Piano Refinishing Issues (Shellac and Nitro Lacquer)

Short version: Should there be any issues with a finishing regimen of Shellac->Solvent-Based Grain Fill->Shellac->Nitrocellulose Lacquer?

I have been a long time reader, but this is my first post. I have recently taken to doing antique grand piano restorations. My current specimen is a 1931 Mahogany Piano. While I have had great successes using modern products on other woodworking projects, I wanted to stick to using more traditional 'non-curing' methods for pianos so as to make repairs easier in the future.


I went through the full refinishing and stripping process, and was starting to wet sand and rub-out the lacquer where I encountered an issue where the finish started flaking. (I nearly cried as this thing looked SO good). I've since stripped it all back again and started over, thinking, perhaps, it was a fluke.


This time I did a couple test pieces being careful each step of the way. Same result.


My steps:
1) I've taken everything back to bare veneer using chemical strippers, scraper, brass brush, and steel wool. (Finishing with a scrubdown of a mix of Lacquer Thinner, Alcohol, and Naptha to make sure it is totally clean.)
2) Next I stain using Old Masters Fast Dry Oil-Based Stain (Rich Mahogany Color)
3) Spray on a wet washcoat of dewaxed shellac (1 lb cut), made within the past 3 weeks from fresh flake and alcohol
4) Grain Fill with Mohawk Black Grain Filler (applied with plastic putty knife and burlap)
5) Once dry, light buffing with Steel Wool to remove any leftover filler
6) Sprayed Wet washcoat of 1 lb dewaxed shellac
7) Multiple sprayed coats of Mohawk Nitrocellulose Piano Lacquer


I have tried two different cans of stain (one purchased 1 year ago, one purchased recently), two different brands of shellac (one purchased a year ago, one purchased this spring), two different orders of grain filler (one purchased a year ago, one purchased recently), and I have used two different orders of Piano Lacquer (ordered 2ish months apart from each other, last fall.)

I have attached an image of what I'm describing. I tried very hard to swab the chipped areas with various solvents to determine between which layers the failure was occurring. I THINK it is somewhere between steps 3 and 7 above (shellac->grain filler-> shellac ->lacquer) but it is really hard to tell as the black grain filler tends to dissolve easily and make it hard to determine where the failure actually is.

Does anyone have any thoughts or suggestions?

Attachment 482717

Link to Google Photos

Thanks,
Aaron

The finish looks exceedingly thick. If so, I'm not surprised it chipped off easily. Are you completely wiping off the oil based stain and scrubbing off the grain filler that's not in the pores? Those are the two things I'd first suspect. Also, you might want to forget the steel wool unless you clean the oil off with Naptha first. I would use a synthetic abrasive pad instead.

John

No advice, but I'll be watching closely. We have a 1933 Steinway M that needs two small replacement veneer pieces. It's Cuban Mahogany, and I have the veneer for the little ends by the keyboard, but have held off because I have no idea how to match the finish, and also a Very long to-do list.

The finish is very thick.

John thanks for the reply,
It is somewhat thick, but Mohawk does specify you can go up to 7 mil on their datasheet, so I wasn't sweating it. The original 1931 finish I stripped off was at least 2x as thick as this. https://www.mohawk-finishing.com/mhk...%20lacquer.pdf

The oil stain was definitely wiped off (It was actually on the darker side of what I wanted, so I was sure to do a quick apply, then wipe off almost immediately.)

This is my first time grain filling, so I don't have a good concept for what is 'normal'. I thought I had the grain filler wiped off well, but It IS certainly making everything MUCH darker, indicating to me that there is dye left behind. I've scrubbed enough with the burlap that it actually started to pull out of the pores and I was having to build the finish and sand back to get it level again. Are there any other things I should try to clean it? (A quick wipe with some Naptha?)

Good thought on the steel wool. Easy enough to move to a pad (or even some fine sandpaper instead.)

Aaron, The Mohawk (aka. Behlen) grain filler takes a long time to cure, you need to let the stuff dry 72 hours, I am not sure if you let it dry enough. One downside of using grain filler, is it will impart additional color to your project, got to believe the black filler will darken the color. The shellac wash coat is supposed to keep the color shift minimal, but I was never happy with the results using the product. Hope this helps.

Quote:

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Originally Posted by Robert LaPlaca

Aaron, The Mohawk (aka. Behlen) grain filler takes a long time to cure, you need to let the stuff dry 72 hours, I am not sure if you let it dry enough. One downside of using grain filler, is it will impart additional color to your project, got to believe the black filler will darken the color. The shellac wash coat is supposed to keep the color shift minimal, but I was never happy with the results using the product. Hope this helps.

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Robert, thanks for the info. I was starting to get that feeling - the first time (prior to stripping back everything) I did have a gray 'haze' on about 90% of the pores whereas the other 10% were jet black. My research led me to believe that was due to not allowing the filler to dry long enough. The datasheet says 12 hours (and I was giving it closer to 24), but you may be onto something. Could it not being totally cured cause this kind of chipping on the non-pore areas?

When you say you were never happy, is that due to the additional color it imparted (which actually worked out well in my case as a I really like the final color) or is it due to other performance problems (like the extended dry time.)

What are you using now?

I'm about to do a test piece of oak (I don't have scrap quartersawn mahogany laying around...) and one of the sections I'll dedicate to a 24 hour dry time while I'll let the others go for a few days. For what it is worth, some of the 'accessories' (like the lid prop) were made of maple (so no pores) and they did not experience these same adhesion problems.

Aaron, the oil based pore fillers like Mohawk do a fine job of filling the pores of the wood. The gray haze is a sign that your filler wasn’t dry enough, also you mentioned it was kind of soft, generally when these fillers are dry, soft isn’t a adjective one uses to describe them.

I was working with some South American Mahogany that had unbelievable chatoyance, the customer wanted the grain filled. Every time I tested the finishing schedule on some scrap mahogany, the filler killed the color and chatoyance. Finally wound up using aquacoat, which didn’t kill the fire in Mahogany, but I wasn’t impressed with its ability to fill the grain at all, even after multiple applications of the stuff.

I also suspect the grain filler.

What happened when you ran the regimen on a test piece? Did you do the fingernail test after each coat?

Even if you get good adhesion, beware that the grain filler rarely gets you all the way there. They shrink, and you need to count on some leveling and extra coats on the top. I have not found the same shrinking when using z-poxy as a grain filler. Some popular guitar makers turned me on to it and I prefer it to both wb or ob fillers if going for that polished look. It also cures quicker than oil based, so I suspect you may get fewer adhesion problems with it. But I have only topcoated with WB products, so I cannot personally vouch for that...

How are you planning to rub out the finish? You might consider automotive compounds and a buffer for that...

What is the temperature and humidity in your drying space?

Robert,
That's what I gathered from my research. On my test board I have it taped off into 8 sections. One of them I am following the same finishing schedule as previously (to hopefully recreate the issues), the other 7 I'm allowing the filler to dry 72 hours AND trying a few different changes for each segment (ex. skipping the 2nd coat of shellac, not topcoating with lacquer, etc).


The Chatoyance can be WONDERFUL on some of the mahogany. That would be so unfortunate to kill that. This mahogany has a little bit in one area, but overall it's pretty 'flat' you could say. Either way, it is still quite pretty.


Prashun,
Thanks - the 'official' test peice is currently finishing it's 72 hours of dry-time. I'll update back once I go from here. Prior to using a scrap of oak to test, I did do a fingernail test on a couple small actual mahogany pieces that I started refinishing. There didn't seem to be an issue with the 2nd layer of shellac, but there was an issue after the lacquer on top of that. Part of me wonders if it is just because the shellac was so thin (1# cut) that it was more forgiving whereas getting a few dry mills of lacquer on top makes it more likely to fail? (Or if the issue was between the shellac and lacquer.) The test board should have more answers.


Thanks for the info on the z-poxy. I do want to try to stick to the more 'traditional' methods on this one as they are so much easier to repair in the future, but for other projects that is good advice.


I was going to rub out by going up through the various grades of sandpaper and then, as you suggested, using automotive compounds to get that final shine.


Tom,
Great question. The drying space, for the most part, stays between about 55-60% RH and 75-85 deg F. It's unfortunately not as controlled as I'd like but also not the worst conditions. I have a dehumidifier running at all times to keep the humidity in check. The area is well insulated from the outdoors, but has no cooling, so I'm a bit at the mercy of the weather.

In case anyone helping (thank you all!) and/or following was curious, I just sprayed a few heavy coats of NC Lacquer this evening - I intend to let it dry a few days and then test everything. As you can see in the photos, my wonderfully old (made in WEST Germany!) RH and Temp indicator said it was ~57% humidity and 82 deg F while I was spraying.

Here are a few photos (the lacquer was still tacky when I took these.)

All were stained and allowed to dry ~22 hours.

Unless otherwise specified:
A) Shellac coats were allowed to dry for a minimum of ~22 hours
B) Grain filler was allowed to dry for a minimum ~77 hours from final coat (I applied 2 coats ~21 hrs apart)

1 (Top Right) - Shellac, Grain Fill, Sand, Shellac, Sand, Lacquer
2 - Shellac, Grain Fill, Steel Wool, Shellac, Lacquer
3 - Shellac, Grain Fill, Shellac
4 (Top Left) - Shellac, Grain Fill, Gray ScotchBrite, Shellac, Gray ScotchBrite, Lacquer

5 (Bottom Right) - Shellac, Grain Fill*, Gray ScotchBrite, Lacquer
6- Shellac*
7 Right - Shellac*, Lacquer
7 Left - Shellac,* Gray ScotchBrite, Lacquer
8 - Shellac, Grain Fill (24 hour cure)

* Items denoted with a star were allowed to dry for longer than indicated just due to finishing schedule of other sections.

https://photos.app.goo.gl/XKJDXKxU1uoFU94f9

It looks clear to me that you do not want to put shellac directly over the grain filler without first sanding or otherwise abrading it. Specimen #3 is the only one that looks rough.

John

John - The scotchbrite definitely gave the best finish and it was definitely rougher on the one I didn't abrade first. That said, that is about where the good news ends.

Long story short, anything with lacquer on top is now chipping, but anything that is just shellac (or shellac and grain filler) seems to be just fine.It does seem to be holding up better than the last go-around, but that may also be because this hasn't dried for as long so it is still more flexible.

Details:
1 - Stain, Shellac, Grain Fill, Sand, Shellac, Sand, Lacquer - Chips
2 - Stain, Shellac, Grain Fill, Steel Wool, Shellac, Lacquer - Chips
3 - Stain, Shellac, Grain Fill, Shellac - Good
4 - Stain, Shellac, Grain Fill, Gray ScotchBrite, Shellac, Gray ScotchBrite, Lacquer - Chips
5 - Stain, Shellac, Grain Fill*, Gray ScotchBrite, Lacquer - Chips
6 - Stain, Shellac* - Good
7 Right - Stain, Shellac*, Lacquer- Chips
7 Left - Stain, Shellac,* Gray ScotchBrite, Lacquer- Chips
8 - Stain, Shellac, Grain Fill (24 hour cure), Steel Wool, Shellac, Lacquer- Chips

Results:
- After 48 hours, all seemed good
- After ~1 week anything with Lacquer chips under the pressure of a marker cap (See below description), anything without Lacquer is good.

Results Photos:
https://photos.app.goo.gl/zm68sLbFYwcAKFXu5

Previously I noticed the chipping issue after about a month, and it would chip under the pressure of my fingernail. I'm not sure if this is actually better or if it the finish just hasn't hardened to the same point yet.

To be a bit more precise, for my final test where things chipped, I used a standard sharpie marker (cap down) held at about a 45 degree angle to the board and pushed/ pulled it across the surface. I used a scale to measure pressure and it was about 15lbs of downforce.

MY QUESTIONS:

1. Are my expectations too high? Is 15 lbs of force on a sharpie cap more than can be expected for brittle lacquer? It still doesn't explain the fingernail chipping previously though.
2. Is it possible that the wood is deforming, the shellac is able to move with it, but the lacquer can't so the bond splits? (I find this hard to believe with just my finger nail, but I suppose the sharpie could be doing that)
3. Is there still something causing a bonding issue? What else can I try here? I almost considering just finishing it out with shellac....
4. I have used two different shellac vendors (1st time https://www.oldworldshellac.com/, 2nd time http://www.shellacfinishes.com/) who both sold me dewaxed shellac, so I find it hard to believe that they BOTH sold me bad product but... This week I dissolved 1/4 lb of shellac in 1 quart of alcohol and let it sit for a few days. There was some white residue in the bottom of the glass jar after a few days. Is this normal for dewaxed? Or is that maybe some water contamination of some sort?

White Residue Photo:
https://photos.app.goo.gl/x5ZWNCUWvAueMKkT9

Thanks everyone!

Have you tested lacquer on the bare wood, no filler, no shellac, no stain.

It is possible you just have bad lacquer that is not drying.

The soft finish problem can also happen because the underneath layers are not drying, which means you could test your complete regimen - but give a few days between each layer before doing the next. I would first rule out that the lacquer is bad.

Usually compatibility problems will manifest for me quicker - as crazing or cracking all over the place. Shellac, if sprayed too thick can trap ethanol in the finish - which will eventually make its way to the surface - all by itself. Again, this will manifest as little pits or pimples (looks like maybe in Pic 1 there is some of that?) .

Quote:

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Originally Posted by Prashun Patel

Have you tested lacquer on the bare wood, no filler, no shellac, no stain.

It is possible you just have bad lacquer that is not drying.

The soft finish problem can also happen because the underneath layers are not drying, which means you could test your complete regimen - but give a few days between each layer before doing the next. I would first rule out that the lacquer is bad.

Usually compatibility problems will manifest for me quicker - as crazing or cracking all over the place. Shellac, if sprayed too thick can trap ethanol in the finish - which will eventually make its way to the surface - all by itself. Again, this will manifest as little pits or pimples (looks like maybe in Pic 1 there is some of that?) .

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Prashun, thanks for the suggestion - great minds must think alike as I just did that today. I took another oak board and sprayed it with just lacquer. I laid it on pretty thick over the course of about an hour (since I'm not too concerned with final appearance, just bonding ability.) I will let it sit till the end of the week and see how it does.

At the same time, I built up the shellac (on test square #3) higher to see if it is a film thickness issue (or if a backup plan can just be to use shellac.) I'll let it sit for the week as well.


I did forget to mention my dry times in the previous post:
A) All were stained with oil-based stain and allowed to dry ~22 hours.
B) Shellac coats were allowed to dry for a minimum of ~22 hours (Square 7 which had no grain filler was actually allowed to dry 4 days between shellac and lacquer)
C) Grain filler was allowed to dry for a minimum ~77 hours from final coat (I applied 2 coats ~21 hrs apart) - The can indicated a 24 hour dry time, so I tripled that.)
* Items denoted with a star were allowed to dry for longer than indicated just due to finishing schedule of other sections.

You are correct in noticing the little pits (I've always called it solvent pop). Those were in the lacquer coat, not the shellac. I built the lacquer a little too fast on that one and it popped.