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I'm on my second attempt at stripping and refinishing, attempting to get a clean nitrocellulose finish. I keep ending up with white residue in the finish. I realize it's from the sanding process but I'm uncertain how to get it off and not ruin the finish. After applying 3 coats, I would lightly sand and then wipe down with mineral spirits. In some cases, I've even used a toothbrush with some mineral spirits to get out of some pores that didn't get completely filled when I grain filled.

As you can see in the pics, I still end up with white patches and specs. Any thoughts or tips?

Can you tell us how you filled the pores? It looks like finish dust from the sanding packed in the poors, but I've also experienced something similar from trying to pore fill with pumice and alcohol. Pumice that doesn't fully dissolve before getting packed into the pores will have a similar presentation.

What James said, pore filling is suspect here in my mind as well.

Many of us use epoxy for pore filling and the best pore filling video out there is on YouTube by Maryland Luthier Todd Stock. Here's a link to the first in a series of videos that he crafted on pore filling. Highly recommended!

https://youtu.be/YYHxMg7n9cI

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Ann Arbor Guitars

Nitro dust residue normally doesn't stay white, and least in my experience. If it were my git, I would spray a reduced, very wet coat of nitro on it.

I am not convinced it is a pore fill issue. I see lots of pores that look good and lots of white specs that don't seem to be pores.....

My first thought is the wipe with mineral spirits after sanding. "Mineral Spirits" can be a blend of many things and almost any solvent will react with fresh nitro. It may have actually embedded sanding debris or rag fuzz into the finish rather than remove it. Did you use paper towels for that wipe down? I swear I can see the paths where it was wiped across in that first pic....

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Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

Yup I can see the transverse wipes across the back, your correct brian .Something in the wipes reacted with the finish, including whatever solvent was used , there is a definite streak line there.

I saw those same 'wipes' and it looked to me like the gun made paths across with little over-lap and coats too light/dry. The lack of gloss makes me think the same. That's why I recommended a good wet coat. But, who knows from a pic?

It seems to me that the streaks are particles left behind not any sort of reaction -- easy enough to test do more wiping. I'd spray straight thinner, if lacquer dust it will melt and blend right in.

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Ken Cierp

http://www.kennethmichaelguitars.com/

I had that same issue when I started to learn how to spray.
For me it was too much material and too high air volume . You can get air bubbles into the material and it sets before it can get out.
I use a retarder and a thinner. set the gun up for less air volume if your finish is drying before it hits the body you can have all kinds of problems

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Appreciate all the comments and help. Let me address the process I used in a little more detail.

Pore filler was done with ColorTone grain filler. It dried for over a day before lacquer was applied. Granted, some of the pores did open back up after sanding but the problem appears either in or out of pores. I have looked at the epoxy filler option and will certainly consider that for my next build. I oiled my first guitar, lacquered the second (which had similar issues as my current build), and french polished the 3rd. (decent results)

Indeed, the "lines" you are seeing are spray lines. I wiped with old t-shirts, wet with mineral spirits, after sanding. I would typically apply 3 coats, 1.5hrs in between, and then dry for a day before sanding. Nitrocellulose has retarder and flattener in it. I'm spraying it with a gravity feed sprayer at 40psi. The final sanding (400 dry -> 800 wet -> 1200 wet) and polishing was done after 10 days from the last spray.

Interesting comment about spraying with thinner. My understanding is, that would take care of nitrocellulose dust, but if it's due to air being trapped as I'm spraying, it wouldn't help, correct?

Did you wet sand?
If so, did you add soap?
I used to wet sand with water and soap.
The soap was left in unfilled grain areas.
I wet sand with mineral spirits these days.
Dan

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Wah-wah-wah-wah
Wah

I did wet sand, but with water only. Interesting comment on the soap. Haven't tried wet sanding with mineral spirits.

I gave this a shot. Just straight thinner all over the back. While I can still see the sweeps, it's more of a haze that may buff out at this point and the spot at the top is no longer visible.




So it would appear that I may have multiple issues. Pore filling is not great... probably should move to epoxy based filling method. Spraying is not great... not exactly sure of my issue here, maybe heavier spray to prevent it from drying too quickly. If I see white appear during coats, stop and hit it with thinner before applying more coats.

Good job.

A bit late in on this on, but I see multiple issues.

First, the pore fill is not great and there seem to be a few unfilled pores, but that is probably the least of your problems. Rectification is more pore filling.

Second, the white/foggy streaks look like blooming, caused by spraying too heavy a coat on too thin a substrate at too high relative humidity. As the solvent in the lacquer evaporates, it cools the surface causing condensation from the air to form on the surface and and a foggy bloom to form. You'll get the bloom forming first on areas that are more heavily sprayed and areas of thin wood, which cool faster. Often you will see a bracing pattern outline as the extra wood of the braces has enough heat capacity to keep that area a little warmer, thus avoiding bloom. If you spray over bloom, it can sometimes be released and rectified, but can also be trapped in and require sanding out. Can be avoided by spraying in lower RH conditions or by using a retarder. Can be rectified sometimes by over-spraying with just solvent, as you've found.

Third, you have entrapped solvent bubbles, called variously pin-holing or solvent pop, depending on the exact nature of the outcome, but the cause and solutions are similar. The cause is by spaying too heavy a coat, which dries on the surface first, trapping evaporating solvent beneath, which forms bubbles within the thickness of the finish film. Sometimes these bubbles pop (solvent pop), causing a crater in the mostly dried surface. Sometimes they don't, but they can be seen as bubbles in the finish that are revealed by sanding and look like the white spots you see as sanding residue fills the bubble holes. It can tend to happen more when spraying thick on hot days. In mild cases, sometimes rectification can be achieved by over-spraying with just solvent (called thinners/reducer depending where you live).


This is why I'm writing. That is not the course I'd take. Spraying thicker/wetter will likely compound your problems. I've found it very difficult to get rid of trapped bubbles by spaying more. Spraying a mix of reducer + retarder might do it, which will take a while to dry, so at the end of the day it seems faster just to sand them out. Then when you re-coat, spray a lighter coat, likely using more thinner/reducer in lower RH conditions, which should solve most of what seem to be the problems. With the spray schedule I use (no retarder) 60% RH is about as high RH as I can spray at.

As lacquer as supplied is a very variable product (some pre-diluted, some not), there are no universal rules. For years I never had any problems spraying fairly light coats at a 50% dilution with thinners at less than 60% RH. Then the manufacturer changed the formulation but sold the product under the same code/labeling which caused a lot of people a whole bunch of different problems, including all the above and more, until they figured out again what worked.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Fantastic information! (the whole write-up, not just this part) While I knew relative humidity could play a factor, I had no idea it would generate this dramatic of an issue. Your comments and guidance are extremely helpful! I know that took some time to write-up and please understand how much I appreciate it. I'm going to work on seeing what I can do to get this finish resolved as much as possible and will work on my mixture and environmental conditions for the next run. (as well as my pore filling)

Thanks again for taking the time to put this together.

I have never seen blush that looked like that (in the pores only).

And i think we have it....it's your flattening paste is not mixed completely with your lacquer. Could be just lack of mixing or an incompatible finish/paste mix......

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Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

I believe Trevor has privided some good info, however I don't totally agree with his assesment. When I attended William's auto paint school here in Michigan I must say that guarding against air borne and in line moisture was a key factor. Fortunately as mentioned a re-spray of a slow evaporating thinner/solvent aka retarder will usually correct the issue. I keep rattle cans of the Behlens product on hand since shifts in temp and humidity can cause unexpected clouding. I would caution against trying to spray coats that are too thin and dry -- in my experience (and how I was taught) wet even coats yield the best results. Heres a pretty good trouble shooter

http://www.woodfinishsupply.com/LacTroubleShoot.html

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Ken Cierp

http://www.kennethmichaelguitars.com/

Hey tjpilson, level sand the back some with 800 girt. If white stuff forms in the pores, it's what we thought.

Interesting. I'm using all Behlen products so I would assume compatibility is good. I'm not sure how to mix other than stir together. I love flat finish guitars so I do need to figure out how to get the mixture correct. I did follow the back-of-can mixture ratio for flat.

Yes sir, I'll do that. I can pretty much guarantee I'll get white fill in the pores. That's been the case so far every time I've sanded.

Had bubbles in lacquer using Behlen finish.
I sent Mohawk an email, next day a nice feller responded and solved my problem.
Would not hurt to ask Mohawk.
Dan

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Wah

I've never seen blush in pores only, either. But on a couple of pics (on my screen at least, check out the pic IMG_8456, lower bout) I can see foggy bands, which look a lot like bloom/blush to me. (Tim called it "haze").

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

One more thing...

I'm not sure what your spray schedule is, but when aiming for a "flat" (matte) finish, it is common practice to spray all the build coats with normal gloss lacquer (no flatting agent), fully level all that when you have enough build, then just spray the last coat or two with the flatting agent added (and very well mixed!)

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au