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Dope
Oct 18, 2004 18:27:59 GMT -7
Post by fox4ever on Oct 18, 2004 18:27:59 GMT -7
Hoping SM can help me with this one. I have been using Sig nitrate dope on tissue covering to hold and shrink the tissue. I really am not happy with the results. It doesn't seem to have the shrinking that I am happy with. I am sure I have the tissue tight when I start doping the open areas. Any ideas? Lee TGD
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Dope
Oct 18, 2004 19:11:29 GMT -7
Post by RetNavySuppo on Oct 18, 2004 19:11:29 GMT -7
I have used Sig nitrate dope with silkspan and it works like a charm. Are you applying the silkspan wet (with water) or wetting the silkspan with water after you apply it dry? Silkspan shrinks tight as a drum either of these ways.
After you shrink the silkspan it will accept dope very well and should stay tight. Silk on the other hand, requires several coats of dope before it tightens up.
Did I understand your problem correctly?
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Dope
Oct 18, 2004 19:20:16 GMT -7
Post by fox4ever on Oct 18, 2004 19:20:16 GMT -7
I apply the tissue wet, stretch tight, dope leading edge, stretch, apply dope as I work out the tissue. The biggest problem is later when I start the color process. Thats when the tightness leaves. I have let it set a couple of days in the sun, it tightens up, but loosens when it gets cool again. I have used a monocoat heatgun on it and it seems to help. I am just looking and wondering if there was a better way. Thanks Lee TGD
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Dope
Oct 18, 2004 21:36:55 GMT -7
Post by Garf on Oct 18, 2004 21:36:55 GMT -7
Sounds like you don't have the grain running spanwise.
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Dope
Oct 18, 2004 22:38:33 GMT -7
Post by tperry2054 on Oct 18, 2004 22:38:33 GMT -7
I haven't used nitrate dope for a long time, but seem to remember that some nitrates don't shrink well. A lot of folks are content with that as they just use nitrate for a filler as it is a little lighter. You can use butyrate over nitrate and it should tighten up fine for you. You didn't mention what you are building but I hope you're aware nitrate is not fuel proof.
Tight lines,
Tom
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Dope
Oct 18, 2004 23:32:12 GMT -7
Post by StuntMaster on Oct 18, 2004 23:32:12 GMT -7
So heres what works for me.....
1) Use Brodak Dope.
2) Dope LE and Tr edges with 3 or 4 coats of nitrate, sanded in between coats.
3) I put the tissue on dry, glueing down with either straight thinner or 50/50 dope thinner worked in with the finget tips. Make sure the grain of the SS runs span-wise!
4) Spray on water to shrink the Silkspan.
5) Spray or brush on a couple of coats of Sig clear shrink dope, either nitrate or butrate. thinned 50/50 with a few drops added Flexall. I'd strongly suggest spraying for an even coat and avoiding runs.
The above should have you wing covering drum tight. SIG Nitrate shrinks and shrinks and shrinks.
6) Now move on to your SIG Litecoat, Color Dope and clear. Let things dry without heat.
Sig materials work well and should stay tight. Brodak works much better since it has more pigments and higher quality binders and thinners but I have done OK with both brands.
If you are using SIG dope, buy Dupont 3608S thinner. It's higher quality than Sig, works faster and is much cheaper
In reality, I seldome use any Nitrate at all, prefering to start and finish with fuel proof materials but it shouldnt make much differience either way.
I don't really understand your problems . Using nitrate under Butrate Color and Clear will sometimes get things TOO tight. Just the opposite of whats happening to you!
SM
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Dope
Oct 19, 2004 4:43:03 GMT -7
Post by fox4ever on Oct 19, 2004 4:43:03 GMT -7
Hmmm, Maybe this old age thing stung me. I didn't know or could see that there was a grain direction on tissue. I sort of looked at some last evening, but couldn't really tell. Looked to me to more of a swerl patteren. I will check more carefully. Could it be that the high humidity we have, or had, could be causing part of the problems. This was on a 540 sq.in. wing with lots of open panels. It has finally tightened and has stayed that way for few weeks. Thanks for the help. Lee TGD
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Dope
Oct 19, 2004 8:01:21 GMT -7
Post by jet on Oct 19, 2004 8:01:21 GMT -7
I was going to give you all kinds of good advice but just realized that I have never used Sig nitrate. It's not clear what your "tissue" is. If it is either silkspan or Sig Plyspan it will tear in strips with the grain but not across the grain. There are several coverings which have the swirl appearance you mentioned. I don't recall which ones. Jim
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Dope
Oct 19, 2004 8:13:16 GMT -7
Post by RetNavySuppo on Oct 19, 2004 8:13:16 GMT -7
The justification that I have heard for using nitrate dope is that it has a higher solids content than butyrate which means it will fill faster.
Also, nitrate dope is "stickier" and does a better job adhereing the tissue to the wood. With silkspan, there is not much of a difference in this aspect but if you use silk, the nitrate does a better job adhering the silk. Thirty years ago, my LHS didn't carry nitrate. So I improved the "stickiness" of my butyrate dope by adding either Ambroid or Duco cement to the dope - just for adhering the covering to the frame.
My favorite brand of dope was the old Aerogloss butyrate dope. It had more pigment than Sig and I could get a glass finish using just a brush. I was never impressed with Sig dope (Aerogloss spoiled me). I have used the new Aerogloss dopes and they are just not as good as the old formulas. Today, I use the brands that are more commonly used on full scale aircraft like Certified Coatings, Randolph or Aero Classic.
Since I only fly R/C, I don't use silkspan anymore (except on the tail surfaces of small models) because it is too weak and too easy to puncture. However, here is a way to check which way the grain is running on a sheet of silkspan. Go to a corner of the sheet and tear it a bit. If the tear is jagged, you are tearing against the grain. If the tear is smooth, you are tearing with the grain.
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Dope
Oct 19, 2004 20:31:02 GMT -7
Post by fox4ever on Oct 19, 2004 20:31:02 GMT -7
Well,,,now that I have ripped up all my silkspan,(haha) I still cannot decide which way the grain runs. Nor can my wife, and women are supposed to know these things. This particular silkspan is probably years old but still seems in good shape. I will test some that has been purchased more recently. I did get some of the dope that is used on full size aircraft, and am giving that a try. I was informed that it can take several weeks for dope to thoughly dry. In fact, this builder told me that he has opened sealed panels on wings months after the plane was built and found the dope to still be wet or tacky. Anyway thanks for all the usefull advise, and I will keep on keeping on. Lee TGD
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Dope
Oct 20, 2004 1:00:39 GMT -7
Post by RetNavySuppo on Oct 20, 2004 1:00:39 GMT -7
On all the silk I get, it is easy to tell the direction of the grain. The silk has a hem along the sides and the grain is parallel to the hem. Plus it passes the tear test.
Silkspan is different as it is made in sheets rather than long rolls like silk. I just had a "tearing orgy" in my shop after reading your post. Someone once told me that the grain of a silkspan sheet is parallel to its longest dimension. Maybe yes. Maybe no. I have a lot of silkspan I have accumulated over the years and I found this is not always true. Can you tell me the brand of silkspan you are using?
On your other point, yes, dope does take time to fully cure, gas off, dry or whatever term you want to use. It will keep shrinking for the life of your model which is why older dope finishes sometimes shatter (some people add plasticizer to the dope to keep it felxible). The trick to speeding the drying process is not to allow any airtight compartments in your model.
Let's take your wing for example. Dope will hold real tight to your wood and every wingbay will be an airtight compartment. These will trap moisture as well as dope fumes. These dope fumes inside these compartments will S-L-O-W down the drying process of the dope. So the key to success is to ventilate your wing.
Here is how I do it. First, I start with the wing ribs. Before I even glue them to the spars, I first cut holes in them - at least one ahead of the spar and at least one behind the spar. Depending on your size of model, these should be at least 1/4 inch in diameter. Sometimes, I make "swiss cheese" of the rib. This also sheds weight, especially if your ribs are cut from heavy wood. A rib doesn't have to be as strong as a tank - it just maintains the shape of the airfoil. I also drill/cut at least one hole in every shear web. Then I build my wing. Most wings have a sheeted center section. I also drill/cut a vent hole here. If the plane is, say, a high wing trainer, I cut a hole in the bottom sheeting, right where it goes over the fuselage (so it doesn't show). Low wing planes would have the hole on top. Actually the sheeting has a hole on each side of the center joint. When you are figuring out where to put the holes, just make believe you are a dope molecule and see if you have a clear way out of the wing.
Now for the next trick. After your first few coats of dope, put the plane out in the sun. The wing will heat up, the dope fume laden air inside the wing will expand and it will flow out of the wing. After a while, take it inside. The wing will cool off, the air inside the wing will contract and fresh air will be drawn back into the wing. Doing this after every coat or two of dope will greatly speed up the problem of the dope staying wet or tacky inside the wing. Your buddy described that problem accurately.
The building in of a ventilation system into your wing is also useful if your plane is a dark color and you fly on a hot day. If your wing is airtight, the sun will heat up and expand the air inside your wing. Where is this expansion going to go if there is no ventilation system?
This ventilation system is also handy if you use Monokote type coverings. Let's say your wing is airtight and you get a wrinkle in the covering. So you hit the wrinkle with a heat gun. What happens? The heat gun heats the air in the wingbay and it expands and pushes out on the covering, expanding it, while you are on the outside trying to get it to shrink. However, if you have built a ventilated wing, the heated air escapes through the holes and the covering can be shrunk easily.
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Dope
Oct 20, 2004 5:29:24 GMT -7
Post by fox4ever on Oct 20, 2004 5:29:24 GMT -7
Some of the tissue is what came with the kits, age could be way back there. I do have a box of Sig that I have just given the tear test. I think that I can tell a grain in it, but is very close to the same. Good idea about the vent holes. I know the the milar coated planes do strange things in the sun. Thanks Lee TGD
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Dope
Oct 20, 2004 15:19:28 GMT -7
Post by jet on Oct 20, 2004 15:19:28 GMT -7
I like not to have any totally enclosed spaces in my airplanes, particularly in the wing, because when they heat up in the sun the air inside expands, and when it cools down it contracts. If the air can vent this process doesn't stretch your covering or put pressure on anything. I make vent holes whatever the finishing method.
Jim
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round1
New arrival
I once flew inverted, now I stand on my feet.
Posts: 4
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Dope
Oct 20, 2004 18:36:42 GMT -7
Post by round1 on Oct 20, 2004 18:36:42 GMT -7
I have worked in print shops for 30 years and you can tell the grain of silkspan just like paper. When you tare it with the grain it tares straighter than if you tare it against the grain. Also the tare is less ragged with the grain. Try it with a plain piece of computer printer or bond paper, you will see. Tissue is a little harder to tell. Never used silk except in my pajamas. Good luck, Jerry
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Dope
Oct 20, 2004 18:44:52 GMT -7
Post by fox4ever on Oct 20, 2004 18:44:52 GMT -7
Ok Thanks Guys, I think I have it. Its hard to tell on some of this tissue, but other brands it is evident. Oh ya,,, please forward my mail to the Okla. State Hospital. My wife is having me committed as soon as she digs me out of all this torn paper around me. And can someone send me some $$ so I can refresh my tissue supplies. ;D Thanks again, Lee TGD
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