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I've been trying to get my head around this for ages, but I can't find simple, clear definitions anywhere.
Would someone mind explaining to me what the differences are between these finishes? They all sound so similar, yet they're so different!
With lacquers you have nitro, pre-cat, post-cat (or acid-cat - are they the same?), polyurethane and polyester. Then it seems you also have polyurethane varnish, but that is totally different from polyurethane lacquer. And then you have UV cured polyester and non-UV cured polyester.
Are nitro and pre-cat lacquer the same thing?
What's the difference between polyurethane and polyester and are they connected to varnish somehow, or is that just to confuse everyone?
At work (furniture maker), we use acid cat lacquer almost exclusively, but used polyurethane lacquer for the first time last week. It seems to build much quicker than AC.
We use an hvlp system. Are you supposed to change nozzles on the gun for different types of finishes?
Finally, is AC lacquer suitable for guitars and do you get witness lines when sanding through coats? We only do matt and semi gloss finishes normally, so it's not usually an issue.

Thanks in advance!

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"I am always doing that which I cannot do, in order that I may learn how to do it."
Pablo Picasso

https://www.facebook.com/FenskeGuitars

All the coatings chemicals you mention have been used and are being used to finish both acoustic and electric guitars. In my view there is no broad brush "simple" explaination for their formulations, characteristics, application process etc.

The key is to decide on a coating, and then use a system and schedule that assures chemical compatibility -- sealer, filler, stains, top coat. The best place to get this information is from the coating manufacturer. Trying to mix and match products using home brew methods and processes has been the self inflicted headache for many, ultimately resulting in a "guitar finish re-do."

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Ken Cierp

http://www.kennethmichaelguitars.com/

There tends to be confusion due to the loose usage by some people of the term lacquer for any clear finish.
Strictly speaking a lacquer dries by evaporation.
Shellac is a lacquer as is nitrocellulose and acrylic lacquer. Catalysed pre or post just adds another form of hardening after the drying process.
Other forms of finish generally cure by chemical reaction rather than by evaporation, either by a two pack process, UV curing or reaction with air or moisture in the air.

All good questions. The problem starts with the fact that "lacquer", "varnish", "poly", etc are not technical terms and are applied to products as the manufacturer sees fit.....

Start here with this article on air dry lacquers http://howardguitars.blogspot.com/2015/02/air-dry-lacquers.html

Varnish was traditionally a finish made of various resins, drying oils and spirits. Originally the resins were all naturaul like Copal, Shellac, Gum Arabic etc bit in gthe 50's synthetic resins began to be used to increase weatherability for exterior use.

Polyurethane is a catch all phrase for virtually any closed polymer finish system. Originally it was a blend of urethanes but today the actual resin matrix may contain lot's of polyester and few urethanes....

Polyester is a synthetic resin and can be found in a number of different chemical configurations mainly having to do with method of polymerization.



Solids content of your product will determine actually build rate. I have nitro lacquer that is 30% solids and some that is 18%. And polyurethane lacquer? that's a new one on me.....




Fluid tip and needle are relative to the viscosity of the product being sprayed. for a thin Isolante I use a 1.0 -1.3. For most of my clears I use a 1.4 and for large flake metallic or high build surfacers I use a 1.8-2.0



Acrylic lacquer has been used quite successfully on guitars, should leave no witness marks when cut and buffed.

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Brian

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

https://www.howardguitarsdelaware.com/

Urushiol lacquers harden by oxidation and polymerization rather than evaporation. They've been used for thousands of years, so the confusion goes way back.

Urushiol (AKA 'Japanese lacquer') was, in fact, the original 'lacquer'. It's the milky sap from a relative of poison ivy, and some folks will get a rash from contact with oriental lacquer ware. The film is totally waterproof: lacquered wooden objects have been found in tombs that had been floating in water for hundreds of years. Basically, everything that calls itself 'lacquer', or has 'lac' in the name, such as shellac, is trying to gain status by association. It's like all the woods that are called 'mahogany' that are not even related.

You could say something similar about 'varnish'. Originally varnish was a concoction made by heating a drying oil with a resin to form a co-polymer. It combined the superior film forming ability and wear characteristics of resins with the toughness and resistance to chemicals and water of the oil. It hardened by an oxidation and polymerization reaction. Over time the word started to be used more loosely, so that violin makers, for example, call the mixtures of resins in alcohol that many of them use 'varnish', even though they harden by solvent evaporation rather than a chemical reaction. Polyurethnae varnish, iirc, works more or less like a traditional one, but hardens by taking up water rather than oxygen.

There are finishes that fall in between. Shellac, for example, initially hardens by solvent evaporation, but over time it seems to cross-link and becomes insoluble in darn near anything after about 75 years. I suspect that's the model for the catalyzed lacquers and so on.

Modern chemistry has helped to further confuse simple craftsmen like myself. I'm sure a lot of the distinctions you've asked about are very important in terms of the chemistry, and probably make a difference in how you use the stuff and even what it's good for. In the end, though, what seems to count is how well a thin coating protects the wood, how easy it is to use, and how it looks. Oh, and one other major consideration: tradition. Sometimes that trumps everything else.

Thanks for the replys. At least I know now I have a good reason to be confused!
Just one more question though. Brian, you mentioned acrylic lacquer - is that just another name for catalysed lacquer, be it pre or post cat?

Sent from my GT-I9300 using Tapatalk

_________________
"I am always doing that which I cannot do, in order that I may learn how to do it."
Pablo Picasso

https://www.facebook.com/FenskeGuitars

Except that our word "lacquer" comes from a Sanskrit root meaning "shellac". Urushiol lacquerware is ancient, but Europeans encountered shellac before they met "lacquerware". We borrowed a word, and we borrowed the wrong one. That's what I mean about the confusion.

Acrylic lacquer just uses acrylic resins dissolved in solvent as opposed to the nitrocellulose resins in nitro lacquer.
It can be uncatalysed or pre or post cat

http://woodtools.nov.ru/books/Understanding_Wood_Finishes.pdf

Here's a very good article. Happy reading!

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I'd like to be able to prove, just for once, that money wouldn't make me happy...

This is an interesting and useful thread. Thanks, Peter, for initiating the conversation and to the rest of you for sharing your knowledge.

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George :-)

Clear as mud?



Ah the joys of finishing and the myriad of finishing products.

Thanks, Eric. I had thought that shellac didn't come into wide use as a finish until the invention of the column still made reasonably pure alcohol cheap and easy to get. That was pretty late in the game: ca 1650, iirc.

Thanks Alain, that looks like a really useful booklet.

_________________
"I am always doing that which I cannot do, in order that I may learn how to do it."
Pablo Picasso

https://www.facebook.com/FenskeGuitars

You're welcome Peter. Someone else on this forum had posted a link a while back. I'm sorry I forget who. It's a great read and I always keep the link handy in my favorites.

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I'd like to be able to prove, just for once, that money wouldn't make me happy...

Alcohol is a convenient solvent for shellac, but shellac has a long history in India as a finish for wood and ceramics of being applied with heat--melted directly onto the surface. It can also be dissolved in an alkaline water solution, and applied that way. Objects can be made of solid shellac. It is famous as a dyestuff. Europeans were aware of lac dye at least as early as 250 AD.

Urushiol lacquerware created a craze in Europe when it first started being imported from Japan ca 1600 AD. Attempts to imitate these products with oil varnishes were "Japanware". It was at least another hundred years before Europeans understood the source of "lacquerware" a craft that goes back at least 9,000 years in China and Japan. Long before the process was understood, the name had stuck--confusing the shellac based products of India with the unrelated "lacquerwares" of South East Asia. This doesn't have much bearing on guitar finishing, but whenever I start to pride myself on my "craftsmanship", I find that a look at the history of lacquerware is a potent tonic.
https://www.youtube.com/watch?v=c-rF1uzYbuI

Double post.

We've been playing with soap finishes, which are easy to apply, give reasonable protection, and leave a nice satiny surface. No toxic/intoxicating solvents! Lots of options beyond shellac, nitrocellulose lacquer, polyester and polyurethanes.

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A constellation only takes shape when one maps the whole.
- Beth Brower

Lac dye is still available, and fairly expensive, iirc. Red dye was always expensive. The 'seedlac' we use is what's left when they wash out the color. Seedlac can then be processed in various ways to purify it and remove more color. What I've found is that the more you process it, the less durable it seems to be, so that 'water white' shellac is not nearly as good a finish as seedlac, or orange shellac in general.

A friend of mine points out that when there are lots of ways of doing something, it mean either than everything works, or nothing does. I think finishing is an example of the latter: there's no such thing as a 'perfect' finish, so you end up choosing the one that has the good features you need and bad ones you can live with.