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.