Sanding Joined Sheeting

Here's a question I have not seen posed on SSWF:

Having been sprayed with Windex several times in an unwarping process, the l.e. sheeting of my wing has become marginally THICKER now than newly attached adjacent sheeting from the same 1/16" piece. If I sand these seams level at the usual time, is there a danger that dope will affect these pieces differently, perhaps swelling or raising grain more on the fresh sheet, creating a step to be sanded down again during the finishing process? I'd like to plan so that I don't have to thin any sheeting too much to achieve a smooth transition. Any special precautions. Perhaps I'll do a test sample. Thanks!

SK

Use your common sense, do it with light coats and see what ha<[ens.[P>Maybe you should wait for a couple of days, and see howthe wood behaves.

Normaly dope will not make the sheeting swell, I would sand it even.

Hi Serge, I would sand them all even now, and when you apply the clear dope it should be as it would be when you do a normal finish.
Good Luck!
Bill <><

Insofar as sanding is concerned, be sure to use a sanding block. Don't try to do it by hand. Since the sheeting is thin, use something fresh and very fine 220 or more, and be patient.

Tools

Unless someone has experience here, I have a feeling we are all shooting in the dark. Wetting wood can raise the grain. This much we know. However, I am also wondering if some of this swellling yhou are seeing could be temporary. And then, instead of the dope raising the wood on the fresh pieces, the others may dry out and "unswell".?? (I don't know. That's why I said, I am wondering.) Now, what would happen if you sprayed the fresh wood with Windex? Would it swell to the same thickness? If so, and if then it later dried out and changed dimensions, it would all do so together. That might be my solution.

Does Windex contain ammonia? If so, this could be part of the problem. Perhaps next time (if there is a next time) just use water.

regular, blue, windex does indeed contain ammonia

LAST EDITED ON Jan-09-04 AT 10:36 AM (CDT)
 
Serge,

I tried a new finishing method on my Critical Mass. I hardened the surface of the wood with medium CA rubbed into the wood with a paper towel. It seems to have worked well. It was an idea of Richard Oliver. The idea was to make a very hard but light substrate for the application of dope based filler.. the airplane is in silver and so far seems to be fine. Now, if I just don't have any problems with masking tape pulling up finish.

In any case Serge, I think you can sans the wing flat, then harden the remaining balsa with CA or thinned epoxy. Alternately, I also use thinned epoxy under my dope filler. When using epoxy, I use one part Zap finishing resin, one part hardener and one part acetone. this makes a very thin brushing consistency and dries overnight to a finish hard enough to sand. I have used thinned epoxy before and intend to use it again on Snaggletooth 2.

>I tried a new finishing method on my Critical Mass. I
>hardened the surface of the wood with medium CA rubbed into
>the wood with a paper towel. It seems to have worked well.
>It was an idea of Richard Oliver. The idea was to make a
>very hard but light substrate for the application of dope
>based filler.. the airplane is in silver and so far seems
>to be fine. Now, if I just don't have any problems with
>masking tape pulling up finish.
>Al

Al,

My experience with using CA on the wood surface prior to painting has not been all that satisfactory. On my Gulfhawk Bearcat, every place where the dope finish knows there is CA on the wood surface beneath the dope, the dope has blistered. I used Brodak paint throughout the process. There are several small spots on the airplane where there is CA on the bare wood surface. I doped over this with clear, then applied carbon mat over the entire airplane, using clear to apply the carbon mat, more clear to fill the carbon, some Brodak primer, mostly sanded off, another coat of clear, then color, then a couple coats of clear. Now, after 3 years, each of the half dozen or so spots where there was CA on the bare wood surface, a blister has appeared. Some of these appeared in the first year. It also might have had something to do with the formulation of the Brodak dope at that time which I have been told might have changed since those earlier days. I have not had a problem where the dope is over CA joints, only in those places where the CA was smeared over the surface of the wood.

This is not a condemnation of the Brodak paint as I intend to continue using it on all of my models or at least until I learn how to use epoxy paint without adding too much weight. It is just that I will be careful to not ever let the dope finish be over any CA covering the wood surface. A coat or two of nitrate dope over the bare wood and then using nitrate to apply the carbon mat would probably solve this problem, as long as Brodak colors would be compatible with the nitrate dope. Using nitrate dope to apply the carbon mat has some advantages as it seems the carbon mat takes on an extra hardness and or stiffness than with the butyrate dopes but that is a whole other subject.

Keith,

Now you tell me, when the airplane is already in silver.

Richard Oliver has had good success with this method, and suggested that it might be a solution to my weight problems. Most of my older airplanes were finished by painting the bare wood with thin epoxy which made a very hard base for fillers. At the time I also used epoxy primer and color.

Now, in an effort to make the lightest possible finish, I tried hardening the wood with rubbed on medium CA in a process described to me by Richard. It was easier and went smoother than I expected and, so far, seem to work OK. I am really fearful of paint weight gain on the Critical Mass because of its size and the necessity to paint it red which is typically a heavy poor covering color.

Over the CA'ed balsa I have been using Randolph non-tauting butyrate dope with a zinc sterate for filler and a bit of silver powder for visibility. To save weight, I had planned to put a coat of Brodak white on next overall to make the white areas and to provide a white base for red. Over the white brodak, I was going to use red and black Superpoxy with Superpoxy clear. I just don't like having nothing but clear over exposed dope but was going to accept it for the small amount of white which would have shown. Now I will probably use Klass Koat epoxy white, Superpoxy Red and black and Klass Koat clear. I am assured there should be no compatibility problems. I have white Superpoxy but all of my hardener has some color now , so it is inevitable that I begin a transition to a new epoxy product. Dave Platt did some quoted testing of the product, and I will probably give him a call before actually spraying epoxy.

Meanwhile, I will probably build new flaps for the Critical Mass. I am pushing my design weight limit and worry more about the Critical Mass's ability to hammer a hard corner without buffet than the Saito's ability to haul the airplane. In fact, I am also considering what modifications might be necessary to the case of a Saito 91 to get it into the Critical Mass. I know I will almost certainly need nose weight anyway. This airplane is the same size as the sea fury and uses the Sea Fury wing of 62" exactly as used to win two NATs with the rather weak ST.60. The new flaps for the Snaggletooth 2 are extremely stiff torsionally and should help airplanes with small wings and marginally high wing loadings. I will say more about the flaps later as Snaggletooth 2 comes together.

LAST EDITED ON Jan-09-04 AT 12:33 PM (CDT)
 

Thanks, guys, for the answers so far.

Len-

>Does Windex contain ammonia? If so, this could be part of the problem. Perhaps next time
>(if there is a next time) just use water.

The windex does contain ammonia, and due also to brittleness induced by the ammonia, I will be using good ol' water alone next time.

>However, I am also wondering if some of this swellling yhou are seeing could be temporary.
>And then, instead of the dope raising the wood on the fresh pieces, the others may dry out
>and "unswell".??

The sheeting is dry and stable. Although the final "de-warping" took place just last Monday, it only involved the outer 4" of a t.e. spar. All four l.e. sheeting pieces having been dry for about eight days. I think I'll pass on "pre-treating" the new wood, since I've had my fill of warps for this winter(!) and would just like to bring this thing to a relatively "stressless" conclusion.

Actually, this question relates to one I posed earlier in the wing-warp thread: what is the net effect of dampening wood? I think that, from my experiences with the warps and their removal, the answer is that the final dried balsa is marginally larger than the initial new stock. I was surprised, however, by how much thicker this sheeting ended up; you can feel and see the resulting step when new stock (from the same original sheet) is butted up against it. Another reason I asked about doping effects was that, when clear dope had been applied to an earlier model, its central sheeting developed surface discontinuities at seams between dissimilar grained/density balsa sheets. These had to be reworked to regain a smooth surface. I do try to learn from experience and am joining like wood where possible. 'looks as though I'll probably end up sanding as usual.

Again, thanks to everyone for the thoughtful responses so far.

SK

P.S. I have a Windex soaked sheet (damaged reject) drying so that I can join it to an untreated sheet to see what dope does to a smooth-sanded joint.

Edit: I see that some very interesting things were posted by Al and Brad while I was writing this. Much food for thought. Many thanks.

LAST EDITED ON Jan-09-04 AT 01:32 PM (CDT)
 
One trick for joining sheeting. Buy a piece of Teflon! Teflon! It looks like clear plastic and is about 1/32" thick. Join all your sheets over the Teflon. Press the sheets tightly together together and down against the Teflon with your fingers (I do not even true the edges anymore since I hacked off the end of my thumb) and tack every 3" or so with thin CA. Then when the entire edge is done, push down on the edge against the plastic and fill in between the tacks with with thin CA. The side face down to the Teflon will show no seam at all (that is to be the side that shows on your 'rplane, mark the glue side "inside" in big letters with a felt tip pen)! It's a snap, and super fast, but you must get the Teflon sheeting for it to work. So you see, if done correctly the “shiny side up” will show now changes in thickness at all.

Note: I recently found a nice alternative to teflon sheeting. It is called UHMW. It comes in various thicknesses and appears to be very inexpensive. A 2’ x 4” sheet at 1/8” thickness runs about $15 to $20. The good news is just about nothing will stick to it! Check your local plastic houses, it should be readily available.

I then sand the finished sheeting with 180 grit and a big sanding block, but only very lightly. 4 lb contest wood (typically used in sheeting) is very soft, and will gouge before is smooths sometimes. It is best to wait to smooth once a layer of Polycrylic has been applied.

I sheet to foam using Titebond 2 Poly urethane glue (available at Lowe’s), I do not recommend any others for sheeting foam, as they are all substantially thicker and foam much less (I have tried Gorilla and Elmer’s).

There are several advantages to the Titebond over epoxy. First of all it is a one part glue and so no mixing is required. Second, it is fully cured in about an hour and the wing is completely done. Third the glue foams which allows the user to remove ALL EXCESS glue as no extra is required. Fourth, since the glue expands and foams, the joints in the sheeting are actually filled by the foamed glue, making the joints in the sheets less critical. Lastly, it is really a snap to use! First, I put the sheeting over a big strip of paper towel (face down) then I just make big "Z" stripes on the sheeting with the PU glue, and then spread the whole mess around with an old credit card until every inch has been in contact with the glue. Then squeegee all of the glue off onto the paper towel. Try to get it all off, there is no need to leave any extra. You will, in fact, not get all of the glue off, no matter how hard you try. Put the cores back into the cradles, straighten everything up and weigh down with a board and something very heavy. Make sure you use a flat surface to support your cradles while they dry. Keep the paper towels that were covered in glue, and when the glue has foamed and turned rock hard, you can remove the cores from the cradles. This usually takes about an hour.

Lightly sand finished cores with 320. Coat with Polycrylic, using only a paper towel to lightly apply. Let that dry overnight. Brush a solid coat the next day.

Ready for paper or carbon.

Serge,

Something to remember is that some of the swelling of the balsa is expansion of the fibers that were compressed when the balsa was sanded initially. Water softens the fiber and makes it swell while dope does not, or at least to the extent that water does. Go ahead and level the work using fine paper 220 or finer, a good flat block and LIGHT pressure. OYou want the paper to cut the wood not recompress it.

I guess I am a low tech guy, because I have used waxed paper to do what Brad suggests here and it has worked well. The "down" side (against the waxed paper) becomes the outside. Now maybe waxed paper has a downside since one could, I suppose, get wax on the wood to mess up the finish, but I have never experienced this as yet.

I've discovered that not all 1/16" sheets are the same thickness! (surprise, surprise) I join sheets on a piece of window glass 36" long, and sometimes have to pre-sand the thicker piece (without much pressure) to match the thinner piece. I use lots of masking tape to prevent "creep" just as Ken Willard showed me once. Also, good old Ambroid or SIG-Ment is easier to sand smooth.
Floyd

Godzilla, as usual, presents a complete and thought-out package when it comes to wing skins.

I would like to add that while teflon sheet must be nice, the backing from MonoKote has proven to be quite resistant to any of my glues, is always pressed into service when needing a smooth surface upon which to work, yet not wanting to make fabricated parts an integral part of this surface.

The technique for actually joining sheets can, in my experience, be improved by simply closing the joint with finger pressure and then "stitching" joint with thin CA and over a 6-inch stretch. Immediately wipe joint with a finger, spreading the CA along joint. Quick and easy. Not a very light way to do it, but it sure is quick. Technique stolen from Bobby Hunt--the man with the most outrageous comb-over in all of modeling--vid tape.

>Godzilla, as usual, presents a complete and thought-out
>package when it comes to wing skins.
>
>I would like to add that while teflon sheet must be nice,
>the backing from MonoKote has proven to be quite resistant
>to any of my glues, is always pressed into service when
>needing a smooth surface upon which to work, yet not wanting
>to make fabricated parts an integral part of this surface.
>
>The technique for actually joining sheets can, in my
>experience, be improved by simply closing the joint with
>finger pressure and then "stitching" joint with thin CA and
>over a 6-inch stretch. Immediately wipe joint with a finger,
>spreading the CA along joint. Quick and easy. Not a very
>light way to do it, but it sure is quick. Technique stolen
>from Bobby Hunt--the man with the most outrageous comb-over
>in all of modeling--vid tape.
>
>Dan

LAST EDITED ON Jan-09-04 AT 01:51 PM (CDT)
 
I have always used waxed paper (with masking tape) too. I'd like to try Brad's method, since he doesn't have to worry about keeping the waxed paper flat, unwrinkled, and stationary.

*idea*

I just ran downstairs to test thin CA on an old sheet of stress relieved polypropylene; they didn't react. Perhaps I've found a new tool.

SK

Edit: I too have noticed the thickness differences and do try to join only wood of like thickness.

LAST EDITED ON Jan-09-04 AT 02:11 PM (CDT)
 
Serge,

Correct me if I'm wrong, but you are not talking about making wing skins you are concerned with the thickness of the swollen balsa in comparison to the new balsa you are applying. Cap strips I'm guessing, or perhaps center sheeting? These being applied to the wing now that the warp has been eliminated.

Good ideas on skin making techniques but not quite applicable to the question as I think I understand it.

You're right, Randy. My question concerned the seams between the leading edge sheeting and fresh sheeting to be joined behind it - in this case, center sheeting, cap strips, and sheeting over the last bay before the tips.

Incidentally, my test isn't proceeding as planned; the newly Windexed sheet is not swelling as much as all four l.e. sheets on the wing. Repeatability...'story of my scientific life.

SK

LAST EDITED ON Jan-09-04 AT 11:00 PM (CDT)
 
Serge,

Not to worry, just built as you normally would and block sand with light pressure. I'm sure the worst mismatch you'll ever get would be recovered when you do your sealer/primer coats if you get any at all.

Hi Serge. I have gone through the same trials you are. The water or Windex will make the balsa expand. In fact many use that method to take out dings, along with white vinegar, and heat applied with a monocote shrinking gun (hair dryer). Dope will cause the balsa to shrink as the dope shrinks. I have a bunch of nice smooth joints, until they got doped and then the joints show up. Frustration it's self. So, light doping, 50/50, light BLOCK sanding, more dope, more light sanding and filler and finally, things are smooth. Lots of elbow grease. "Non carborundum bastardi". Keep smiling.

Serge ,I believe all of us have experienced what you are describing and I for one have gotten quite flustered about it over the last few planes. Several folks here have described the possible and plausible with inconsistent sheeting thickness being the main culprit in my opinion.

One other idea is that where the LE planking meets the back edge of the spar is critical joinery if you dont want the seams to show. This has happened to me repeatedly and usually shows up despite all filling and sensitive sanding and labor intensive finishing technique after exposing my shiny new bird in the hot sun. Paint shrinks or something expands and my perfectly fussed over joint shows through. Nuts! I have yet to get cap strips to meet the sheeting at the same height either. I use blasa filler at every strip and seal with thin CA on a Q-Tip after sanding to desired perfection so the dope wont shrink the filler.

Getting the very back edge of the sheeting adhered to the spar is important as it will curl up slightly if not sealed down.Prior to applying the sheeting I triple check rib and spar height alignment and run a block down the sparand back fill with thin slices of balsa as necessary. Subsequent center section sheeting is cut oversized and the contact edge is sanded with a slight bevel and lightly pressfit into place and then sanded flush. Despite all this fuss there has always been an anomally somewhere and it invariably shows through sometime down the road.

Bottom line is getting used to fussing with fine grit on sanding blocks and not going to far with it causing a weak spot or dip in the sheeting that will show through once finishing begins. Also, silkspan and in particular carbon veil helps hide a multitude of sins where as often monocote just accentuates them. Ray Firkins idea of short foam sanding blocks has helped me here as to large a block and you sand areas you dont wish to or gouge somewhere else.

Good luck!

John Sunderland