Official Luthiers Forum!

Hi!

Have an 80 yr old instrument.

Was told that new nitro readily bonds with old nitro-- is this true?

In fact, I am working on a piano at the moment-- but the luthier community is large and experienced, and guitars used nitro for a long time, so you guys might know.

More specifically...
I already resprayed the legs, using a friend's spray equipment. First time using a paint gun; it was not a bad experience.
They were in bad shape, with flaking, crackled finish all over. I sanded them very aggressively, even down to bare wood where the finish was coming off-- and also where veneer repairs had to be made.
These came out beautifully.

The music board was in even worse shape (burned...? doesn't matter) so it's even more aggressively sanded down.
I don't expect anything but good results here, too-- spraying later today.

Now, on Thursday if I can find the time, I want to rework the entire body of the piano itself, having pulled off every peripheral bit of finished wood that I could.
Much of it is not in bad shape. Not like the legs, music desk, or fallboard anyway-- not half so bad.
So... I'm wondering if I can just clean off any possible wax with alcohol and steel wool,
sand it very lightly with 220,
and spray away?
On the basis that the finish is showing crackled texture, but is not, in fact, flaking off, across much of the body of the case. Lots of the damage seems sun-related, you see, and that hit some portions worse than others. The fallboard had physical dents all over, the music desk was burned, but the rest of the finish repair is crackling just from sun exposure. It's a fair guess, because the lyre (where the pedals are) was the only part of the whole piano whose finish was in perfect condition-- and it's the only part completely shaded too.

Can this just be cleaned, lightly scuffed, and sprayed over? Or must it, too, be sanded down fairly far? I'd prefer to do the minimum, if only because there's probably 40 square feet of total surface left. The peripherals were small in comparison.


Secondary question.
So far done just the legs.
The piano is mahogany. Going for a satin finish, but still, as smooth as possible.
I remember from my french polish dabbling that pumice made an excellent grain filler.
I figured I was refinishing, and never thought twice, but it seems that some of the texture of the grain showed up in the resprayed legs. Some of that is because portions were taken down to bare wood (I stained these parts to match-- it worked very nicely), and I'll be honest, I just forgot about filling grain and didn't think of it.
But I'd swear that grain marks showed up, light ones, even over the old finish that was sanded down a bit to blend but left intact....

Is there a way to fill these marks AFTER spraying? Between coats of nitro, or after the last coat?
Or, just sand them out when I'm all finished, and buff it out...
Or maybe the final coat of paste wax will fill them in?




thanks for the help!!

I'm no expert on lacquer, but I've been around here enough to know that new lacquer does bond quite well with old lacquer.
The crazing that you've attributed to sunlight (that's partially correct, it's usually caused by rapid temperature changes) is probably fine to overspray.
I don't have an answer for the open pores, at least not one you haven't already considered.
It would probably be worth it to wait on one of the nitro experts, they can answer your questions from thier personal experience.

_________________
A man hears what he wants to hear, and disreguards the rest. Paul Simon

Roger Knox wrote:
"The crazing that you've attributed to sunlight (that's partially correct, it's usually caused by rapid temperature changes) is probably fine to overspray."

Actually, I think that while short-term crazing is often caused by temperature swings, over the long term sunlight can do a lot of damage to nitro. The finish itself is partly chemically unstable, and sunlight can provide enough energy to kick over the reaction that breaks it down. The yellowing you see in old nitro is one of the first signs of this, and it's why Detroit stopped using the stuff as an auto finish pretty early on.

Is it the UV in sunlight that provides that energy, and not just the heating from direct sunlight?

I've got an old guitar that was refinished in the early 70's, and now it's completely crazed. It happened slowly over a period of years, maybe decades. While it's seen more than it's share of temperature swings, mostly from inside the case, it has hardly seen the light of day.

I'm certainly not going to dispute that sunlight may cause crazing just because I've got one that wasn't caused by exposure to sunlight.

Back to the OP's question, is it OK to overspray a crazed finish as I led him to believe? I'm not nearly as confident in that advice if it's cause is chemistry powered by UV.

_________________
A man hears what he wants to hear, and disreguards the rest. Paul Simon

Old cracked nitro can be repaired with a minimum of muss and fuss.....well compared to some other finishes anyways. First a thorough cleaning is in order to make sure the surface is free of dirt and wax etc. Naptha works good for this with some clean cotton rags and some small brushes. The crazing from what I have seen is usually from too high of a dry film build. Straight-line cracks with the grain can be due to temp or humidity or impacts or a dozen other things. Extreme temps often will cause it to flake a bit as well. Once you have it clean all you often need to do is re-flow the existing coating. This can be done with a number of things like Cellosolve, Butyl acetate or straight retarder. Start spraying coats of the solvent onto the surface and keep an eye on it, when it flashes coat again. In short order the coating will start to become liquid again. Be careful! It will get wet like it was in the can if you go to far and drip right off. You want to get it a bit beyond jelled and let it go. It will flow back out. It will be tacky for days, let it dry, don't mess with it. Careful with a fan, you may get tiny bubbles in your finish. When dry you can buff, but it should be almost smooth as glass. Or you can apply more coats if needed. Even if I am just going to re-coat an old finish and buff out, if it is more than 10 years old I will generally re-hydrate the existing finish with some of my slowest reducer to get a better "bite"

_________________
Brian

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

https://www.howardguitarsdelaware.com/

thanks for the advice!

Not sure if I'll have enough time in the 'booth' to take 10 days to rehydrate, the body of the case will have to be moved out before then...

but good to hear that I can definitely just repair the old finish.
also thanks for naming naptha as the solvent to use for cleaning. I really didn't know what to use.

Unfortunately, folks on the piano forum were very quick to just say, 'strip it all! start afresh.' when I LIKE the old finish, just want to freshen it, and clean up 80 years of dents and dings.

But it's the body of the piano that's already in good shape, and won't be sanded quite so thoroughly, that I wanted to be sure of my prepwork for.
Sounds like it won't be too bad!

Will post photos in 2 weeks