Official Luthiers Forum!

I'm sure it depends on brand and film thickness, but what is an average cure time for PU?

I assume you're talking about the non-UV cured stuff. What I spray (simtec) I can buff in 2-3 days.

_________________
ELEVATE || Next Level Lutherie
http://elevatelutherie.com
&
http://ensorguitars.com

Wow! 2-3 days is amazing!

As far as I know, the UV stuff is usually always polyester, which I'm not into.

I want to try a polyurethane...

Generally 72 hours minimum to cut and buff. like any other chemical process, max hardness is reached at about 28 days.

Why the prejudice against polyester? Polyester and polyurethane are very closely related. Polyurethanes are in the class of compounds called reaction polymers, which include epoxies, unsaturated polyesters, and phenolics

_________________
Brian

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

https://www.howardguitarsdelaware.com/

The prejudice against polyester comes from two things, mainly.

1) past experiences with how my guitars sound with it. The guitar you did with conversion varnish sounds substantially better than ones I had done with uv poly. Was it the 003.5 vs .007 film thickness, or the material?

2) Trevor Gore specifically mentions polyester as something to avoid. He seems to have been basically right about everything else in the books, I see no reason to doubt that he's right about that as well.

I know that some people make guitars that sound good with uv poly, but I can't help but wonder if they wouldn't sound even better with a different finish.

I also don't have the ability to do a polyester in my basement, but I may be able to swing a polyurethane.

I would not spray a catalyzed polyurethane in the basement of
a home. Even if you are able to remove the over spray while you are spraying the instrument, the open time (amount of time that passes before you can no longer smell the solvents flash off ) is so long with urethane, you will likely make your family and neighbors very unhappy, nauseated or worse.

_________________
jw ( o)===:::

No spraying involved...my paint room is 8ft from the furnace...

I'm going to try the gloss version of the wipe on poly I use for my satin guitars. Just a lot more coats...

Ah, gotcha'. That's an entirely different beast. Sorry but I have zero experience with that. Hopefully someone will chime in with some experience soon for you.

_________________
jw ( o)===:::

We'll see what happens...fingers crossed!

My 72 hours to buff was for 2part urethanes...... what you are using is anybody's guess. Polyester would be far superior to any air-dry polyurethane from the hardware store, that stuff is soft! Soft = damping of vibration.

_________________
Brian

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

https://www.howardguitarsdelaware.com/

Another option is brushing on EnduroVar, which is a WB polyurethane which can be brushed or sprayed. John Greven uses brushed EnduroVar over Silvertip epoxy filler, and with 60-70 guitars a year and six years or so using that system , has enough instruments in the field to be comfortable with the finishing system's durability in service. We use it on necks and some bodies where the customer's body chemistry degrades lacquer.

EnduroVar is sandable in 24 hours, but waiting 21 days gets to a fully cured, hard film which buffs nicely. Hardness/wear are 123 Koenig and 96 Taber. Another option might be General Finish's High Performance WB acrylic-modified urethane floor finish at 59 Koenig and 34 Taber...softer, but more wear resistant than EnduroVar, but for which we have very little in the way of experience re: instruments.

Keep in mind one part urethanes will likely be significantly softer than catalyzed polyurethane or catalyzed polyester, but will still be significantly harder than nitrocellulose lacquer. If full cure hardness numbers are available for your other options, it might be useful to compare, as the Koenig hardness test appears to be a good indication of the damping of a finish, as it uses a pendulum test to measure the degree of elasticity of the collision between the steel ball and the finish.

_________________
For the times they are a changin'

- Bob Dylan

Right, well we'll have to see. No idea how to get that technical data Woodie mentions.

As for damping, my satin guitars sound great with the satin PU, so we'll see what adding enough to buff does...

Lowden is smart, still gets 7k+ for satin...every other brand I know of starts buffing at 2k..

Isn't the biggest shortcoming to polyester resin long-term shrinkage, a.k.a. dimensional stability?

Ovation has used polyester resin to finish their tops for years and IMO, it's the main cause of the top cracks commonly found on these guitars.

I've re-topped a few Ovations and almost all of them show signs of curling once the top is removed from the body. Below is an example. The curling seen on this top is as extreme as it is because the cross braces have been remove from the back of the top. Even when the braces aren't removed, the tops show this type of curling.

John Hall has a wipe-on polyurethane experiment going on another forum - hope it is kosher to cite them:

http://www.kitguitarsforum.com/board/viewtopic.php?f=6&t=8498

Ed

Encouraging...

I wonder how many coats he put on...

I'd have to read it again to be sure, but I think he put on 3 a day with leveling with 600 each morning before starting a new group of 3. He did that for 4 days, let it sit for 1 week, then buffed

The ovation Fan brace pattern I believe is the main cause of the curling of ovation tops, Taylor tops do not do that, same basic finish with a traditional type bracing...... in fact I have never seen a case where it appeared that dimensional change of a wood finish actually cracked the wood!

Polyester has incredible stability over time, the main reason it is used as the binder in fibergalss. I have instruments dating from the 80's with both nitro and polyester finishes and the polyester ones have held up better. They also telegraph joint lines a lot less leading me to believe they seal better as well.(these are electric guitars)

In fact I bet beyond testing composition with acetone once you have determined it is not nitro most do not know what the finish actually is, be it urethane, poly-uretane or polyester. Even I often do not know this and do not worry about it With all the things that effect tone production like choice of wood, exact carving of braces, tightness of joinery, etc to then blame a few thousandths of an inch of plastic for vast differences in sound between instruments is quite amusing. Especially when one delves into restoration work and see exactly how some of these coveted vintage instruments were actually finished.... Every 50's & 60's Gibby I have ever restored should have been a dead sounding dog, but they weren't.

_________________
Brian

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

https://www.howardguitarsdelaware.com/

Believe me Brian, if you could hear the guitars I have with the different finishes, you'd not be so amused. Yes, every guitar sounds different. Yet, my satin brand has a very distinct tonality across all guitars from dread to parlour, mahogany to rosewood. I've built nearly 250 of them over the last five years. The ones with gloss sound entirely different, even though the identical voicing techniques were used. It ain't subtle. You'd have to be deaf not to hear it.

I always thought the biggest issue with polyester was toxicity. Not a garage or basement finish. I also thought the drawback to polyurethane was witness lines and repair-ability. I would love to use polyurethane on a guitar but I'm afraid.

_________________
Hutch

Get the heck off the couch and go build a guitar!!!!

Me too. But gotta do something. I'm going to try Post-cat royal-lac as well...

Not sure how a fan brace pattern would cause the type of curling shown in the photo I posted, especially in areas where there are no braces.

In any event, Ovation has many different brace patterns, including several different styles of X-braces, A-braces, K-braces, Quintad braces and the double fan braces used on their classical guitars. I've got several tops that exhibit this curling, all with different brace patterns and different species of wood. The only common denominator in all of these is the finish-- polyester.

I've been molding fiberglass parts for the better part of 30 years. Polyester resin is usually the last choice for quality parts. It's cheap and fast, but it's not stable over time. For that, you need either epoxy or vinyl ester. The exception is prepreg polyester SMC, which is molded in steel hot compression molds. (think: Corvette body parts)

If you've had good luck with polyester resin finishes, great. Personally, based upon my composite experience, I would choose something other than polyester to finish an acoustic guitar.

Is post cat Royal Lac something different then regualr Royal Lac? I used Royal Lac only once on one of the last guitars I built earlier this year and I rather like it. There is a learning curve and I have no doubt that I will get better using it but it's a pretty easy application.

I used a wipe on poly on the very first guitar I ever built over 25 years ago now. It actually still looks pretty good. I think it's a good choice for guitars. I am definitely one to believe that the finish has a big impact on the sound and I'm not sure if it has only to do with the thickness, or the thinness, of the finish or the finish material, or both! But something is definitely going on there. The very best sounding guitars I have ever made were FP. IDK if that is because I only started FP a few years ago and as such my building has gotten better but still that fact remains.

Interesting to hear that you see a difference in satin vs gloss, I never would have thought of that.

Let me be clear about something in that regard. The very thinnest gloss coat I've ever had applied was conversion varnish that came out around 3.5-4mil. The best sounding gloss coat so far. All others came out at 7-10.5, the latter being UV poly. Those sounded noticeable not as good to me, the primary audible difference being a strong reduction of trebles a comparatively sluggish feel. Still fine sounding guitars all, but notably different.

By comparison, the satin I do comes out at 1-1.5, if that. So thin that you can't actually measure it. With gloss, you can always measure the film thickness by how deep you have to go to get to clean wood while installing the bridge, either chiseling or routing it off, measuring the depth or measuring the flakes. With my satin, it takes about three strokes with 100 the clear the wood.

So for me, I'm also still uncertain whether it's the material, or the film thickness, or both.

In a perfect world, it'd be great to find someone with the competence to get a 4-4.5 coat in a variety of different finishes over a number of guitars of the same size and wood combos by the same builder. That would establish a baseline for more reasonable comparison, but that's unlikely ever to occur.

Yes, post-cat Royal-Lac is different than ordinary Royal-Lac. PCRL is ready to sand and buff after four days, vs the 30 day cure time of the regular stuff. At some point I'm going to need to learn to FP. I've heard it's not as hard as you'd think...

I have only used Nitro and Ktm9 for gloss finishes and it didn't seem all that difficult, relatively, to come in around 4-5 thou. It's hard to believe professional finishes are coming in at 7-10.5. Seems thick from all I have read and heard.

I have done several guitars in the past with no finish at all. If anything, it was good trebles that were noticeable. Caveat, a small body of work..

_________________
Pat

That is my assertion so far, that the largest audible effect is a damping of the trebles.

I agree that 7-10.5 is too thick for a professional finish that you pay for.