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  What ever glue you use make sure you put some pressure on the stack to squeeze out any air. For balsa blocks, even white glue would work and sand well, but tight fitting joints are a must.
  Type at you later,
  Dan McEntee

Fist time making tear drop wing tips. I have to stack up 4 pieces of 1/2" balsa to get the thickness I want. After I carve the outside I figure the center joint will be thin CA just in the area I'll hollow out  first so I can split it in half to make it easier to hollow out. Just wondering what to glue the top and bottom slabs with. I think Sigment would be easiest to sand but not sure it's good for laminating. I've got sigment, epoxy, Zpoxy, titebond, gorilla glue and CA. \

  I would certainly use cyanoacrylate (Hot Stuff Super T), if I was out, I would use Titebond (I). As long as the glue is hard, and not rubbery when cured, it's always much softer than silicon carbide sandpaper and with proper technique you will have no problem smoothing it out.

   There is some other air-drying glue that seems to be the source of Pica Gluit/Luthier white glue, but I forget the name right now. That works OK, I will dig around and find it.

   Consumer epoxy is usually rubbery.  Gorilla glue is very difficult to sand because its even more rubbery, same with Titebond (II) and (III). Sigment/Duco/UHU is a bit soft and will take a week to cure in a laminating situation, and for sure, don't do final sanding for about a month, and is prone to reactivating when you put solvent on it later, causing it to bulge or sink. Ambroid will do the same while also tending to bleed orange into dope, at least.

  Of course, anything will work and hold it together, so it doesn't really matter that much. People tend to worry about perfecting something, when good enough is by definition, good enough.

      Brett

Brett.
I've had trouble sanding seams in balsa sheets that I used CA on.  If I didn't sand them within a few seconds of glue up, they'd end up "raised" no matter what I did to sand them.  Would you describe the proper technique for sanding CA'd seams? 
Thanks,
Rick

    For the most part, if you are sanding square across it, nothing special needs to be done, and the bond line is only a few thousands of an inch thick. If you are sanding it at a low angle, this is where you have problems. In any case, to sand it the same as you would sand any combination hard/soft surface; that is, with the coarsest sandpaper you can get away with, and the lightest possible pressure. Use fine paper and press hard, and you will have hollows on either side.

     I pretty much never use anything finer than about 240 grit on any bare wood or construction sanding, there's really no point to any finer if you are going to put silkspan or carbon veil and dope over it. And for the most part, until the very last bit of sanding the primer, I stick with 240 or 320. Using finer will just create shiny but lumpy surfaces. The one exception is finish sanding a solid surface that will have Monokote on it, for that, I will use 400 and the lightest of light touches to prevent it getting lumpy over the varying hardness surface.

        Of course, always use a sanding block that you have *glued the paper to* and is perfectly rigid. I use the Hobbico bar sanding blocks that are 11x2 and 22x2" and use 3M77 to stick it to the block, but anything that holds it flat to the rigid surface will work. Don't wrap the paper around the block, or use a rubber block, or, God forbid, just loose paper.

    As always, the most important thing it to make the bond line as small as possible, so fit everything as closely as you can. If you get a puddle on the surface, you aren't going to be able to sand it smooth with sandpaper and a block, and usually I just shave underneath it and use filler or a balsa patch.

   Sanding CA counts on it being so brittle that it easily chips when the grit hits it.

     Brett

p.s. Another thing - if you put too much CA on and it squeezes out of the joint, for goodness sake, DON'T TRY TO WIPE IT OFF! All that is likely to do is spread it into a flat puddle or film, at which point it will set and then be very difficult. Let it squeeze out, "zap off" into raised bumps or bubbles. Those you can carefully scrape off with a credit card or something like it. If you try to sand the bubbles, you will break them off and they will tend to roll along the surface, grooving it up massively. Fix it with water and an iron to raise the grain, let it dry for a day or so, then go back to sanding.

I guess I only have to hollow the inboard tip to hold the line slider. The outboard tip gets the weight box anyway so why make it lighter.

Motorman

For adjustability.  Maybe the weight of all that balsa in the wingtip is too much weight.

  If using blocks for wing tips, you always hollow them out to reduce weight at the tip and reduce barbell effect, especially on a large model. Tip weight box should be in the wing and forward of the balance point. it's easier to build the structure with a hatch or cover in the wing than in the tip. There have been guys in the past that used a hollow tube that exited the leading edge and used lead shot for tip weight, and plugged the tube with a nice, bullet shaped end.  On the twister I am building, I have provided for flat weights to just bet bolted to the wing end plate. Easy to service and change and I didn't have to build anything into the wiong for a hatch or cover.
  Type at you later, and HAPPY HOLIDAYS!!
  Dan McEntee

   If the weight is the same distance from the CG, the moment of inertia for a given balance moment is the same, since the radius of gyration is the same. It doesn't make any difference whether it is balsa or lead. It *is* a generally bad idea to put all the "heavy wood" in the outboard wing, but as long as it's all at the tip, its no different.

     I think you generally want to put the tipweight in line with the CG, not in front. You don't want to create, on purpose, "products of inertia", or put another way, shift the principle axes of the airplane. Of course, not many people play any real attention to this effect in other ways, so it's coming out along the normal axis defintions by luck, if at all. There are interesting effects of intentionally shifting the principle axes, but I think in general you want them lined up rather than skewed.

    For those unfamiliar with the effect of products of inertial, it's the difference between balancing a car tire statically VS dynamically. That's why the spin the tire to balance it, and why you sometimes need weight on both the inside and outside rims.

   Brett

No actual suggestions about glue then?