How does CWF & dope compare?

Discussion in 'Techniques' started by ek-hornbeck, Jun 6, 2019.

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  1. Jun 6, 2019 #1

    ek-hornbeck

    ek-hornbeck

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    When I read about filling and smoothing balsa on the internet these days, what I see is that everyone uses Carpenter's Wood Filler or some equivalent, thinned down with water. Back in the 70's, when I was a teen building model rockets, the recommendation was to use dope sealant/filler on balsa. These are very different materials. What are the tradeoffs? Do people use dope anymore?

    Also, I note that hobby stores sell two *different* kinds of dope sealant: "balsa fillercoat" and "sanding sealer." What's the difference?

    (I'm a pretty classic BAR case: rediscovered model rocketry by virtue of having my son get to an appropriate age, and am enjoying seeing how the internet has assisted the propagation of expertise...)

    EKH
     
  2. Jun 6, 2019 #2

    kuririn

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    Back in the day instructions for building kits said NEVER use enamel paint, use dope instead. Reason was enamel would add considerable weight compared to dope. Rockets were smaller and engines were A-C for LPR, so keeping weight down was a priority.
    BTW dope is not a sealer, but a colorant. We used sanding sealer to fill the balsa, and dope for color.
    Nowadays there are superior products, and weight is not as much of an issue with bigger and more powerful motors.
    There are still a couple of places that sell dope and sanding sealer ( remember Aerogloss and Pactra?).
     
  3. Jun 6, 2019 #3

    neil_w

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    Biggest advantage of CWF over the old stuff is that it's very benign. No fumes, easy to handle, and it works in a variety of circumstances beyond just filling balsa. Downside is it's water-based and you have to manage its tendency to warp balsa (*usually* not a problem if you do each side symmetrically), also it's a bit grainy and sometimes leaves pits that you need to go back over and fill a second time.

    These days I paper most of my fins, and do everything else with CWF. I usually (but no always) use high-build primer to provide the last layer of filler. Everyone has their own preferred techniques; most of them work fine if applied correctly.
     
  4. Jun 9, 2019 #4

    OKTurbo

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    Yes. Dope is a good sealer for balsa. I use Randolph nitrate dope on old school stick and tissue rubber band powered planes all the time. There are 2 basic types of aircraft dope...nitrate and butyrate. The butyrate is ‘hot fuel proof’. Nitrate is not. You can pretty much paint with anything over cured nitrate, but can’t put nitrate on top. It will eat thru it.

    I buy it from aircraft spruce. Very old school traditional coating for fabric covered aircraft.

    Recently I needed to make a fin mold for a composites project I had. I wanted a mirror smooth finish on a 1/4” thick balsa fin ‘plug’. The easiest for me was to coat with nitrate dope. It drys super fast and you sand between each layer. Multiple layers later you end up with a super smooth surface.

    It smells to high heaven, so you have to have a well ventilated area, but it works excellent as a balsa sealer.

    1FD8909C-41BB-4066-8D84-6446B9A2B7B2.jpeg B4830031-A5AC-451D-BBED-F75A663A82F4.jpeg 1D0BA33E-145A-4817-A286-2DEDD1281A61.jpeg 19C58D12-C412-41B2-80D3-455A3DF9C508.jpeg

    That being said. When building rockets, I usually go with a few coats of rattle can rust oleum grey auto primer...sanding in between. I suspect that a dope covered fin would go up in a ball of flames if it burnt thru the top enamel coat on the launch pad. Don’t have to worry about that on a rubber powered plane.
     
  5. Jun 9, 2019 #5

    Nathan

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    ???
    Dope is a sealer for wood, fabric, and paper building materials. It can be colorized but doesn't have to be.
    https://en.m.wikipedia.org/wiki/Aircraft_dope
     
  6. Jun 9, 2019 #6

    BABAR

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    As https://www.rocketryforum.com/members/nelie61.29348/ pointed out on a different thread, dope has advantage of being unlikely to warp the balsa, compared to water based CWF.

    from what people have posted on this forum, warping not too much of a problem as long as you treat both sides of the fin at the same time. I can tell you from experience that you will get major warpage of balsa if you DON’T do this.

    Nelie61 described great combo results using sanding sealer to seal the balsa followed by CWF to avoid warping.
     
  7. Jun 12, 2019 #7

    ek-hornbeck

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    OK, so that raises the question: what *is* "sanding sealer" and how does it relate to "dope"? Are these the same thing? Variants of some general kind of substance -- that is, is "sanding sealer" a *kind* of dope?

    When I read about CWF & dope, I always see the same summaries: CWF is much more pleasant to deal with than dope. But then I see little hints that CWF might not be *quite* as good, in terms of providing a glass-smooth surface, as dope. Water-based CWF can warp a thin balsa sheet (such as a fin). It's not waterproof. You might need more coats.

    Given all this, I wonder: why deal with CWF? I used dope in the 70s. I didn't use much -- it doesn't take a lot to coat the fins and nose cone of a small model rocket. The solvent evaporated into fumes, but I did not think it was a big deal, as long as I worked in a ventilated space. If CWF is almost-as-good as dope, then, heck, use dope. CWF "fixes" problems with dope that I don't find to be problems.

    Or am I missing something here? (That is, a reply of the form, "Do whatever works for you," is not a useful reply. The point of this thread is to explore the merits and tradeoffs of different sealants.)

    I also hear weight comments -- some sealant is lighter or some other sealant is too heavy. I'd really like to see some *data* on this. Has anyone used a *scale* to weigh how much these sealants add to the weight of a rocket? It would be really fantastic to know what the weight cost of a range of sealants is, in g/m^2. Then you could take the surface area of your fins or nose cone and work out what these sealants cost, in terms of mass footprint. I'm going to be a little skeptical of weight claims, pro or con, until I have some actual numbers. After all, if you do it right, you are going to wind up with the thinnest layer of sealant imaginable.

    Finally (and this is relevant to my weight comments above), why not just seal with one of the laminating epoxy resins used to lay up fiberglass? You could put on a really thin layer because these resins are so liquid. You'd get a very hard, smooth, tough surface. Epoxy is relatively heavy, in this space of possible sealants, but, again, you are going to sand it all away, leaving only a tiny amount.

    Everything I've said above is conjecture. I'm just getting back into messing with model rockets after a forty-year absence. One thing I'd like to do is check out some of these possibilities, with measurements, to pin down some answers.

    But if anyone has done it already, that is much better.

    EKH
     
  8. Jun 12, 2019 #8

    jqavins

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    Let me start with two disclaimers. First, I've never used CWF, thinned or not. Second, I've never done the measurements of mass you suggest and, while I would also like to see the results of such measurements, anything I say about mass is conjecture.

    Dope, as I understand it, is a type of sealer. (I remember from my stick-and-tissue airplane days of 45 years ago, it even seems to make the paper shrink a little, pulling tight over the frame, though I can't think of how it would do that.) It soaks into the wood a little, sealing the pores and strengthening the surface a bit. But it's not a filler. OK, let me restate that, since I just implied that it fills pores. It doesn't fill grain. More precisely, it doesn't fill the valleys of the grain, but only strengthens the peaks.

    The strengthened and stiffened peaks can be sanded more effectively than untreated balsa fibers, which means you can sand the peaks down until you reach the valley bottoms. Reapplication of dope is advisable on the way, since you may sand off all the impregnated material before you're done.

    A filler fills the grain valleys, so you don't have to remove as much wood before getting to smooth. The more filler goes in, the less wood you have to remove. CWF, as I understand it, add so much that it fills the valleys completely up so that you may not be removing any wood at all.

    Which comes out weighing more, balsa with the peaks removed until you get a smooth surface at what had been the valley bottoms, or balsa with the valleys filled so you get a smooth surface at what had been the tops of the peaks? Obviously, the filled piece comes out weighing more. Grams per fin or milligrams per fin? I couldn't say.

    Aerogloss sells both colorless dope and milky dope/filler. The latter has suspended solids that fill valleys at the same time to dope is strengthening peaks. It's not a thick filler, so you probably end up meeting in the middle.

    Some people use water-thin cyanoacrylate (CA) glue* as sealer and strengthener. Your suggestion of thin resin or epoxy would amount to the same thing. A fine idea, but I haven't tried it.

    When I restared building rockets 15ish years ago (Wow, that long and I've build so few. :() I used dope on my fins. Dope-sand-dope-sand-dope-sand-paint. I got very nice results. But I've switched to a light weight filler that also gives me very nice results and with less work, fill-sand-paint. My filler is porous, so I might do better with fill-sand-dope-sand-paint.

    * CA is better known as superglue or Krazy Glue. Mostly you see it in a medium thickness, and there are thick or gel versions also in hardware stores, but you won't have seen the thin type outside of hobby stores. It's very handy stiff for many purposes. Use it with care, since it runs like water and bonds like Krazy.
     
  9. Jun 12, 2019 #9

    neil_w

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    In my experience, the problems with CWF are more theory than practice. In practice it's quick and easy and warp-free the vast majority of the time. I started enjoying it more when I decided to apply it with my fingers rather than a paintbrush.

    The fumes from dope, on the other hand, are reality. In my house, they're a dealbreaker.

    OK, I won't say it, but I'm thinking it. Both approaches can yield smooth-as-glass results.

    As a kid in the 70's, I used dope. As a BAR, at least initially, I used CWF. Now, I usually paper my fins. I don't think any of the techniques has gotten me noticeably better results than any other; I just like the *process* of papering the best, and it's fume-free. As often mentioned, papering does have the additional benefit of adding significant strength to the fins.

    I think dope might yield the smoothest surface before painting, but I only care about the surface after painting.

    I've heard this mentioned in the past but can't think of any examples of anyone doing it, at least not "in public" on this forum. Seems like it should work in theory, don't know what the downsides might be. At minimum, I would venture to say that it's an uncommon practice. Unless some knows a compelling reason to avoid this, why not just give it a try, and then please post about your experience and results. I'm certainly curious.

    Oh, and some folks use stuff like wood hardener, others (a, I see jqavins just posted about it above) use lightweight spackle. There are many many ways to get essentially equivalent results.

    I can't speak to the different weights of the different finishing techniques. Someone somewhere did do a bit of testing in that area (maybe the Rocket N00B? Not sure). However, weight is only one aspect of the problem, unless you're doing competition and shaving fractions of grams wherever possible. Whichever technique is most comfortable for you is something only you can decide, which means just trying stuff and seeing what you like. There's no downside to experimentation, it costs only a few pieces of balsa and a bit of time.
     
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  10. Jun 12, 2019 #10

    prfesser

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    One downside of laminating epoxy as a sealer is that it's pretty hard to sand and tends to gum up the sandpaper.

    I heard of a hint for reducing the work of sealing balsa, when using sanding sealer or dope; I don't know how well it works. Rub talcum powder (talc, not cornstarch) into the balsa, apply sealer/dope, sand, and repeat. The talc fills larger pores and grain cracks pretty well, so it takes fewer coats to get a good finish.

    If you're looking for a solvent-based sealer (for fast drying), dilute auto body spot filler---the red goop in a tube---with lacquer thinner or acetone to the consistency of paint. Store in a glass jar with a good tight lid.

    Best -- Terry
     
  11. Jun 12, 2019 #11

    jqavins

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    That's more or less what the Aerogloss milky filler/dope is. I don't know what solid they use, but it's the same idea.
     
  12. Jun 17, 2019 #12

    djmartins

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    Ok, silly question for a first post but if you are papering or heavily filling balsa wood why not use
    bass wood or thin ply instead? I'd guess that the balsa when done is close to the same weight as the ply or basswood
    and it might be less work finishing those and you get greater strength.
    Clue me in as to why I see so little used on rockets as my only guess would be the weight of the stuff.
     
  13. Jun 17, 2019 #13

    neil_w

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    It’s all about weight. Papered or filled balsa is still much lighter than bass or ply.

    Basswood is sometimes used in larger LPR and MPR, while plywood is commonly used in MPR and HPR.
     
  14. Jun 17, 2019 #14

    jqavins

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    (Quick tangent: I have an Apogee LPR kit that uses 1/8" plywood. In that case plywood was used to deliberately add weight, in order to give it a slow liftoff.)
     
  15. Jun 17, 2019 #15

    neil_w

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    That is a weird way to add weight. Then again, that's a weird kit (although a pretty cool one!)
     
  16. Jun 17, 2019 #16

    mbeels

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    Yes, it seems that 3/32" papered balsa is at least as strong as 1/8" ply (if not stronger, I didn't test to destruction. It is certainly stiffer), and significantly lighter.
     
  17. Jun 17, 2019 #17

    neil_w

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    Got any documentation of that? I know papered balsa is strong, but I wouldn't have expected *that* strong. Also, I assume that's for glued paper; label papered balsa is significantly less strong (although still much stronger than un-papered...)
     
  18. Jun 17, 2019 #18

    Karl

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    I've used this sanding sealer in the past and whilst it does give a good finish, I find it takes infinitely longer than CWF. I tend to use it more on intricate builds where time isn't of the essence and it is far easier to brush on than wood filler on smaller pieces. 20190617_152114.jpeg
     
  19. Jun 17, 2019 #19

    mbeels

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    I don't, I should back off that statement. I'll say that when flexed between my fingers, 3/32" papered balsa felt at least as stiff as 1/8" plywood. I don't know anything about strength, because I didn't break them. But now I'm curious, maybe I'll make some test pieces and try to measure the actual strength. For sure, the papered balsa is lighter.
     
  20. Jun 18, 2019 #20

    jqavins

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    And actual stiffness, please, once you're taking the time to make measurements.
     
  21. Jun 24, 2019 at 12:01 AM #21

    Alan15578

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    OK, I'll toss in my $0.02, mainly because nobody really addressed the epoxy issue that you raised. But first...

    Weight of finish is important, but it is not the overriding factor. Possibly the most important issue is to minimize ugliness drag, and ugly is in the eye of the beholder. There are many factors to "explore the merits of". Just a few are: weight, cost, time, availability, smell, ventilation, durability, coloration, availability, water proofing, drag reduction, wet sanding and other personal preferences. So yes, whatever works for you, is probably the right answer. I guess I'm an old doper from the late 60's, so I guess that sets some of my biases. I actually started in Control Line airplanes before model rocketry. To me the ugliest finish is when someone brushes on coat after coat of sanding sealer, until the balsa grain is undetectable, and never sands. Don't be that guy.

    Cost is not a huge issue for me, but honestly , nobody wants to read the instructions in that new $5 starter kit that says to assemble and and finish this rocket properly you will first need to buy $100 of supplies. On the other hand, If you have lots of prime automotive finishing products at hand, why not use that? I used to be a serious contest flyer and I would typically have to design ,build, and finish say 10 models in 10 days, while working full time. Classic dope finishes work well in that scenario. I rarely have time to spray on some sandable filler/primer and let it dry a couple days between sanding, although I did that once on a scale model.

    The most time consuming and weight critical models are boost gliders, rocket gliders,and helicopter models. PD,SD, altitude, etc are quick builds, but still have to be done; weight is not as critical and you can often get by with plywood fins. The lightest way to add color to balsa for visibility is to use a dye, next best is ink, then colored dope, then colored acrylic, enamel, lacquer, and finally epoxy. I like to cover all but the lightest of my models with colored Jap tissue. This adds strength and stiffness, but it may just be one of those personal preferences. You can also use something like Monocoat, but that gets rather heavy for typical contest builds. The dye and ink should still have a thin coat of dope or sanding sealer to smooth the surface and offer some protection against water and moisture. I reject general finishing with CA under personal preference, so I have no idea where it fits in mass wise. It would be good to have all of the viable finishing options quantified with grams/square meter values, but such numbers would vary considerably by the individual doing the quantifying, and even by different samples from the same person. Mark Bundick wrote an excellent article for The Leading Edge Newsletter about dope finishes. I don't remember the title or issue, but they are available on line, and perhaps someone can provide the link. The thesis is that dope is evil and how to use as little of it as possible.

    The lightest glider that I have built many of is the Bumble Bee with a 1/16 inch thick wing. I'll cut out the wing and sand the lower surface smooth, then I'll paint 3/4 of an inch of the training edge on the lower surface with clear dope or sanding sealer. Let it dry and sand off the fuzz. This is to help keep the paper thin trailing edge together. Without this step the trailing edge will eventually separate along the deep grain, looking like a separate feather. After sanding the airfoil, the trailing edge will warp down just a little, increasing duration performance. About a 1/4 inch of the leading edge gets one coat of dope or sanding sealer to help toughen it against gliding impacts. This might be a place where CA could be used instead of dope. On larger, heavier gliders I may add a leading edge of hard balsa, bamboo, basswood, or even spruce. I'll finish the Bumble Bee wing with some magic marker coloring, and then give it a final smoothing with 600 grit sandpaper.

    Something like a Delta Katt typically gets two coats of clear dope to seal and smooth the surface, but no attempt to fill the grain. A Jiskra (evolved) gets the wing tissue covered and the tail sealed with 2 light coats of dope. A Groundhog, depending on size may get one layer of tissue on the inner 2/3 of the wing, and a second layer on the inner 1/3 of the wing. The stab gets tissued if I add a pop up DT. I could go on, but my point is that the finish is intimately tied to specific models and requirements, not one solution fits all.
    to be continued
     
  22. Jun 24, 2019 at 4:01 AM #22

    Alan15578

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    As a general rule, the harder the surface is the more mirror smooth it can be polished to. So if you want that mirror smooth reflective surface, you probably want an epoxy finish. Of course that hard surface will be difficult to rub and buff. You will probably need a metal polish and powered buffing wheel. On the other hand, dope is relatively soft and you can rub and polish the surface smooth by hand with a common rubbing compound and an old T-shirt. Wax is not really a part of finishing, but it helps protect the paint from oxidization. Wax can sometimes restore the luster of a blushed paint job. If you have sealed the wood, but not filled the grain and painted it with a dark color, wax can partially fill the deep wood grain giving the appearance of white veins. This may even reduce drag, at the expense of calling attention to your poor finishing.

    In my rushed contest building, I often do not have time (often due to weather) to spray the color coat on my rockets. I take them to the contest and paint then as soon as I arrive at the contest field. I'll hit them with a spray can of red or orange Pactra or Sig dope. Unfortunately, I cant find spray dope anymore. I don't want to drag out an airbrush, and then have to clean the airbrush. The dope dries fast in the hot summer sun, I rub down the surface with rubbing compound, and I'm ready to fly in short order. You can't do that with an epoxy finish.

    There are two schools of thought on epoxy finishing that I will not be addressing, HPR composite construction, and ultra light FAI models. You can find discussion of those elsewhere. Rather, I am a fan of the old Hobbypoxy and K&B products.

    I have used Hobbypoxy Formula II for finishing. Formula II was a thickish 50-50 mix epoxy that had lot of uses. I used it on the balsa fins of a k-36 Saturn V. This gave me a hard durable finish with sharp edges. I have also used it for finishing molds for vacuum forming, I would turn hard oak on a lathe, turning it a bit below my final intended surface, and then slather on a layer of Hobbypoxy Formula II. You do have to keep rotating the part until it sets up, so that it does not flow or slump. However, the real secret is that before you can do final machining in the wood lathe, you need to heat cure it. Put it in an oven on warm (140-160 deg.) for a couple hours. So epoxy finishing works great for some applications. Hobbypoxy is long gone, but I'm sure another suitable epoxy could be found. I also liked the HobbyPoxy Stuff filler.

    HobbyPoxy and K&B both had two part epoxy paint that was sprayable. The thinner would dry off quick enough, but the epoxy paint itself takes a while to set up and cure. During this time it can collect dust etc.; a heat lamp can help speed up the cure. I used the K&B paint on one test rocket. Meh. I also used it on a sailplane fuselage where I thought the durability would help on landings. The product that I truly loved was the K&B epoxy primer.

    more later
























    &
     
  23. Jun 24, 2019 at 8:27 PM #23

    beeblebrox

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