Sealing the wooden airframe......

Discussion in 'Finishing Techniques' started by MadProfessor8138, Jan 3, 2019.

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  1. Jan 3, 2019 #1

    MadProfessor8138

    MadProfessor8138

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    So this is the first time that I've completed a wood aircraft on my own from start to finish and I have a newbie question.....
    When the airframe gets sealed do you varnish everything or do you try to avoid areas that glue and covering will be placed?

    Kevin
     
  2. Jan 3, 2019 #2

    Wanttaja

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    You *must* avoid areas that will be glued together. Otherwise, you're just gluing two coats of varnish together; the glue is not soaking into the wood fibers.

    Mask off the areas that will be glued, and varnish the rest. When you're done gluing, re-coat everything with varnish.

    Ron Wanttaja
     
  3. Jan 3, 2019 #3

    MadProfessor8138

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    There's only a few areas on the airframe that will be inaccessible to varnish once everything is glued together and I have that part covered as far as when to do it.
    Yes,I know not to varnish anything that is to be glued together because the glue will not penetrate the wood fibers.
    My question pertains to the covering aspect of the build.
    What rule of thumb is followed during the varnish phase in preparation for the covering phase?
    Is it alright to varnish everything and then proceed to covering?
    Or does the same rule apply to the covering process that applies to the building........do not varnish anything that will be glued to secure the fabric to ?
    Simply put,will the fabric glue stick to a surface that has been varnished?

    Kevin
     
  4. Jan 3, 2019 #4

    Pops

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    This is two coats of polyurethane varnish and a top coat of Poly-Fiber Epoxy varnish, all brushed on. The MEK in the Poly-tack fabric glue with attack the Polyurethane varnish, but not the epoxy varnish. Light sanding between coats to knock the fuss down. Covered and painted with the Poly-Fiber process.
    Other set of wings I built in 2007 looks as good inside as the day it was built.
     

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  5. Jan 3, 2019 #5

    dcstrng

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    This is a process that I've never been clear on -- do you pre-varnish all the components (except the aforementioned joining/gluing/epoxying surfaces) or do you wait until full assembled... I had a fully build (wood portions) Buttercup wings at the raw-wood, but otherwise done, and was never sure where t go next -- solved it by selling the project, but that may not be the favored solution.

    Perhaps an equally dumb question, but on something complex like your pictures (geodetic ??), can you spray the varnish...

    Thanks
     
  6. Jan 3, 2019 #6

    Dan Thomas

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    This is right. Poly-Fiber warns about using the wrong varnish under their coverings, as the chemicals in the process can damage the varnish and cause it to delaminate from the surface, leaving the covering loose. Very bad on a wing or any control surface. Epoxy is the stuff to put immediately beneath the covering. And that applies to steel tube structures, too.
     
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  7. Jan 3, 2019 #7

    FritzW

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    Be sure to check your covering manual. As mentioned Poly-Tac (and Super Seam Cement) will make a mess out of polyurethane varnish. Stewart Systems Ekobond might be compatible with polyurethane varnish.

    I used epoxy varnish on the surfaces that were going to get Poly-Tac (no polyurethane varnish under it) and polyurethane everywhere else. First coat was brushed, second coat was sprayed.

    I only pre-varnished the stuff that was going to be inaccessible when it was assembled (LE skins etc.)
     
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  8. Jan 3, 2019 #8

    davidb

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    Is there something special about Poly-Fiber Epoxy varnish that helps it adhere to polyurethane varnish? Generally speaking, epoxy doesn’t adhere on top of polyurethane but polyurethane does adhere on top of epoxy.
     
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  9. Jan 3, 2019 #9

    Pops

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    No problem.
     
  10. Jan 3, 2019 #10

    cluttonfred

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    A few thoughts....

    On wooden boats I prefer "boat soup" (linseed oil, turpentine, japan drier, a bit of pine tar). Even though it doesn't last as long as varnish, you can just rub some more on when needed with no sanding or scraping. I wonder if any traditional aircraft used something like that?

    By the same token, modern one-part polyurethane spar varnish is pretty good stuff, and I'd be tempted to finish a wooden structure with just that except glue joints or surfaces to which fabric must stick. Under the glue or fabric adhesive (whatever the system) I'd just leave the wood bare and count on the glue or adhesive for basic protection.

    Either way, I'd expect my plane to see less sun and moisture in a lifetime than a boat does in a year.
     
  11. Jan 4, 2019 #11

    Dan Thomas

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    It would make glued repairs impossible. The linseed in the wood would discourage glue adhesion. With varnish, you can sand it off and fix the crack/break/delamination or whatever. Oil soaks in and keeps soaking in.
     
  12. Jan 4, 2019 #12

    TFF

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    Epoxy resin goes over polyurethane, at least resin, better than polyurethane over polyurethane. Polyfiber wood varnish is designed to go over any previous varnish; at least it says it on the can. Lots of planes used regular varnish that had to be covered with it to make it safe. The glue in Polytach uses MEK as the solvent. It will penetrate anything not bullet proof to it. You don't know if it acted like paint stripper where you can't see. I learned that Polytach over auto epoxy primer was a no no. It melted the PPG stuff like it was just pored out of the can. Just like the wood varnish, the stuff that goes over 4130 is not really primer in the classic sense. Its really a hard shell epoxy paint that happens to stick straight to metal. They know their products, if you are going to sub, make sure you know the ins and outs of why they made it like it is.
     
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  13. Jan 4, 2019 #13

    Pops

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    Yes, it's on the can. What I like about Poly-Fiber products is the fact that there are no surprises. Read the instruction manual and do what it says and you will be pleased in the results. Yes, I know, you don't like the smell. That can be worked around. I have a HVLP turbine spray outfit that also supplies 2 fresh air hoods. No smells. Putting the fabric and tapes on I open the patio door and garage door of the hanger for a good airflow and work upwind and also a box fan close for no smells.
     
  14. Jan 4, 2019 #14

    lr27

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    What's wrong with using epoxy instead of varnish for all coats?
     
  15. Jan 4, 2019 #15

    TFF

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    Nothing wrong with using epoxy only. Pops has his system that works for him. On my plane I had to paint over the one part varnish with epoxy for using Stits. The best thing is follow the instructions, unless you can handle the problems of coming up with a new system. Pops can; he has tons of experience. I would have just done the epoxy, but nothing wrong calling like you want it. What you are doing with any varnish is killing the mold spores in the wood. They are in the wood as the tree grows. The varnish is killing or encapsulating or whatever that keeps the wood from rotting, by not letting the add water, mold grow, trash part. Add to it, you are now making it a structural because you have to glue fabric to it. The varnish is now an anchor. Use good stuff.
     
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  16. Jan 4, 2019 #16

    Pops

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    I may be wrong, but the varnish seems to soak into the wood a little better and takes longer to dry than the epoxy varnish. Thinking of farther the varnish soaks into the wood fibers the better.
     
  17. Jan 5, 2019 #17

    lr27

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    Couldn't it just be epoxy instead of epoxy varnish? At least if it's going to be protected from the sun? You can also heat up epoxy so it flows easily.
     
  18. Jan 5, 2019 #18

    TFF

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    The Stits epoxy varnish is thin as water. Straight epoxy is too thick. You will add pounds for the surface area needed. I tried System Three Clear Coat. It's thinned T88 but it is still thicker than I liked. It has the consistency of laminating epoxy. If you just have to be an experimenter try this https://www.jamestowndistributors.c...MI0YfNmO7W3wIVT1uGCh0Q9QabEAAYASAAEgLZUvD_BwE It's the closest you will find to the Stits varnish.
     
  19. Jan 5, 2019 #19

    Pops

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    Don't see how you could beat Stits epoxy varnish. Its designed for the job and is an excellent product. I like the epoxy primer so much, when I painted my 1981 Chevy 3/4 ton truck, I epoxied primed it and painted it with Stits Urethane paint. With airplane paint, it has to be a fast truck :)
     

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  20. Jan 5, 2019 #20

    FritzW

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