscoe
03-18-2016, 11:33 AM
Thanks for the responses, there are a few things to consider. I would like to eliminate the cracks so perhaps I'll try removing the soundboard finish (as suggested by Beau) and have another go at spraying it this Summer. I think I will stick to a Tru Oil finish on my future instruments!

For the record, I did use a sealer/primer as recommended by Bolger.

The cracking occurred whilst I was abroad for 2 weeks. The temperature in my house during that time was about 12-14C rather than the normal 20-24C, so this superficially appears to be the cause of the cracking.

Michael N.
03-18-2016, 12:50 PM
Doubt it. If a finish cracks because of a 10 C temperature drop it's a hopeless finish. A big swing in humidity might have done it. Nitro cellulose is known for cracking and although over the years it's certainly improved in that respect I still see signs of it. It's one of the problems with hard finishes, they tend to be brittle and don't move with the wood.

Red Cliff
03-18-2016, 01:10 PM
I would say that in cumbria a 10C temperature drop is normally accompanied by a change in humidity, so one may be a good proxy for the other. Cumbria is also called the lake district for a good reason, there is sometimes a lot of moisture about, and sometimes not.

sequoia
03-18-2016, 08:15 PM
Greenscoe please send pictures for these sorts of things. A picture says a thousand words. We could actually see what is going on. Is this cracking or crazing? Just from your description it sounds like you didn't get good layering and things separated. Solution: Ditch the nitro. Send Pictures!

Allen
03-18-2016, 10:23 PM
That temperature is not a big enough swing to have been the problem.

My hunch is that it's been applied far too thick. It's unbelievably common for people to think that they are doing the right thing when they are spraying. Getting that gloss up by building finish and sanding back, and repeat. The end result is a film build far more than they planned on, or didn't realise they were doing.

If after a week or so of you buffing out the instrument to a high gloss in lacquer that is a nice thin finish, I would expect to see the grain lines of the timber to be telegraphing through on a soft wood soundboard. This comes from normal fluctuations in the ambient temperature and relative humidity. If you don't see this, then the finish is too thick. Being too thick exacerbates the flexibility problems with a hard finish.

I get emails at least a couple times a month from aspiring luthiers asking questions and advice about these sort of issues. And when we get to the exact steps they took, and the reduction of the material they were spraying, it comes out as the problem in almost all cases.

For instance. The lacquer I use recommends to spray with none to at the most 20% reduction. I tried one instrument like that and noticed right away it was way too heavy and uncontrollable. I switched it 100% reduction with 5 % of that being extra slow thinners, and have no issues at all. And my instruments are all over the world from the very hot and dry climate of Central and Western Australia, very humid Malaysia, and all across the USA / Canada, Europe and South America.

greenscoe
03-19-2016, 12:39 AM
Allen/BlackBearUkes

You are confirming what I suspected and asked in my first posting, namely whether too thick an application would result in cracking. I realise that for the cracking to have occurred there must be an underlying problem, hence my use of the word 'superficial' in attributing the temperature/humidity change as the cause.

The manufacturer recommended the use of up to 5% thinner. I thinned more than this (maybe 10-20%), but built up quite a few coats, knowing I would be removing a fair amount to get a mirror finish. The soundboard doesn't have that washboard appearance of a thin finish. This may not be the whole story: I may have made other mistakes.

Lack of proper equipment and knowledge/experience often result in failure. I did my homework before using this finishing technique and it appeared I'd done OK until the cracks appeared. Clearly there's more to learn if I should choose to use this finish again.

My reason for posting was to ask about rectifying the existing problem: I think I have the answer. It looks like I need to remove the finish and reapply fewer thinned lacquer coats rather than trying to repair the existing finish.

Sequoia, I wasn't able to take a photo that clearly showed the cracks, that's why I didn't post one. (There are only 3, all along grain lines)

Beau Hannam Ukuleles
03-19-2016, 04:25 AM
If after a week or so of you buffing out the instrument to a high gloss in lacquer that is a nice thin finish, I would expect to see the grain lines of the timber to be telegraphing through on a soft wood soundboard. This comes from normal fluctuations in the ambient temperature and relative humidity. If you don't see this, then the finish is too thick. .

Yes indeed.
like this- http://www.beauhannamguitars.com/Finishes.html

Hluth
03-19-2016, 07:33 AM
Here’s a spruce top assembled at 45% rh that was in as low as 35% over the winter (top photo), then placed in a 55% environment for a week (bottom photo). The early wood in softwood grain is a moisture sponge that can rapidly acquire or release moisture.

Another example of this is when fresh glue under the bridge actually causes the early wood around the bridge to swell temporarily and show the grain while the newly buffed surrounding area remains flat in appearance.

89529

sequoia
03-19-2016, 06:18 PM
Thanks Greenscoe for trying to send pictures. My reason for asking was to see if the finish was cracked or crazed. From your description; "The finish is cracked along the grain lines" sounds more like the wood has moved under your finish. If the wood moves, no matter how good/thin/thick your finish is, it is gonna crack. Maybe this is a wood issue and not a finish issue?

The pictures that Jerry posted almost make my skin crawl. Hammers home the reality that wood is a dynamic substance that expands and contracts in ways that we can't always control. Such is the nature of the beast.

Michael N.
03-19-2016, 11:44 PM
It's only a wood issue if the movement is way out of the norm. Either wood that wasn't seasoned properly or huge movements in relative humidity. I very, very rarely see cracks in shellac finishes (it can happen) but I've seen countless dozens in nitro. Given that some of the shellac guitars are 100+ years old they are highly unlikely to have been kept in humidity controlled environments. On the other hand nitro has a reputation for cracking, largely due to it's hard and brittle nature.
Next time you spray do a test strip at exactly the same time. Measure the thickness of the wood before and after you have completed all the coats. Of course you'll need an accurate caliper for this. There's probably a target thickness for nitro.

Hluth
03-20-2016, 06:50 AM
Thanks Greenscoe for trying to send pictures. My reason for asking was to see if the finish was cracked or crazed. From your description; "The finish is cracked along the grain lines" sounds more like the wood has moved under your finish. If the wood moves, no matter how good/thin/thick your finish is, it is gonna crack. Maybe this is a wood issue and not a finish issue?


Maybe, perhaps the cedar wasn't fully dry (stabilized) before gluing and that the wood is actually cracked under the finish? An inspection light inside the body would rule this out.

BlackBearUkes
03-20-2016, 12:35 PM
All lacquers will crack over time, it is what they do. IMO it is not such a bad thing. Even the new poly finishes, which are harder than lacquer, will crack. If this fact bothers you, I would suggest staying with a finish that is more to your liking. I also like it when a instrument looks its age. If a 50 year old uke looks like it is brand new, it tells me it was under a bed and not used. This may be OK with a collector, but not a player. A finish is there to protect the wood, nothing more.

mzuch
03-20-2016, 02:32 PM
All lacquers will crack over time, it is what they do. IMO it is not such a bad thing. Even the new poly finishes, which are harder than lacquer, will crack. If this fact bothers you, I would suggest staying with a finish that is more to your liking. I also like it when a instrument looks its age. If a 50 year old uke looks like it is brand new, it tells me it was under a bed and not used. This may be OK with a collector, but not a player. A finish is there to protect the wood, nothing more.

This is one of times I wish there was a "like" button in this forum.