MIMF

I'm using hot hide glue more and more. For me, the biggest challenge is keeping it hot enough until clamped. I've worked this out with practice and warming parts with heat gun for smaller parts. Biggest part to date is a couple fingerboards I glued down recently.

But how does one glue an acoustic guitar top or back on with HHG? More specifically, how do you keep the glue hot enough through the fitting and clamping operation? This typically takes me five minutes to complete, but the glue gels up in 30 seconds or so, even if the parts are warm.

There are several ways to do this that I've been involved with. The first time was when we needed to re-glue the top on an upright bass. We got all the clamps ready and a crew of four, and then heated the room up to about 95 degrees F. This is actually pretty close to the way they did things in furniture shops in the old days. It works, but it's no fun.

By the next time I needed to do a bass I had learned a thing or two. I dry clamped the top to be sure that it fit correctly, with the overhang even, using spool clamps made of large dowel and thread rod. Once everything was set I removed the clamps from corner to corner in the bass side C-bout, worked glue into the gap, and clamped it. Once the other C-bout had been done the same way it was fairly easy to remove the clamps from one end, except over the end block, and work around a few inches at a time from the corners toward the ends. This sequence is more or less SOP in fiddle shops.

One 'cello maker (JIm Ham, iirc) spreads hot glue on both the liners and the edges of the plates and allows it to dry. Then he dry clamps everything. Then he releases clamps sequentially, but instead of working in glue he just injects steam to reactivate the glue that's there. This has got to be the neatest way.

It's said that Torres used to glue the back braces into the guitar, and then put on the back. Working on a solera and building upside down this makes sense. The braces were notched into the back liners, and then arched. Hot glue was applied to the braces and allowed to gel. Then the back was put down, using string to clamp it in place and working outward from the waist. Once it was down he'd use a paint brush to put lines of alcohol across the back where the braces were, and light it on fire. The heat would melt the glue and stick the back to the braces. Apparently he got a little carried away with the alcohol sometimes and the heat darkened the backs deeper than he could scrape it out. That's the story, anyway.

Interesting! I like the steam injection idea. Here is video where the top was quickly clamped down to the sides in a go-deck. Took about two minutes from the application of HHG to fully clamped down. He preheats the wood, though in my experience, not enough to really make much difference. Wood is a good insulator and though it may feel momentarily warm on the surface, this quickly through off and there isn't any residual to make up for it. The 95 degree room would be a help.

https://www.youtube.com/watch?v=y2SuYAYJ5oQ

Saw another where the builder had taken a large single screw press and modified the business end with a caul that pressed the top to the sides evenly. This was a big rig, must have weighed at least 500 pounds. Once the glue was applied and the top was aligned on the sides, it took 3-5 seconds to run the screw down, done. Can't find that one now though.

As an aside, i was told by one French builder that the Sicilian ex pat instrument builders in Paris in the 30s and 40s made their laminated, molded arched backs by applying HHG to the veneer, placing the stack over a mold in a box and then filling the box with hot sand a couple feet thick. They through some metal ingots on top for good measure. A couple days later when the sand cooled off, out came the molded back. Or so the story goes......

I do this similar to what Alan describes.

First dry clamping the entire back in place as best as you can. I like to start with the neck and heel blocks first and usually let that dry before proceeding. Sometimes I will drill small holes through the back into the neck and tail blocks, then use the tiny drill bits as alignment pins. The holes can be easily filled later. Then the rest, remove a few clamps at a time, jack it open slightly and work the hot hide glue in with a palette knife. I don't worry too much about any glue clean up at this point, unless it gets out of hand. The clean up can be left until the clamps come off. Soft cloth, damp wet with hot water followed by a dry cloth. Be careful though to keep the clamp pads clean. They can become glued to a fragile finish and pull it off when the clamps are removed.

I use Infra Red lamps for my heat source which gets things quite hot. Adjust the distance closer or further away to control the amount of heat but leave them on until your glue session is done. I use hot hide glue a lot and spent the money on a Herdim glue pot...money well spent. It keeps my glue a constant temperature all day.

With hot hide glue, you want to put as many variables in your favor as possible before starting; clamping/ alignment strategy, heat source, clear area, everything ready... you get the idea.

I turned my bathroom into a hot room once, it had a forced air ceiling heater. Did the glue up in my under pants and felt like I just came out of a sauna when I was done but it worked great.

Maybe I should have fessed up to why I want to use HHG for back and top in the first place. I'm actually pretty happy with Titebond for this most of the time, but occasionally I find that I miss my top or back tuning targets and need to yank the back off to rework things, hopefully before binding. Titebond comes apart with a hot knife but is tedious. HHG in other joints I've opened up is much better. Popped a fingerboard in a repair this weekend, it was HHG and came right off, 30 minutes, no sweat. So thinking of using HHG for the back at least.

It occurs to me that since I use a 15' dish for most of my backs, I could use it as a clamping caul. My guess is once I had clamps at the head, tail and waist, I'd pretty well have it down in place and could follow up with the rest more leisurely. I used to do it that way, not sure why I changed.

Another option might be fish glue which I use more often now, but for a joint like the back, I'm still worried of high humidity causing a failure in this rather critical place. It does seem moisture sensitive. I have not reversed many fish glue joints but here it comes apart like HHG.

I do what Craig describes with my dishes for both the top and the back. I put a piece of stiff foam in the dish to pressure past any irregularities. First, I put the top or back on dry, clamp it, and look for gaps with a strong flashlight from inside. I fix all of the gaps, either by refitting, or by using additional clamps I heat the rim with a heat gun until all of the gluing surfaces are warm to the touch. I brush two coats of glue around the rim surface. Then I put the rim over the top or back in the dish, clamp the head and the tail, put additional clamps where my earlier testing showed they were needed, and I'm done. It's usually less than two minutes from glue application to clamping and no rushing is needed. I carefully test the glue joint all around the next day and locally moisten, heat, and reclamp any place where I didn't get a good bond. This process has been working for me for years.

Sounds like this is for a new build? I do repairs and don't have any new builds under my belt. Should be much easier with the extra plate overhang that will be removed. Lining up a removed back on an existing instrument comes with extra baggage.

Titebond and related glues are best removed with acetic acid. Household vinegar is about 3%, and agricultural vinegar is, I'm told, 10% concentration. I once used photographer's stop bath, which is either 28% or 38% (can't remember which) acetic to remove a bass top that had been put on with Titebond. It 'only' took five hours... Use a stainless steel knife unless you like black wood... Museum restoration folks add methyl cellulose to the vinegar to make a gel, which stays put and doesn't evaporate as fast.

We glue up in a go-bar deck using a perimeter caul (1/8" temper board with texture on one side, trimmed to guitar shape and center removed to within 1" of rim. We use a 1" Taklon artist's brush or a linden bast to apply a coat of HHG on the linings, blocks, etc., then a second coat to get a slightly glossy surface...warm with heat gun, then back goes on with caul. To get even glue line pressure, we use three bars per inch - no more than (back thickness + caul thickness) x 2 for distance between bars. Once the bars are on, then use the heat gun to warm the side/back joint...look for a bit of squeeze-out. Helps to have two folks - one setting bars and the other warming the rim area. After 30 minutes of cooling, remove the bars, caul, and body, and clean up any squeeze-out inside the body. Swap dishes and repeat process to glue top in place. I use an old Milwaukee heat gun, but just about any gun will work...a flared tip helps focus the airflow.

Mario Proulx used to talk about this a lot. he has a nice video on Youtube showing how he gets it done.
https://www.youtube.com/watch?v=TC78Z9zFG0I

I also use a go-bar deck with a rim protector. Parts are warmed with an IR heat lamp and then the lamp is used to keep the area warm during the glue up. I'll usually warm up the shop so it's closer to 80F then 55F. Dry fit and practice so you can move quickly and everything is at hand when you need it. On the rare occasions where I moved too slow then a bit of steam takes care of it easily. Putting the glue on quickly is key for me; Mario's technique works well for me but did take a bit of practice.

He preheats the wood, though in my experience, not enough to really make much difference

Well, in my experience, and since that is my video, the preheat makes all the difference....

Edit: Hadn't watched either of those videos in years. To clarify, I had been preheating the rim for a few minutes prior to starting the video. The camera I had at the time only took 3 minute videos....

Craig Bumgarner wrote: Another option might be fish glue which I use more often now, but for a joint like the back, I'm still worried of high humidity causing a failure in this rather critical place. It does seem moisture sensitive. I have not reversed many fish glue joints but here it comes apart like HHG.

To my understanding (as an amateur with little experience) fish glue is not worse than HHG regarding humidity. There are, however, possible addons to Hide Glue which make it more resistant against humidity. I do not know wether these can be applied to fish glue as well.

BTW: acetic acid also helps to open joints glued with fish glue; in my current repair projects i am using it all the time (25% concentrated vinegar essence) both on PVA glue and on fish glue.