I've heard of getting a low-strength joint because it is "glue-starved". If I remember correctly, I read somewhere that this can result from too much squeeze-out, from too much clamp force. Am I remembering wrong? Does anyone have any experience with this?
This topic comes up regularly at school.
Using clamping cauls can significantly reduce the number of clamps you need, speeding the glue-up process while improving results. This is because the forces exerted by clamps are radiated along a 45-degree vector from the clamp face; using cauls of appropriate thickness enables clamping forces to overlap while using fewer clamps. Using fewer clamps also imposes self-regulation re: max clamp pressure that can be applied.
Using appropriate clamps is important because different kinds of clamps are capable of exerting different forces (see manufacturer's literature for each clamp). A starved glue joint is less likely when you select appropriate cauls and clamps to begin with. A clamp that's too weak will result in an inconsistent or incomplete bond with voids and bubbles. A light-duty clamp will distort, and can be permanently damaged, if overtightened. A high-pressure clamp will begin to crush wood fibers when overtightened, a clue that you may be using too much pressure.
When I'm clamping a multiple-layer lamination, or attaching a top skin to a bass body, my clamping tool of choice is a vacuum bag with a torsion-box platen; under perfect conditions (which none of experience), a vacuum bag is theoretically capable of exerting 14.7 psi at sea level. I've never experienced a starved glue joint using a vacuum bag - provided that I use the right glue and apply it correctly.
When gluing multiple layers, selecting adhesives that offer abundant open-time and don't "creep" is important to me. I like urea-formaldehyde adhesives like Unibond-800 and Weldwood plastic cement, and epoxies; Resorcinol is also a contender if a reddish-brown glue line isn't objectionable.
You are likely to experience a glue-line failure that
mimics a starved joint if the proportions of resin to hardener (or water to powder) aren't correct, or when a two-part adhesive hasn't been mixed correctly. You'll see the same thing if you don't apply enough glue, if the glue skins over before you clamp your project, and when your glue hasn't been applied in a uniform film thickness. I use a short-napped roller to quickly apply a consistent film of adhesive.
Good luck with your project!
Learning how to glue neck lams without slippage, warpage, and gaps is good practice. Use too many clamps. More is better. Figure out how to keep things from slippin' and slidin'.
