DIY Papered Balsa Plywood - Curved or Flat

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lakeroadster

When in doubt... build hell-for-stout!
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I made some curved fins and documented the technique on this thread and wanted to document it on the "Techniques" sub-forum since the end product came out well.

A 5-ply construction was used: (3) pieces of 11 mil printer paper and (2) pieces of 1/16 balsa.

For curved fins I made a form out of 3" pvc pipe. For flat fins, simply use some scrap plywood as the form.

Gorilla Wood Glue was used, with a little water added.

After allowing the part to dry in the form for 72 hours the plywood was removed and then clamped just to the o.d. of the form, to allow another 24 hours of drying.

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After 96 hours the balsa plywood was removed.

The measured finished dry thickness = 0.158". Calculated thickness = (0.0625*2) + (0.011*3) = 0.158. The glue did not add any thickness to the joint.

The end result:
  • amazingly strong balsa plywood, with almost zero flexing with 20 lbs. of weight.
  • is lighter than raw non-papered basswood sheet which is 0.289 ounces per cu-in.

Papered Balsa Data.jpg



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Use epoxy and 60lb virgin (brown) kraft paper and prepare to have your mind blown.
 
I paper my normal balsa fins for LPR, usually 3/32" and 1/8" balsa, but I use TitebondII and normal copier paper. I do think heavier paper is beneficial for strength. I use TitebondII because I have lots of it and it is easy, but I think epoxy would create a stiffer fin. I've been sandwiching the fin between a couple of pieces of wax paper and a couple of flat pieces of wood, but I've noticed that the wax paper can still react adversely to the tiny bit of moisture left in the glue joint. For my last set of fins I tried a layer of clear packing tape on the pieces of wood and that worked better. I've seen woodworkers use that packaging tape trick for gluing panels and in my case the glue didn't stick to it.

I have a separate thread here on my papering and some informal tests that I did. The fins did not come out as stiff as I wanted and I think this was mostly because of thin paper and maybe because Titebond doesn't dry to be very hard. If I wanted to go to the next level I would buy some light cardstock and use that.

My biggest problem with paper on balsa is the water in the glue causing the paper to want to wrinkle. I use very little glue and squeegee the paper flat before clamping it but still have occasional little wrinkles. I don't know how you managed to use gorilla glue plus water and not have problems with the paper.

Engineering principles would say that your center layer of paper would not have any benefits for strength or stiffness, you could build the sandwich out of 2 layers of paper plus 2 layers of balsa, or 2 layers of paper and one layer of thicker balsa at least for flat fins. 2 layers of thin balsa might conform to the curve better than one layer of thick balsa. Another thing I did- if you've ever tried tearing paper you might see that it has a grain direction- paper tears better one way than the other. When I paper fins I put the paper grain direction perpendicular to the wood grain direction much like the layers of plywood are alternated.
 
My biggest problem with paper on balsa is the water in the glue causing the paper to want to wrinkle. I use very little glue and squeegee the paper flat before clamping it but still have occasional little wrinkles. I don't know how you managed to use gorilla glue plus water and not have problems with the paper.

Engineering principles would say that your center layer of paper would not have any benefits for strength or stiffness, you could build the sandwich out of 2 layers of paper plus 2 layers of balsa, or 2 layers of paper and one layer of thicker balsa at least for flat fins. 2 layers of thin balsa might conform to the curve better than one layer of thick balsa. Another thing I did- if you've ever tried tearing paper you might see that it has a grain direction- paper tears better one way than the other. When I paper fins I put the paper grain direction perpendicular to the wood grain direction much like the layers of plywood are alternated.

There are some small wrinkles. Rustoleum sandable primer and light sanding takes care of them with ease.

Engineering principles would say that your center layer of paper would not have any benefits for strength or stiffness, you could build the sandwich out of 2 layers of paper plus 2 layers of balsa, or 2 layers of paper and one layer of thicker balsa at least for flat fins. 2 layers of thin balsa might conform to the curve better than one layer of thick balsa. Another thing I did- if you've ever tried tearing paper you might see that it has a grain direction- paper tears better one way than the other. When I paper fins I put the paper grain direction perpendicular to the wood grain direction much like the layers of plywood are alternated.

I normally place the wood pieces so the grain directions of subsequent pieces of wood are perpendicular to each other, and don't use any paper between the pieces of wood. In the case above, the pieces are curved and I couldn't do that (I'm not a fan of using ammonia or steam bending), so the wood grains are parallel. My thought process was if I placed the paper between the pieces, in the event there were any microcracks or future cracks in the wood, the paper would help to minimize propagation of the cracks from the one wooden piece into the other.
 
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