Best glue for spruce ?
 

I haven't used spruce in several years and don't recall which glue worked best with it. (attaching spruce spars to balsa or ply ribs). What do you guys recommend ? Thx. 'buff

TiteBond, Gorilla Glue or one of the epoxies!

If you have a little extra time I find TightBond or equivalent is the best. Of course every job requires a little different application.

Ken

Laminating epoxy for all hardwood structural applications.

Scott

Epoxy isn't needed for attaching to ribs. Sure, it's the way to go for firewalls, landing gear mounts, and wing mounts. But the joint between spar and rib only has to be as strong as the balsa ribs themselves. And since that joint is only under compression stress along with a tiny bit of vibration, it really doesn't need much in the way of glue. CA works fine, as did Ambroid and as Titebond does currently. You could probably use white school glue and never have one come loose.

I guess I didn't read the original post very well. You are correct Jester, to attach spruce to balsa CA is fine. I was referring to hardwood to hardwood structural joints.

Scott

I used to use generic PVA with cyano 'spot welds' if I needed to hold stuff together.

IMHO Titebond is better but it was hard to find in the UK at that time.

If you use Ca on hardwood, always rub a little soda (like your wife uses in the kitchen) on the area to be glued before gluing it as hardwoods tend to be acidic which CA does not like to work with. The soda neutralizes the acid and lets the CA perform at full strength. Yes, I know that balsa is a hardwood but it does not seem to be nearly as acidic as oak, spruce, pine etc. are so does not require the soda. However, soda even improves the bond with balsa. You will find soda to be a real asset when using CA. You can even fill a hole with soda, apply thin CA, and drill and retap the hole for a metal screw.

I thought that soda acted like an accelerator, which is precisely what you wouldn't want, when gluing hardwoods. CA needs to soak in, for a strong bond. .Accelerating the curing process prevents this.

Actually the whole " soaking in " deal is pretty much a myth. Pretty much the same level as " nose heavy is always more stable ". The strongest glue joint is one that the parts fit tightly, you are able to maintain .005 bond line and use the minimum amount of adhesive to fill that .005 gap. For out purposes, we could use CA, epoxy or Titebond and not have any issues with the bond strength when bonding ply to ply, ply to balsa or balsa to hardwood.

Penetration of the adhesive is unimportant ?

I do remember, that back in the 60s/60s, we used to pre-glue parts. Maybe you're right.

The bond is only as strong as your base material. In this case wood. No matter how much glue soaks into the wood the bond is still going to only be as strong as the wood. IMO using more glue in an expectation that it will penetrate and make for a stronger bond is just adding excess weight. A good example is sheeting a foam wing. Some guys swear by using polyethylene glue because they feel it penetrates into the foam. I use laminating epoxy resin and also use much less. Recently I sheeted a wing that is about 700 sq in in area. The sheeting process took 90 grams of resin total for a finished wing weight of 10 oz prior to hinging the ailerons.l

In cases where an end grain is present as when "lacing" a stick fuselage the end grain needs a double coat of glue so you don't wind up with a dry joint. Every joint of my Super Fli build had the end grains "double" glued and I still had the occasional dry joint.

I see the same thing said about epoxy when coating a boat or, less often, gluing parts together. Some feel that a thicker layer will hold parts together better, make the material stronger or seal the wood better. Another "unfounded" belief is that by thinning epoxy with alcohol the epoxy will soak in to the wood and therefore make the joint stronger. According to a couple of epoxy manufacturers, thinning with alcohol(or any other solvent for that matter) only degrades the epoxy and adversely affects the epoxy's ability to do the intended task

Ken, that makes perfect sense. The first application appears to seal the wood especially since glue will really penetrate an end grain cut. Then the rule of only use just enough glue to do the job would apply once again. There is always that balance of not too much but also not too little. We need to have some glue between the parts being bonded for maximum strength. At work we sometimes will all glass beads of a certain diameter st that when the parts are clamped together for curing we will have a bond line of adhesive the same thickness of the glass bead diameter. When we CF wind SRB cases we have points where we measure the resin content in the fiber strands. Too much resin creates just as many issues as too dry.