I did a little experiment several years ago where I cut and planed some black walnut into strips about an inch wide and 3/4" thick. I cut these into pieces about three inches long and glued pieces together using various adhesives to see which would give me the most unobtrusive glue joint after sanding, and after applying various stock finishes.
Titebond II was hands down the best. Polyurethane was a very close second except where squeeze out or foam out stained the joint. It worked very well where you could sand off enough to remove the mess. It's also messier to handle and harder to clean off of your hands. But I've been told it works well on wood that may have traces of oil contamination. I found a thin 4:1 mix boat builders epoxy also gave a pretty invisible joint, while regular Acraglas was just a bit more visible. A thicker 90 minute Duco or Devcon 1:1 epoxy was most visible leaving a very visible amber line that only Watco Danish Oil finish did a good job of covering. Among the epoxies, it seemed that the greater the viscosity, the more visible the glue joint was. I think this is because fillers like silica are used to thicken them.
The Titebond is nice because, depending on your clamping, much of the squeezeout can be quickly wiped off with a wet rag as long as you don't saturate the glued joint. If you try that with epoxy and denatured alcohol or lacquer thinner, you may have thinned epoxy soak into the surface around the joint and later interfere with stain or finish if you can't remove it by sanding. Forget about trying to clean off the squeezeout or foam out from polyurethane. Best to let it set and cut it off flush after it hardens. This foaming does a great job of gap filling, but if you want a really invisible joint, the joint must be very close before glueing. Surgical rubber tubing works great for wrapping around split wrists, etc. without marring the surrounding wood or checkering. I stretch the rubber tubing and wipe on a thin coat of paste wax on it so that the glue or epoxy won't stick to it, and it can be re-used. I'd rather not fill large gaps with epoxy when I can replace missing walnut with matching walnut, so the only place I ever used Acraglas Gel was for bedding rifle barrels.
