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Building Tips and technical articles. => Paint and finishing => Topic started by: jfv on November 09, 2015, 07:37:08 PM
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Just wondering what procedure is best for eliminating the joint line between two pieces of balsa from showing through the finish after a period of time.
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Ambroid.
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ambroid shrinks just like dope does,,and as such it will come back,,,, titebond and a GOOD fitting joint is your best bet IMHO
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carbon veil
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Just wondering what procedure is best for eliminating the joint line between two pieces of balsa from showing through the finish after a period of time.
How long a period of time?
Charles
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Check out how the wing skins are done for full sheeted wings, Bob Hunt has lots of information on this posted. The basic approach as others have stated is to fit the sheet joint as tight as possible. Use titebond type wood glue or medium CA, be careful not to apply to much glue as it will just be squeezed out and cause a hard spot. Fit the joints together on a flat surface (sheet of glass is great). One way of keeping the joint tight is to use masking tape on one side to make a hinge, apply the glue and fold shut. Carefully wipe off any excess, if possible use a piece of wax paper to cover the joint and put a stiff straight piece of wood or steel and weight it down. Let it dry a day or so then remove the weight and wax paper and start block sanding with 220 grit paper. After you get the edge knocked down go to 320 and sand smooth. Flip it over remove the tape and repeat the process. Once you install the wood cover with silkspan/dope or silk/dope or carbon veil/nitrate.
For thicker pieces (1/8" and up) you and do the joint on the flat surface an add the weights to the sheets themselves.
Best, DennisT
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I have this problem, even after lots of help from Bob Hunt, both on the phone and in his video. I think I have been squeezing the joints together too hard. They form a peak which gets worse with sanding. I backed off on the pressure squeezing the joints together on recent projects, and the joints seem to be better. I had a bad case of this problem on a wing of last year's dog. In desperation I built up the area on both sides of the joint with dope and zinc stearate. Sanding that carefully fixed it. Finally, I made sure the peak wouldn't return by running the wing into a traffic cone on takeoff.
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Years ago I had the privilege of spending many hours in the basement workshop of master modeler Ralph Cooney. Ralph was flying some CL stunt with us at the time but is better known as an accomplished free flighter. At that time he was gluing up sheet balsa skins that were then covered with very thin aluminum sheet. These skins were to be used on FAI power FF ships. The skins joined edges would sometimes show under the aluminum skin and Ralph really didn't like to see that. His solution to help alleviate this was to note the relative angle of the grain as it passed through the sheet's thickness, that is, top to bottom. Unless you have the purest of pure A grain sheet with the grain passing absolutely vertical through the sheet, there will be some observable angle to the grain. This grain angle can be seen by looking at the butt end of each sheet. Ralph would then be sure the adjoining sheets were matched up with the relative grains angling in the same direction. This produced much better results than when the peaks of the adjoining grains were fighting each other at the joint. A tip I've carried in my head all these years.
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Although it seems unlikely, I've made very good butt joints with 1/16" balsa by using thin CA. I cut the ends to be joined on an angle to avoid a stress riser, pressed them firmly (but not hard) together, taping along the seam, and flipped the assembly tape-side-down against my wax-papered table surface. I light-sanded across the seam (keeping it beneath the sanding block/bar) and then dragged the fine CA applicator (blood pipet) along the seam. The CA permeated the immediate area without puddling on the surface, but a quick swipe with a paper towel seemed a good precaution. The seam usually set immediately ('can use accelerant), allowing an immediate light sanding across the seam, which did not take just the balsa down along the seam, as might be expected. One fuselage like this was covered in doped glass cloth and never revealed the line. Two others covered with epoxied fiberglass and naturally did not reveal the line. My original P-Force kit fuselage, with it's thicker, joined, fore and aft sides, was also assembled this way and covered with lousy modern silkspan, but it's seams never showed over several years.
Here's one of the profile-fuselage sheeting joints. I'm sorry that I don't have a photo that reveals the smooth surface well, but you can see the arrangement. Mike Alimov showed me this method years ago, and it has always worked - no visible seams after finishing, despite mismatched wood.
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Thanks for all the replies. I generally use Ambroid for joining sheeting because I like the way it sands. And I'm very fussy about the joint. Also, I typically use carbon veil, glass or silkspan over the surface. What seems to happen is the seam will shrink slightly and become visible some time afterwards. I had one seam that would show up about 5 days after the primer was sprayed and sanded. Seam was perfect for about 4 or 5 days and would just come back ever so slightly. This happened on a 1/8" thick rudder. Re-did the outboard seam about 10 times before I gave up. The funny part about it was the inboard side of the seam finished perfectly the first time and still remains that way 6 months later. The frustration comes because I can't predict when it will happen. I may try Titebond next time. I was told to slightly moisten the edges of the joint and to thin the Titebond.
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FWIW, here's what you get with immediate gratification - the instantaneous, strongest bond with the least force concentration and no seams showing. These two fuselage pieces were finished over 12 years ago and have set outside in the hot sun and cold on numerous occasions over the years. The only blemishes are dents from our 2013 Christmas tree stand leg. I've set the pieces next to the plan, so that you can see where the fore and aft fuselage pieces joined on each side. That's a doped finish, using the crappy modern silkspan and tautening Brodak Butyrate dope without retarder.
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Polyurethane glue is allegedly supposed to sand easily if it foams up. About as easily as balsa.
Has anyone been using or tried it for sheeting?
I know several sailplane guys that swear by it for sheeting foam wing cores, either with 1/16 balsa or 1/64 ply.
R,
Chris
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I use polyurethane glue when I vacuum bag my balsa sheeting over a solid foam core. No need to glue the skin sheets together. Just tape them tight and the expansion fills the seam. Never had any issues. Never liked it though for general gluing as it's too hard to control where it goes. And, I'm not particularly fond of how it sands.
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That is one of the things I like about the glue, well two thing, it will find every pin hole/pore in the balsa and sands easily.