More glue line questions

[StickMark]

Used Smooth-On to attach a hickory backing to a hickory board. I had originally used Titbond 3, but changed mind.
Using a thin, heated knife, I worked the @70" backing off. I flipped it, and used Smooth-On.
Question. As the backing cupped ever so slightly in a few places, my glue line is visible in a few places. Bow length is over 68", target weight below 45 at 28". Can Smooth-On, having filled the millimeter or less gap, work? I think it can, looking for internet assurances. Thx.

[BowEd]

The best holding and elastic glue line is a thin one.Sizing the material good with the proper amount of pressure.I'd try to take the cup off of the backing before glueing again.Too thick of glue line becomes brittle.Sanding surface with at least down to 100 grit.Most times I use 36 grit surface applied length wise before glueing.Smooth on does better with a rougher surface more so than tite bond.
Slick way of taking it apart with heated knife.

[StickMark]

I am tillered out to 20", and I am going finish,  shoot it in soon. Made it for a friend, who shoots 70# plus compound, and wisely wanted below 45#.
I figure that Smooth-On job is not reversible. The overall length is flat, solid, just a few areas. Might even sinew those down.

[StickMark]

Did a much better job on another gift bow. Learning.

[Hamish]

A good epoxy like Smooth On can handle a thick glue line.  You will nearly always get a visible glue line with epoxy, as its a gap filling glue. Its more of a cosmetic thing, don't worry.
TB3 is a very good, easy to use glue, if you want thin, virtually invisible glue lines for laminations. You can use more clamping pressure without the risk of starving the joint. Its not gap filling so a thick glue line is not what you want.

[Dances with squirrels]

I'm going to have to disagree with epoxy usually leaving visible glue lines. There are simple tools and techniques that  make them virtually invisible, every time.

[bownarra]

I'm going to have to disagree with epoxy usually leaving visible glue lines. There are simple tools and techniques that  make them virtually invisible, every time.

Agreed 100%
I am very fussy about gluelines being perfect and the glue used has no effect on the thickness of the joint - glueline thickness is all about surface prep. If you read the Smooth-On data sheet for EA40 they recommend prepping surfaces so they mate perfectly with 100 grit.

[Stick Bender]

I also agree perfect matting surfaces coupled with good solid clamping system will show a good invisible glue line, smooth on does not like smooth surfaces ,I usualy do lams with 60 grit , I have had a bow delam do to poor clamping system it only took one for me to learn the hard way ! also good air curing at least 24 hours is good but heat curing will give a stronger bond according to smooth on but have never had a air cured bow have any issues !

[stuckinthemud]

I use water as a tell tale. Wet one surface,  press it on the other and the.water will show the high spots. Knock the high spots back and repeat until the whole.of the.surface is wetted

[Hamish]

The viscous, gap filling nature of epoxy means you don't want a joint clamped as tightly as you would with say TB3. If you do, you risk starving the joint, which is why an even slightly visible glue line is ideal with epoxy. You can tell 100% that you have a good bond.

You can get still get great results from epoxy with 40 grit mating surfaces,(deep scratches) which allows you to clamp tighter than normally without as much risk as starving the joint, as the glue won't get squeezed out of the scratches. An elaborate clamping press, with air hoses, like they use with fg bows will give excellent results due to the even clamping pressure all along the bows length.

You can get similar results from using a toothing plane , because the epoxy will fill the deep grooves and stay in them even with high clamping pressure.  If you look closely you will notice the ridges in the glue line.

It also depends on how invisible your definition of invisible is. For instance if you glue a dark timber next to a light timber, eg walnut , and maple then the glue line no matter the type of glue  will be harder to see.


 Glue 2 pieces of maple together,  and even a good glue line will be more noticeable, if it was glued up with TB3(which has a slightly brownish glue line). Use a more transparent glue like a white or yellow pva and you might say the glue line is invisible to the naked eye, but its actually no thinner than the TB3 example.

[Stick Bender]

Here are some examples of different material glue lines first 2 maple/maple ,maple/walnut both 5 cores & a single core hickory on bamboo !

[Dances with squirrels]

Hamish, if the toothing plane is ran parallel with the length of the core and backing the entire time, and the sharpened end of the blade is very slightly convex shaped, it will still have the toothing plane glue prep advantages, but will show no ridges and virtually no glue line along the limb edge.

[Hamish]

Depends upon the how much taper there is to the limbs, and whether the belly is flat or rounded. The more taper to the width the more visible the grooves will be, same again with a rounded belly. Parallel limbs would show very few grooves. I don't mind the little grooves showing, but I usually use 40grit when gluing with epoxy.