Teach Me About Gloss Finishes

[Erithon]

Hi all,
I'm not asking about gloss vs satin vs French Polish, etc. That thread has occurred numerous times. I'm asking about the different types of gloss finishes:

What is the difference between Nitrocellulose Lacquer and Polished Catalyzed Polyester? How does Catalyst Urethane fit in the picture?

I have guitars with all these finishes and they do feel slightly different, but how do these different gloss finishes affect a guitar--both in terms of sound and protection? What are their advantages and disadvantages? Is one easier to apply? Cheaper to apply?

[Simon Fay]

Hi Erithon,
You've asked a very complicated question. Here's the basic gist of it tho'

1) Lots of different "types" of finish and most can be applied as either a satin or gloss. There's spirit varnish like shellac (when hand rubbed this is called a French Polish). You have nitrocellulose lacquer which is viewed as the traditional steel string finish. You have modern finishes like urethane (like the clear gloss coat on cars) and polyester. You also have oil varnish finishes (think "violins").

2) All of the finishes serve to protect the guitar and slow moisture exchange between the wood and it's environment. The modern finishes tend to be more durable. Polyester is used by Taylor and most other high-end factory guitars. It is well suited for this application and offers the best durability. In terms of durability, urethane is better than lacquer which is a bit better than oil varnish - all of which are much more durable than a French Polish finish.

3) Oil varnish and French Polish are usually regarded as having the best tone. A lot of this is that these finishes can be applied very thin. If the other finish types are applied so that you end up with a thin finish, then they can protect the guitar but not harm the tone all that much. But even with the same film thickness, I notice my guitars sound a better with a French Polish than they do with a polyester finish. This tends to be a universally acknowledged perspective among luthiers that oil varnish and shellac will give you a bit more tone and it's why pretty much every high-end classical guitar has a French Polished top. Classical guitars are more susceptible to tone loss from the finish.

4) Broadly speaking, when it comes to tone - you want the finish to be as thin as possible while still protecting the guitar. About the thickness of a sheet of typing paper is a good ballpark to aim for. The cheaper guitars tend to have thicker finishes.

[Simon Fay]

To follow up:

Why is polyester good for factory settings?
Well it can be sprayed as higher solids content - meaning more finish gets on the guitar and less solvent evaporates. The catalyzed stuff can be nasty to work with tho'. Nitro lacquer is a bit more user friendly and why a lot of small shops use it instead of urethane or polyester.

Oil varnish and French Polish guitar finishes are much more time intensive. Not really suitable for the factory mindset.

The bottom-line is that all of these finishes are suitable for guitars. The main thing is that they are properly applied.

My personal favorite is polyester. It can be applied very, very thin and still give you excellent scratch protection. No other finish is as durable. Another advantage is that the finish doesn't sink into glue lines over time. This is an issue with pretty much every other finish type.

[Erithon]

Thank you for chiming in, Simon! You've given me a lot to think about, and I appreciate you sharing your knowledge.

[runamuck]

Originally Posted by Simon Fay

...My personal favorite is polyester. It can be applied very, very thin and still give you excellent scratch protection.

Can you recommend a brand that's worked well for you, Simon?

[MC5C]

A catalyzed finish is one where the finish undergoes a chemical process called cross-linking as part of a cure. There are pre and post catalyzed finishes, a pre-catalyzed finish has the catalyst as part of the finish and it activates some time after application, usually a day or two. A post catalyzed finish is a two part product, and a catalyst is added to the finish just before application, a good example is an epoxy finish, or a lot of automotive finishes with hardeners. There are other variations, a good example being a catalyzed finish that is activated with a UV light - it can go from spray to fully hard and ready for polish in an hour or so, and is more typical in production factory settings.

Many (not all) modern waterbased lacquers are pre-catalyzed. You don't need to mix a hardener, they are quite non-toxic, you spray your coats over a few days, then the finish dries, cures, hardens and you can polish after 2 - 4 days. One key to this process is that new coats can "burn in" - fully chemically bond - with previous coats until the cure takes place. After that, only a mechanical bond can occur. Other waterbased finishes are non-catalyzed, so a cross-linking cure does not occur, and new coats can bond to the old at least until the finish is fully dry and hard. Manufacturers are not fully open and transparent about this, clues are if they discuss cross-linking as part of a hardening process, if they require sanding for recoats after a period of time (2 - 4 days), then they are pre-cat. If they discuss excellent burn-in of coats, or optional hardeners, they are likely non-cat. Post-cat is obvious, you get two parts and have to add the hardener, and you get around 30 - 60 minutes of gun time before it hardens to a solid mass in your gun and you buy a new one. Of brands I know, EM6000 is non-cat, Britetone is pre-cat.

Nitrocellulose finish is a non-catalyzed finish that takes a long time to fully dry, and you can almost always count on new coats burning in even after years on the instrument. It's what I think of as "a live finish", it continues to evolve for the life of the instrument. You can drop-fill dings or chips on a 70 year old finish, and the new finish will blend in beautifully, with care. Nitrocellulose is an organic compound dissolved in solvent, which carries the organics and then evaporates (and is quite toxic), the organic resin left behind never fully and finally cures.

[B. Howard]

Apples,tomatoes and oranges..... They are all fruit. The finishes you mention are all clear.

Nitro is an air dry and 100 year old tech. IMHO it has seen its days.... Here are my thoughts. https://howardguitars.blogspot.com/2...cquer-era.html

And an explanation of the technology and how it works. https://howardguitars.blogspot.com/2...-lacquers.html


Catalyzed polyester is basically fiberglass resin.There is more to the chemistry and not all resins are suited for guitars.

There are also acid catalyzed amino-alkyds like conversion varnish and pre & post cat lacquers...... These are non disolvable when finished but must partially air dry before the chemical cure reaction starts.

2K urethane is more like an epoxy and while solvents do evaporate they are strictly a vehicle. The cure is pure chemical reaction and this stuff will harden up in your gun if not cleaned....

The future is UV cured materials. Virtually solvent free and almost instant drying.More on that here. https://howardguitars.blogspot.com/2...r-guitars.html

[Erithon]

To both Brians: thank you for adding your knowledge. I've read both of your posts numerous times and, in conjunction with some internet searches, now feel like I have a much better understanding of the chemistry behind finishes.

I also read several articles on your blog, Brian Howard. It's generous of you to spend your timing writing and posting those. Very, very informative!

[AshrafO]

Thanks for the info, everyone. And a question for Simon: French polish OVER polyester!? What is the purpose of that? And why would it sound better? Another question is who does your polyester and exactly how thin are they getting it?

[TomB'sox]

Originally Posted by AshrafO

Thanks for the info, everyone. And a question for Simon: French polish OVER polyester!? What is the purpose of that? And why would it sound better? Another question is who does your polyester and exactly how thin are they getting it?

I believe he was saying his French polished guitars sound better than (over) his polyester finished, not that he does a French polish over polyester.

[Simon Fay]

Originally Posted by TomB'sox

I believe he was saying his French polished guitars sound better than (over) his polyester finished, not that he does a French polish over polyester.

Listen to this man !!!

I went back and clarified my original statement. I don't do my own polyester finish - I'm just not equipped to deal with the dangerous chemicals involved in spraying some of these modern finishes. Even spraying nitro would be a challenge in my small shop. Also, I'm fairly health conscious and unless I had a state of the art spray facility, I'm not sure I would want to mess with any of this stuff. And so I outsource my finish work to folks who are very good at spraying guitar finishes.

French Polish is a different matter and that is something that I do myself and do a pretty impressive job of as well. I could look into oil varnish as that is something I would consider doing myself but quite honestly, I just prefer the advantages of polyester. IMO, it is without question, the current best finish for guitars is sprayed very thin.

I have however, started doing more guitars with polyester back/sides/neck and a French Polish top. It's sort of the best of both worlds but the player needs to be careful with the top. The nice thing about French Polish is that it is quite easy to repair.

[jaycal]

How does the Bourgeois “aged tone” cyanoacrylic finish fit into all this?

[MC5C]

CA is best known as a glue, it's a catalyzing liquid that catalyzes in the presence of water. It's used as a lacquer (a lacquer is just a hard finish in modern terms) in wood turning of pens and knife handles quite a lot, apparently. It's well known as a finish repair for drop filling. It can be mixed with linseed oil to slow the reaction speed, and control of humidity would also affect reaction speed. In terms of a finish you could apply it quite thinly, polish it, it dries hard extremely quickly, it sticks to about anything that isn't waxed or extremely glossy. You'd have to learn how to apply it, I imagine that requires some special techniques. No clue about number of coats, one guy talks about applying 20 coats to a pen barrel in 4 minutes.

One of the really neat things about CA glue is that it does not have a solvent to carry it, there is no evaporation or drying element to it's cure. It's a polymer that gets separated into it's monomer elements, as soon as it's in the presence of water or alkalines it recombines into polymer form almost instantly, and boom, job done.

[jaycal]

Thanks very much! Interesting stuff. I’ve got a Bourgeois with the Aged Tone finish, but beyond saying that it’s cyanoacrylic they seem pretty tight-lipped about it.

[mercy]

And then there is the look. I dont like the plastic look of the modern finishes. Its not that Im a traditionalist, its just the impression that those finishes give me. French polish is the best looking and nitro the next best. I would not buy a guitar with the other finishes mentioned.