Hardening Bridge Caps

[Ron Nossaman]

> When cutting a new bridge cap I typically paint on a low viscosity epoxy to
> both harden and seal the cap.  I do this after drilling and notching but
> before pinning, sanding back the bridge top flat again before inserting the
> bridge pins.  
> 
> I'm considering doing this same process with a thin CA glue which seems like
> it will offer some advantages: cures faster, wicks into the wood better,
> requires less sanding afterward.  
> 
> Any thoughts on the differences in terms of providing a seal and adding
> density to the wood between the two substances?
> 
> David Love

I've done a fair amount of head scratching over this stuff, 
and have condensed an observation or two out of it. Epoxy 
doesn't soak into solid maple side grain very well or very 
deeply, so I question the benefit of top application for 
compression hardening solid wood bridge caps. Most of the top 
of the bridge doesn't need hardening anyway. If the wood 
wasn't crushed at the notch edges, the rest of the top would 
remain undamaged however soft it is. I think the reinforcement 
ought to be where the compression stress is, which is at the 
pin. The 0.6mm maple laminations I use for bridge caps are 
flitch sliced, so they don't have the grain integrity of solid 
wood, and epoxy soaks through them fairly easily. They're 
quite porous, which makes them ideal for this use. Installing 
pins in this cap, I use thin epoxy as a pin driving lubricant, 
and an additional wood hardener that will soak into the maple 
just below the cap as the pin is installed. The cap is already 
thoroughly impregnated in the laminating process, of course, 
and isn't going to soak up any more epoxy, but the pins go in 
a whole lot easier with the lubrication.

With solid caps, I think I'd soak as much thin CA into the 
holes in the drilled and notched cap as it would absorb, and 
drive the pins in immediately - one unison at a time, in a 
stiff cross breeze so I could see and breathe while doing it. 
I don't much like the concept of hitting CA soaked *anything* 
with a hammer (splash potential), but it will go into the end 
grain in the hole easier and penetrate quicker and farther 
than epoxy. It will soak mostly into the end grain of the 
hole, and travel along the pin row, doing the most 
reinforcement directly under the area where the pin meets the 
string, which is exactly where we want it most reinforced.

My take,
Ron N