Thread: old nitrocellulose finish on a guitar

Rod,

Lacquer is easier to repair then varnish, because lacquer dissolves and varnish will not. As lacquer ages, it may lose it plastercizing feature and may not reflow back, and could cause problems when you try to coat it.

I sugesst you buy an aerosol can of gloss lacquer, and spray a coat on a small area on the side, watch the lacquer closely to see if it opens up the lacquer on the side, and then closes up without any wrinkling or cracking. If it flows back your in business.

If not, then try padding it with shellac to see if it works and your getting adhesion.

Once you know, come back and let us know.

Good Luck

Thank you for your help, I assume you are suggesting nitrocellulose lacquer, in an arosol can? This seems to be unavailable anywhere I have tried to buy it here in Australia, perhaps someone here can tell me where to try. Do you think the finish would be compatable with shellac? perhaps I could do a sort of french polish finish? that is something I am a bit more familiar with. I am also wondering about filling and colouring the damage (chips and scratches etc). Their is very little wood grain showing through the origional finish, so I am thinking maybe I can match the colour with acrilic model paint, ( including faking the grain of the wood)

Yes, I was referring to nitrocellulose lacquer.

Yes, it should be compatible, and you should be able to frennch polish on the nitrocellulose lacquer.

You first need to dewax and clean the finish, you can do this with white spirits and some clean cloth, keep wiping until the clothes are clean.

You could try increasing the alcohol (meths) in the french polish and work in circles this may remove the crazing in the coating, once the crazing is out, if it comes out, then pad in the conventional method. Do this on a small area to see it you can get it to work.

To try repaiiring the defects you might want to try filling the defects by using a small brush and dripping in the shellac and slightly over fill the defects, then allow the meths to evaporate and harden. Then with a sanding block and sandpaper level the dry shellac. You might want to do your coloring and graining before you do the filling. Then the French Polish can be used to pad over the defects and blend in the damages and the guitar.

Do some practicing in a small area until you get the feel.

Good Luck

Mac, I appreciate your advice, I think I will try the french polish approach as I feel more confident using shellac.I will probably be some time before I get to it as I have several other repair jobs to get done done first, however, when I do it I will let you know the results.

Many thanks,

Rod

Stop!! don't try to mix shellac with nitro cellulose lacquer. You will end up in a terrible mess. Shellac is used as a finish by itself, it is dissolved in metho and is usually only used (other than a finish) to coat over other finishes to seal them off.

The N/C lacquer is originally a fairly clear coating the shellac is brown in comparison. Metho will slightly dissolve the N/C and you will be rubbing the two together and will probably get a mess.

There is a proper way to work N/C lacquer. Being a lacquer it will redissolve if its own correct thinner thinner is put on it and this feature lends its self to working the lacquer as you would shellac if you were french polishing.

The method is described here;

http://www.woodworkforums.ubeaut.com...durwood&page=2


Read this carefully and try it first on an area which is not important until you get the feel of it.

You won't find N/C lacquer in pressure pack cans easily if a t all and don't use Acrylic lacquer its a different material especially under the N/C lacquer to colour it unless you apply very thin coats and leave it to dry for several days . N/C is still available along with its thinner but in a tin and it will have to be applied by a spray gun. You could brush or dab small chips with the N/C lacquer to build up the hollow and then fine sand and rub with the pad as described.

The crazing may repair itself if redisolved so try it also with the pad. I personally would spray a thin coat of N/C lacquer onto the crazing but I would not recommend you do so unless you are very at ease spraying thin lacquer or you may end up with runs and sags.

Stopp!!
I agree with durwood but will go one step further,
If this is a vintage guitar dont touch it without consulting an expert in old guitars.
Messing about with the original finish can reduce its value to almost zero.

Astrid

The Guitar is not particularly valuable ( a few hundred dollars maybe). I am aware of the concerns about old instruments, so I will be proceeding with caution. Thanks for your comment.

Rod

Durowood,

Lacquer and Shellac are compatible coatings, in fact, the Lacquer Sanding Sealer which is used under the Lacquer is made up fron a Shellac mixing Lacquer.

When finishes are contaminated with silicones, Shellac is used as the "barrier coat" to seal in the silicones, and then Nitrocellulose Lacquer is used over the sealer.

Shellac, is used to "pop" the grains" of some woods, then Lacquer is used as the final clear coats.

I could go on and on about Shellac and Lacquer being compatible and used in combination.

Let him do it your way, your materials may not be the same as they are over here.

MacS
There is definately a big difference in sanding sealer in the USA and Australia. I don't think there are any N/C sealers with shellac in them here . In the industry here there has always been a distinct difference between using lacquer based products and shellac. Shellac is always the cheap, easy, least effective way to produce a durable lacquer finish. If you want to produce a first class finish you use lacquer sanding sealer and then the clear lacquer.

Your silicon must also be different but I don't see how it could be. Silicon is a natural repeller so painting over it produces the usually seen fish eyes. Paints don't stick to it so painting over it means the end result is a finish with poor adhesion to the surface. Sealing fish eyes with shellac comes under poor practice in my book if its a worthy restoration then only removal of the silicon is acceptable before proceeding.

Durowood,

The finest furniture made by our best woodworkers are not done in Lacquers, but are French Polished, or done with Wiping Varnishes.

French Polishing certainly is not considered a "cheaper" finish. It’s the Lacquer that is the cheap finish, Lacquer is the commercial coating used in production finishing.

Many restores over here, first use a chemical to wash down the surface to try removing the silicones, then they use the Shellac as a barrier coat, and then either French polish or apply a few coats with lacquer, they have been doing this for years. You know that you do not strip off the finish of antiques or other valuable pieces; this is one way of doing it.

Meths are one of the solvents that are used in nitrocellulose over here, I don’t know about your products. I will try to find some MSDS and see if I can find out what's in your lacquers that is not compatible with Shellacs.

I've used shellac over and under lacquers with good success.

I suggest Durwood study Bob Flexner's ? works on finishing.
He's an industrial chemist as well as being a woodie

If you just want to repair a few chips you can use nail polish which is nitrocellulose lacquer.
Just do a drop fill, leave it a few weeks then scrape sand and polish to blend with the old.
Calling shellac "Compatible" with NC lacquer is a bit misleading, sure you can use it under or over lacquer but you do not mix it.
Shellac is metho (alcohol) soluble
NC is acetone soluble

I'm not suggesting that you don't use shellac, as a finish its great in the right place such as on antique furniture but if you use it in conjunction with other finishes ( and you can use it with any type thats why its always suggested as a sealer) it weakens the finish,

If you wanted to use it on a surface thats OK but N/C lacquer is used when the limitations of shellac are a problem. French polish requires lots of hand finishing to produce its full gloss. N/C lacquer is a lot faster to do to get a gloss. You can easily compound and buff it using a machine something thats not an option with shellac you have to work it by hand. One of the reasons fine furniture has shellac as a finish it fits in with the hand craftmanship of the piece.

On modern furniture or guitars etc N/C is used to give a better more servicable finish than shellac does. For instance on a table where you don't want to have to put down coasters or table cloths to protect the surface from heat or sunlight.

If you introduce shellac into the process you degrade the performance of the final resulting finish. Thats why sanding sealer is used instead of the old system of sealing with shellac. Furniture which would fail the hot cup test with shellac as a sealer become a better finish.

Same would be the case if you used shellac as a sealer under polyurethane or any other clear you weaken the result because the shellac is so poor where the finishing process is designed to upgrade the end performance of the finishes

There have been several threads here on the forum recently where shellac has been the reason for the failures of finish. Sure its a good sealer but its not as good as the newer ones designed to go under the more modern tougher finishes.

I'm not a chemist but I have worked and had contact with many and have have over 40 years in the paint industry and on numerous occasions have come across failed finishes which have had shellac used in the process. It is never a good idea to substitute a better sealer by using shellac because of its poor heat and UV resistance. It will work great on a piece of furniture ( antique furniture is a good example of that) if it is kept inside out of the sunlight and away from heat but put it under any finish where that finish does get heat or UV exposure and it will degrade the possible life of the finish. Do a simple test put shellac under N/C lacquer on a piece of timber and then do another piece of timber using its correct sanding sealer and see how long they last outside in the sun, the shellac will always fail first.

Having conducted paint technology classes for 33 years the easiest way to show students how good or bad finishes were was to do all manner of combinations on timber and metal and leave them on a test rack. Any finish that had shellac on or under it wasalways the first to fail. Ask a chemist to rank finishes and shellac would be on the bottom of the list. I don't know od a worse finish for performance.

As an example straight shellac lasts less than 3 months tops,(worse in summer better in winter) N/C about 6 months, Estapolmaybe 10 months acrylic lacquer 5 years, Urethane enamel 10-15 years depending on type.

Shellac under any of the others, less than the produce without it.

Echnidna, why would you put it over lacquer?

To repair small defects instead of stripping the whole lot and starting from scratch.
Shellack also enables the use of oil based stains under lacquer which gives you a far wider choice of stains plus enabling making your own oil base stains from pigment powders, yet finishing with lacquer.

And buttonlac can also be a nice colour base at times.

I don't use indoor finishes on outdoor furniture.