Guing over epoxy coated surface

TEST ON SCRAP FIRST! If your question is about repair work, either regluing or refinishing, please post it in our Repair Section.

Guing over epoxy coated surface

Postby Enrico Schiaffella » Fri Nov 01, 2019 7:49 am

Hello,
let's see if any of you did this experiment. Otherwise I will have to try it. I was thinking to coat the whold soundboard with epoxy. Including the area where the bridge will be glued. Question is, if I sand back to the wood that area, it will still be impregnated with epoxy. Have you ever tried to glue with titebond over this area? Will titebond have adhesion problems?
Thanks Enrico
Enrico Schiaffella
 
Posts: 19
Joined: Tue Aug 28, 2012 9:46 am

Re: Guing over epoxy coated surface

Postby Bryan Bear » Fri Nov 01, 2019 9:16 am

I don't have an answer to your question because the only time I put epoxy on a soundboard, the bridge area was masked off. I will say you should re-think putting epoxy on the top. Do lots of tests on scrap.

I accidentally got Z-poxy on a WRC soundboard and thought I would just coat the whole top with thinned Z-poxy to even it out. My rationale was that this is what is typically done on back and sides to even out any sand-through areas after pore filling. The Z-poxy soaked in unevenly and became very blotchy. I should have anticipated that because that is common with stains on softwoods.
PMoMC

Take care of your feet and your feet will take care of you.
User avatar
Bryan Bear
 
Posts: 1183
Joined: Fri Jan 06, 2012 1:05 pm
Location: St. Louis, MO

Re: Guing over epoxy coated surface

Postby Brian Evans » Fri Nov 01, 2019 11:42 am

Epoxy is a post catalyzed compound, which means it gets a catalyzing agent added and then chemically cures and cross-links. After it's cured, you can only bond to it mechanically, not chemically. Normally the routine is to abrade with 80 grit to create a basis for a mechanical bond. If you try that in your experiment, the titebond might find something to grab. You could always glue the bridge with epoxy.
Brian Evans
 
Posts: 844
Joined: Sat Aug 30, 2014 8:26 am
Location: Tatamagouche, Nova Scotia

Re: Guing over epoxy coated surface

Postby Barry Daniels » Fri Nov 01, 2019 1:35 pm

Scrape the epoxy off, down to the wood, and then your glue will grab fine. Epoxy does not soak into the spruce below the surface to any significant depth.
MIMF Staff
Barry Daniels
 
Posts: 2370
Joined: Thu Jan 05, 2012 10:58 am
Location: The Woodlands, Texas

Re: Guing over epoxy coated surface

Postby Enrico Schiaffella » Fri Nov 01, 2019 2:44 pm

Ok, thanks guys!
Enrico Schiaffella
 
Posts: 19
Joined: Tue Aug 28, 2012 9:46 am

Re: Guing over epoxy coated surface

Postby Clay Schaeffer » Sat Nov 02, 2019 5:11 pm

To be perfectly honest with you, I can't think of any benefit of putting epoxy on the soundboard. Spruce is a close grained wood and doesn't require filling or sealing. For soundboards I just apply lacquer topcoat and "self seal" with that. That will give you better clarity and adhesion than using epoxy or vinyl or sanding sealers (it just won't build as fast). It's easier to scrape off than epoxy if you don't mask for the bridge.
Clay Schaeffer
 
Posts: 1339
Joined: Fri Jan 06, 2012 12:04 pm

Re: Guing over epoxy coated surface

Postby Bryan Bear » Sat Nov 02, 2019 10:10 pm

To Clay’s point, I guess we never asked why you were wanting to do this. And, why you wanted to epoxy the bridge location instead of masking.
PMoMC

Take care of your feet and your feet will take care of you.
User avatar
Bryan Bear
 
Posts: 1183
Joined: Fri Jan 06, 2012 1:05 pm
Location: St. Louis, MO

Re: Guing over epoxy coated surface

Postby Marshall Dixon » Sun Nov 03, 2019 4:39 pm

I've been using some local sugar pine for guitar tops. It is light, well quartered with no runout. You can plane it from either end.

The issue with it is the resin channels. What I did was to apply a coat of Smith's epoxy that LMI sells, as it's made to bind with oily woods. My thinking is to fill the resin channels thereby sealing in the resins. I sand back the epoxy to bare wood, just using it as a filler. This seems to work for that purpose.

But as far as glues go; the only failures I've had were with epoxy and generally don't like to use it. It has a rubbery characteristic that makes removal with sandpaper incredibly tedious. One time I removed epoxy with a chemical stripper which left a contaminant residue in the wood that caused failure. Live and learn, but what a hassle.

If I had an extra 50 cents in my budget I'd wager it all that titebond won't stick to epoxy. Hot hide glue sticks to glass. Maybe there's something to that. If you do test it I think you'll need to see how it holds up under heat.

Have you seen Frank Fords work up on glue testing? Not what you're talking about doing but good information.

http://www.frets.com/FretsPages/Luthier ... etest.html
Marshall Dixon
 
Posts: 25
Joined: Tue May 21, 2019 8:58 pm
Location: SW Oregon

Re: Guing over epoxy coated surface

Postby Enrico Schiaffella » Mon Nov 04, 2019 2:21 am

Hello, to answer some of your questions. I have been using epoxy on the soundboard for a long time now. I am talking about classical guitars. Just a thin layer. I like it because it works very well as a sealer for shellac. I french polish my guitars. Moreover, it stiffens the soundboard. In my opinion it makes the wood stronger. I build double tops now. So my top is quite thin. Therefore it is a good idea to strenghten the wood. Since I install the rosette when my soundboard is already very thin, I would like to coat with epoxy just after thinning the soundboard. Almost at the beginning of the process. It protects the soundboard. I cannot afford to sand it more than nothing before finishing. The problem is that if I coat the soundboard at the beginning of the process, I do not know the exact location of the bridge. It's a complicated story. I tried several ways of doing this. And I was thinking to try this for my next build. Anyway, I am just doing a test on a scrap piece of western red cedar and see what happens. If I manage to have titebond glue well in an area that was coated with epoxy, but sanded to wood, I might very well do this in the future, that is coating the whole soundboard with a thin layer of epoxy. I hope I was clear. I am very rusty with my english.
Enrico Schiaffella
 
Posts: 19
Joined: Tue Aug 28, 2012 9:46 am

Re: Guing over epoxy coated surface

Postby Enrico Schiaffella » Mon Nov 04, 2019 2:29 am

Marshall, I just love epoxy. Except for its toxicity, I would build and paint the whole guitar with it :-)
Few things. First of all you have to be extremely precise when mixing the two components. I always use a decimal scale and weigh them. If you are precise it hardens like glass. One time I made a small mistake and stayed rubbery.
You are right, it's a nightmare to sand. But have you tried scraping it? it's like butter.
One last thing. Even if applying epoxy is very hard. Especially sanding, To my knowledge it is the best pore filler in the market. At least the result compared to other pore fillers. Especially when french polishing. Many french polishers use epoxy. Did you see the videos by Pablo Requena on youtube?
https://www.youtube.com/watch?v=Uf4z14Vjkxk&t=2s
Enrico Schiaffella
 
Posts: 19
Joined: Tue Aug 28, 2012 9:46 am

Re: Guing over epoxy coated surface

Postby Alan Carruth » Mon Nov 04, 2019 2:29 pm

Brian Evans wrote:
"After it's cured, you can only bond to it mechanically, not chemically. Normally the routine is to abrade with 80 grit to create a basis for a mechanical bond"

Although there is some disagreement about it, I understand that the predominant school of thought these days has it that most of the strength in a glue bond is chemical. Abrading the surface is not done so much to provide a mechanical 'tooth' for gluing as to raise the 'surface energy' and yield a better chemical bond. This was worked out during WW 2 at the Forest Products Lab, when they found that gluing within 15 minutes of working the surface made a stronger joint. They also showed that smoothly planed surfaces bond better than sanded ones.

When you remove material from the surface you are breaking chemical bonds, and for some time afterward there will be 'open' bond sites looking for something to glom onto. This is referred to as having 'high surface energy'. If you put glue on within that fifteen minute window it bonds chemically to the surface. There is a simple test for this: spritz the surface with a light most of water. Water is a polar molecule, and will be attracted to the slight charges of the open bond sites. If the surface energy is high the water will spread out into a thin film, but if it's low it will bead up. They actually measure the surface energy by looking at the angle the edge of the drop makes relative to the surface.

Some time ago I made a guitar with a mammoth ivory bridge. When I got the blank it had an information sheet regarding gluing, which was said to be tricky; they recommended Titebond. It occurred to me that after 10,000 years in the ground most of the chemistry might have already happened, producing a material with inherently low surface energy. A spritz test confirmed this; the water beaded right up. Scraping the surface raised the energy a lot, but a few minutes exposure to air started to reduce it a lot. When I glued the bridge down I made sure everything was well fitted and warm, and I had the clamps all ready. I gave the bridge bottom a quick scrape to remove a thin shaving, slapped on some not hide glue, and clamped it all down expeditiously. No problems.

I found out a lot about surface energy from an article on assembling fiberglass parts in home built aircraft. The mating surfaces come with a 'peel ply'; a thin layer of fiberglass tape epoxied onto the surface. Peeling it off just before assembly raises the surface energy. This can also be enhanced by sanding if there is no peel ply; they recommended a fairly fine sandpaper, and said that the objective was to scratch the entire surface lightly before using the spritz test.

As I say, there is some discussion as to what part of the strength of the glue line is mechanical and what chemical, but my experience suggests the latter is more important.

I'll note that even with high surface energy the glue you're using may not bond well if it's not chemically compatible. As always, test on scrap....
Alan Carruth
 
Posts: 919
Joined: Sun Jan 15, 2012 1:11 pm

Re: Guing over epoxy coated surface

Postby Marshall Dixon » Mon Nov 04, 2019 10:43 pm

Alan,
Thanks for that informative high (surface) energy explaination. Looks like I'd loose that 50 cents!

Enrico,

I think if you scrape the surface to the wood there would be no problem. But I have read of concerns related to introducing moisture into the thin skin of some double tops. I recall the inner layer was a honeycomb nomex sort and the maker used a polyurethane glue.

I also use a french polish, though I've taken to spraying an acrylic lacquer on the backs and sides. I use epoxy as a filler; after trying to fill with the traditional pumice method, only to have it develope a case of small pocks months later. I don't see any shrinkage on the 2 sugar pine tops after 3 years.

I'll use regular scraper and a razor blade to scrape but as my surfaces are irregular I usually resort to sanding at some point.

You mentioned small mistakes and I can't seem to avoid them. All the failures with epoxy were my fault I'm sure. Although I recall reading that rosewood is troubling with some epoxies. I've taken to using one brand and measuring carefully.

Years ago I mixed a 3 pound cut of shellac with Smith's epoxy (from LMI) 50/50 by volume. Spread it out; it dried smooth and hard, not brittle. Often thought to try it again.

Thanks for the link. I'll take a look.
Marshall Dixon
 
Posts: 25
Joined: Tue May 21, 2019 8:58 pm
Location: SW Oregon

Re: Guing over epoxy coated surface

Postby Mark Wybierala » Tue Nov 12, 2019 2:03 pm

Much thanks to all for the science in this post. I need to reread this a few times. There is nothing better than facts if you can remember them. What I do know is that not all epoxy is the same and it also has a shelf life. I've encountered a fair number of lifting bridges on my bench and the overwhelming majority of them had finish applied to the soundboard prior to gluing the bridge in place. How much of a relationship to a true epoxy these finishes have is anybody's guess but the failure of the joint has always been the finish/soundboard rather than the glue used. Less expensive Ovation/Appause are the most frequent I've seen. Despite the structural damage to the soundboard when these bridges depart the surface, I've never had one return after cleaning up the surfaces to bare wood and using Titebond.
Mark Wybierala
 
Posts: 442
Joined: Fri Feb 03, 2012 10:14 am
Location: Central New Jersey


Return to Glues and Finishes

Who is online

Users browsing this forum: No registered users and 4 guests

Your purchase from these sites helps support the MIMForum, but only if you start at the links below!!!
Amazon music     Amazon books     Amazon tools     Rockler tools     Office Depot    

The MIMF is a member-supported forum, please consider supporting us with a donation, thanks!
 • Book store • Tool store • Links •