Butt Jointing Balsa Sheets

[Dallas Moore]

I'm trying to learn how to best butt joint balsa sheets so the seem is undetectable.  CA or epoxy leaves the joint much harder than the surrounding balsa.  Then when sanded, an uneven surface is made (hard joint/seem high), even with a hard sanding block.  I watch a video where the gentleman showed how to do it, but I still have the same issue.
Thanks for the help in advance.

[Dave Rigotti]

Use original Titebond glue. Sands perfect.

[Doug Moisuk]

Used CA for decades with no problems. What type of CA are you using? 

[Brett Buck]

Used CA for decades with no problems. What type of CA are you using?

   Me, too, no issue. It probably depends on the quality of the fits. Fit it up tight enough and there should be negligible glue at the surface, no matter what kind you use.

     I also note that it is generally desirable to use low-angle cuts, to make sure there isn't just a straight line perpendicular to the grain - typically a 12:1 angle, 5ish degrees. Trying to do that in one diagonal cut wastes a lot of wood, so I usually do multiple cuts, like finger joint used in woodworking. This also has the effect of breaking up any straight lines that a glue line would cause. The more random, the better, probably. They don't have to be terribly accurate, eyeball it on one side of the joint, then use the first part as a template to cut the other part.

    If you are really concerned about glue lines, use model cement instead of Hot Stuff.

   This is a great way to put patches in sheeting, as well. Cut the edges of the patch with a bunch of grainwise low-angle cuts, slap it on the airplane and cut it using the patch as a guide to make the hole match the patch, then just glue it in. We had to do that with Paul Ferrell's airplane at some NATs (maybe the year he won Senior), when the spinner came off in a practice flight and poked a hole in the wing right before we left. 20 minutes in Ted's garage, it was all patched up and structurally sound, sand it down and finish it later.

      I might do some example cases for illustration later, I have LOTS of scrap in the balsa scrap box.

     Brett

[Mike Quinn]

Hi

Using CA works but needs practice and it is easy to oversand and create ridges like you say.

The way i do it you need as close to 90 degree on each edge of sheet.  Butt join and run sellotape (Scotch tape in US ?) along the join.  You can now turn the two sheets over and gently bend to open up the gap but not too much.

Then run a bead of Aliphatic (yellow) wood glue down the opened join.  Weight down, wipe off any excess and leave to dry overnight.  Pull the tape off in the morning 😀.  Secret is not to use too much glue so that it doesn’t ‘squeedge’  through onto sellotape side.

Aliphatic yellow wood glue generally dries hard and sands easy.  Used that technique for years on FF, RC and CL models.  Works well.

Not used Titebond but probably same thing.
https://deluxematerials.co.uk/products/aliphatic-resin-500g


Mike

[Paul Smith]

Use original Titebond glue. Sands perfect.

Yeah.  Titebond II.  Water cleanup.  You need beer if your want to get high.  No toxic glue fumes.

[Dave Rigotti]

No...Titebond original. TB 2 is more flexible, ie rubbery, so sands crummy. ONLY Titebond original.  Gorilla Glue yellow glue is great too!

Yeah.  Titebond II.  Water cleanup.  You need beer if your want to get high.  No toxic glue fumes.

[Dave_Trible]

I find very little reason to butt joint balsa sheets with one exception- fuselage sides.   Otherwise common balsa sheet is plenty long enough.   On the fuselage sides I make the joint forward of the wing and let the doublers tank care of the strength.    Butt jointing is pretty tough due to the end grain soaking in the glue.   Here I'd pre glue everything and let the first application soak in.    Edge joining sheets I try to get a pretty tight fit then put tape on one side down the joint.   I can then open the joint on the other side and apply glue.   Then down flat on the table,   wipe the extra glue off an let dry.   Regular cement is easy to sand away the glue ridge.   I do use Cya some here if i'm in a hurry and can get the ridge under control once I start doping and stiffening the wood around the joint.

[Ken Culbertson]

I do the same as Dave except that I always use CA.  Push it together as hard as you can, tape one side, lay it on the bench tape side down and run a bead of thin CA down the seam. I do LE skins the same way except the grain is opposite.

Ken

[Steve Helmick]

Seems to me that I watched a Windy video that showed how to do it, which I recall as having the two edges (wing skins) slightly overlapped (as needed due to any curvature), cutting both sheets at the same. This appears to me as a good way to cut the two sheets so they match, as the knife cut angle will be identical. Shim under the top sheet, of course, with a 3rd sheet of the same thickness. When in doubt, email Bob Hunt and ask nicely.

As for the CA glue hardening the sheet problem...sounds like way too much glue to me. A drop of thin CA every inch should suffice, with the tape on the opposite surfaces. Anybody tried this with the blue or green masking tape instead of the mylar "Scotch Tape"?    Steve

[Donald R Olson]

I use gorilla glue. A very thin layer. If you have any squeeze out wipe it off right away. It will sand almost as easy as balsa. Closer than any other glue I've tried. Works for edge gluing too. It is great for laminating balsa to foam cores. It does take a little getting use to. It has a thicker viscosity. The trick is to get it a consistent thin layer, but don't squeegee it away completely. 

[Mike Griffin]

Years ago I asked Tom Morris about this and he showed me how he did it.  A matter of clarification here; when you say "butt" joining, I think of putting two pieces of 1/16th end to end and gluing.  I have never attempted this because I had no reason to do so.  If you are "edge" gluing, say two pieces of 1/16th to make a wing skin, then this is how I was taught by Tom to do it.

First you need to get the two edges straight by whatever method you like so they fit together perfectly flush. Once they are butted up against each other, run a piece of blue masking tape the length of the sheets and flip it over.  Bend the sheets apart and run a bead of Elmers white glue down the reveal and then bend back flush and weight the sheets down flat and let dry.

Once it is dry, sand smooth.  Tom always used white Elmers glue and I did too and it worked fine.

Hope this helps

Mike

[Juan Valentin]

Hello Dallas
                             I found out what I think is the best way to butt join balsa that will work real good. the first thing to do is to dilute original titebond wood glue with water. then using a small brush you paint the butt ends with the diluted glue and let it dry that way the glue is absorbed by the wood fibers. Once dry you can use full strength titebond to join the sheets. Place them in a flat surface and remove the excess glue from the joint. I use Qtips to remove excess glue.  I place wax paper under the joint and on top and place some weight on top until it dries. You can practice on some scraps of balsa. I used some soft balsa 1/8 of an inch and there is no need to use a doubler to reinforce the joint as it come out very strong. I broke the test piece and it broke but not at the glue joint.
                                                                                                                                                         juan