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Paint bubbles up
http://mowrystrings.luthiersforum.com/forum/viewtopic.php?f=10101&t=51301		 Page 1 of 1
Author:  Spyder [ Mon Dec 03, 2018 7:38 pm ]
Post subject:  Paint bubbles up
OK folks, time to call in the experts. Still having trouble finishing an instrument, and I have no idea why. This time I went with all known, good stuff. used the same mix-myself shellac I've used successfully for years, and coated it with black lacquer from Stew Mac. No more off the shelf stuff, no more taking chances. All went well except for one area on the back of the mandolin I am rebuilding. The finish looked great, except it didn't stick in one area. Like a chemical compatibility issue. I finally managed to get a good picture of what it looks like, not so easy on a black instrument.

Today I sanded this back down, recoated with several coats of shellac, and painted it again. Same thing happened, just not in exactly the same spot (but same general area.) strange thing is, first coat of paint went on fine, it was the second coat that crinkled up.

So, I'm open for suggestions here. I am obviously missing something, and have no idea what. Why would the first coat go on well, and the second coat look great except in one area? What is going on here?

Any help much appreciated, right now I am stumped.
Author:  Glen H [ Mon Dec 03, 2018 8:13 pm ]
Post subject:  Re: Paint bubbles up
Hmm. How long did you let the first coat of black dry before second? I’m assuming nitro black lacquer? Black tiny can slow down the cure/set time. I’m just thinking out loud here. If you had sprayed on top of something other than shellac, I’d say it was lifting the undercoat.
Author:  Spyder [ Mon Dec 03, 2018 9:15 pm ]
Post subject:  Re: Paint bubbles up
Attachment:
DSCN6604R800.jpg
I used nitro lacquer from Stew Mac. When this first happened, I tried sanding it down. That went well, better than i thought. Top of bad material sanded right off.

Attachment:
DSCN6612R800.jpg


That resulted in the photo in the original post. Today, I started over. First I cleaned the entire back with naptha, then used lacquer thinner to make sure everything was stripped as we as I could get it. Then I sanded the whole area down again.

Attachment:
DSCN6633R800.jpg


Then coated the entire area with shellac. I used multiple coats to make sure everything was covered. I gave the first coat a light sanding as it raised the grain. Later coats were shellac only, to make sure I took no chances of adding anything to the surface.

Attachment:
DSCN6635R800.jpg


When that was dry, I painted the back again. First coat went on light, but well. I waited 2 hours between coats, as per directions on the can. after the second coat, more areas came up. Not in exactly the same area, but along the edges of it, in some places where it was bad before is good now, where was good is now bad.

Attachment:
DSCN6641R800.jpg


Attachment:
DSCN6647R800.jpg
Author:  Glen H [ Mon Dec 03, 2018 9:32 pm ]
Post subject:  Re: Paint bubbles up
Phil, looks like you’re doing everything right. I’m at a loss . Maybe someone else can shed some light. If not I’d contact Stew mac.
Author:  StevenWheeler [ Mon Dec 03, 2018 10:03 pm ]
Post subject:  Re: Paint bubbles up
That looks like what you would get if you sprayed nitro lacquer over acrylic lacquer. Was the back stripped to bare wood before you started finishing?
Author:  Spyder [ Mon Dec 03, 2018 10:14 pm ]
Post subject:  Re: Paint bubbles up
StevenWheeler wrote:
That looks like what you would get if you sprayed nitro lacquer over acrylic lacquer. Was the back stripped to bare wood before you started finishing?


If you look at my pictures, you can see how much of the original black paint was left each time before I applied new. There was actually more black paint on the first time. I figured the stuff would flake off from the look of it, but it didn't, as you can see in one photo. I did my best to seal the whole back with shellac, making sure to cover the problem areas well. I don't know what kind of paint was on it before, I do know it was at least touched up around 30+ years ago. But if it is one type over another, why does it only react this way in small areas, and not the whole back? At least 85% of the surface is fine.
Author:  Glen H [ Mon Dec 03, 2018 10:42 pm ]
Post subject:  Re: Paint bubbles up
Phil, I completely missed that the black in the first pic was existing finish. I thought that was your first coats. I would lean to my first suspicion, lifted finish. The solvent in the nitro was hotter than whatever that original black was. I once built a house and sprayed the ceilings with latex, then nailed up crown mold and sprayed lacquer undercoater on the crown, overlapping the ceiling an inch or two. 18 months later, the latex lifted off in a sheet from the ceiling.
Author:  StevenWheeler [ Tue Dec 04, 2018 7:32 am ]
Post subject:  Re: Paint bubbles up
The problem areas seem to be at the feathered edge of the old finish. The obvious suggestion is to strip all the original finish.
Author:  Haans [ Tue Dec 04, 2018 8:31 am ]
Post subject:  Re: Paint bubbles up
Ugh...
What kind of mandolin is it?
Author:  B. Howard [ Tue Dec 04, 2018 8:36 am ]
Post subject:  Re: Paint bubbles up
Surface crawl..... can be due to to heavy of a coat, drafts in drying area, low temp drying. Also looks from the pics that the surface prep is a bit to smooth which doesn't help.

What is happening is the coating is drying faster on outside than inside the coating causing the surface to crust up while the wet underneath keeps flowing.

If you do not have it here is my handy problem solving chart https://howardguitars.blogspot.com/2015 ... rt_18.html
Author:  Brad Goodman [ Tue Dec 04, 2018 9:59 am ]
Post subject:  Re: Paint bubbles up
The fact that it keeps happening in the same general area could be silicone contamination-which usually results in fish-eye.

If it happened to me, I would add fish-eye destroyer to the lacquer, which of course you can't do with the rattle can....
Author:  Bobc [ Tue Dec 04, 2018 10:11 am ]
Post subject:  Re: Paint bubbles up
I would sand the strip the whole back down to bare wood and then seal before spraying any more lacquer.
Author:  Clay S. [ Tue Dec 04, 2018 11:26 am ]
Post subject:  Re: Paint bubbles up
"The fact that it keeps happening in the same general area could be silicone contamination-which usually results in fish-eye.
If it happened to me, I would add fish-eye destroyer to the lacquer, which of course you can't do with the rattle can...."

The downside to fisheye eliminator is that it is silicone you are adding to the lacquer and it contaminates the spraygun. You then - always - have to add fisheye eliminator to your lacquer when you use that spraygun. It's good to keep a cheap spraygun around for the times you want to use fisheye eliminator.
Author:  Spyder [ Tue Dec 04, 2018 11:29 am ]
Post subject:  Re: Paint bubbles up
B. Howard wrote:
Surface crawl..... can be due to to heavy of a coat, drafts in drying area, low temp drying. Also looks from the pics that the surface prep is a bit to smooth which doesn't help.

What is happening is the coating is drying faster on outside than inside the coating causing the surface to crust up while the wet underneath keeps flowing.

If you do not have it here is my handy problem solving chart https://howardguitars.blogspot.com/2015 ... rt_18.html


Brian, I think you may have the answer. I was actually starting to wonder if that second coat was too thick. As for smoothness of surface, sanding to bare wood, etc, could be. But first time, sanded lightly and all went away, it was clearly on the top surface only since what was underneath didn't chip, scrape off, etc. buffed that down with steel wool, it crinkled again. Second time I sanded to bare wood, sealed all with shellac. What was taken to bare wood came out fine, edges did not. Before, edges were fine. That's what's had me scratching my head so much, the fact that the crinkled areas move. And didn't happen until the second coat, which was much heaver than the first coat.

Thanks again, and i appreciate the link to your chart. Think I'll try this again when the weather improves.
Author:  Spyder [ Tue Dec 04, 2018 11:31 am ]
Post subject:  Re: Paint bubbles up
Bobc wrote:
I would sand the strip the whole back down to bare wood and then seal before spraying any more lacquer.


Big question is, seal with what? I figured shellac would work. What is the best sealer/primer for use with this stuff?
Author:  Spyder [ Tue Dec 04, 2018 11:49 am ]
Post subject:  Re: Paint bubbles up
Haans wrote:
Ugh...
What kind of mandolin is it?

This one is worth the effort, and worth saving. It is a Gibson A. Not sure what number, think it was built pre-number designations. But it is a top of the line, including Handel tuners. I have only seen one more on the net that had more inlays. From the serial number, best info I have found is that it was made in 1905.

I have done a lot of work to this thing, and learned a lot. It was a truly interesting trip through the history of mandolin making. Saw many things that are no longer done, most for good reason. A tough job, but also a great experience.

When I got it, looked like someone had stepped on it. The top was cracked, but sides that were rotten and too thin to start with may have been the biggest problem. But it is close to being able to sing once again, after all these years.

I hope to play it at least once during my concert on Dec. 16th, so I am running out of time. Think I'll buff the back and string it back up, and work on the finish when the weather improves. We're about to hit a cold wave this week, no way to keep the shop warm enough for painting since I won't be able to do any other work while the paint dries.

Thanks again to all for the help, I'll try this again, and hopefully not have to go with a "distressed" look. [:Y:]
Author:  Dave m2 [ Tue Dec 04, 2018 12:04 pm ]
Post subject:  Re: Paint bubbles up
Brian that problem solving chart is really useful. Thanks for making it available. Dave
Author:  Haans [ Tue Dec 04, 2018 12:31 pm ]
Post subject:  Re: Paint bubbles up
That you are seemingly getting crinkling next to where you sanded makes me think it is oil varnish that is causing the problem. Those mandolins were finished with oil varnish and FP over. Looking at photo 5 and beyond is clear that you sanded and the crinkling migrated to the orangey areas. Lacquer over oil varnish doesn't work. A couple coats of shellac won't work as the solvent will work right through it.

Strip the back with zip strip (careful not to get it on the binding) and just finish in lacquer if you must. BTW, black most likely is not the original color, likely it was red/brown (cordovan) dye.

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