Finishing Aircraft plywood (birch) fins

[Fore Check]

I'm about to start finishing/smoothing my first set of plywood fins. I have no experience with plywood fins - always have used balsa or plastic. Any recommendations for sealing and finishing? Is the balsa filler stuff the way to go? "Dry rubbing" with carpenter's filler?

I appreciate your suggestions. These are 1/8" 5-ply birch plywood fins.

 

[Zippy]

I like to put on a full coat of Bondo spot putty on all my wood fins. It's sandable in about 20-30 mins. Then I tuch up where needed with more spot putty, primer and wet sand wich really shows up the remaining flaws for yet more spot putty, primer and wet sanding. 2 or 3 coats of paint wet sanded between coats and then a final coat of clear and then Turtle wax. Then I take the rocket out, fly it and get the fins all banged up.

 

[Fore Check]

Thanks zippy,

but I was hoping for an easier method.....

 

[lalligood]

I've experienced great results by using water thinned Elmer's Fill 'n' Finish on plywood.

Add enough water to give it a paste-like consistency & then spread it on with a putty knife, popcicle stick or even a latex gloved finger! (Oh yeah...almost forgot. If your mix gets a little thick while using it, just add H2O to thin it back down.) Let it dry, sand it smooth, & then I don't think I've ever had to lay down more than 2 coats of primer before I could start with the glossy finish coats...

Just my $0.02. I'm sure there are dozens of excellent techniques out there as well.

 

[Zippy]

It isn't hard, just time consuming. Of course you could just spay them with rusty metal primer, sand and then paint and you'll be done in an afternoon.

Zippy

 

[astrowolf67]

I sand them with 220 grit, then with 400 grit. I primer the rocket with 2-3 coats of primer, thick on the fins. Then sand till smooth, usually till the wood is barely showing on the fins. One more good coat of primer on the whole rocket, then sand and start painting as normal. Another option, is to use Kilz primer. One to two coats of Kilz is usually enough to fill fins and spirals.

 

[Fore Check]

I just received my big new 1/8" 5 ply birch plywood fins, and they have a bit of a WARP to them! AAAAAAAAAAAAAAAAAAAAAAAAAHHHHHHH!

Any suggustions on how to get the warp out??????

Along the root edge (which is 10" long) they are warped between 1/16" and 1/8" from top to bottom.

I paid quite a bit for these, and I don't want to break them or mess them up.......

HELP!!!

 

[jetra2]

Big, heavy books should do it! What are the fins for??? Inquiring minds wanna know! Leave the fins under the books at LEAST overnight, a full day would be even better! DO NOT pick up the books to check on them, either...just leave them ALONE!

My 2 cents...

Jason

 

[Fore Check]

Let's just say the fins are for a "healthy" LPR.

Once I get the warp out, I'll start finishing them - that should be the most time consuming process. Once I get the correct centering rings in (I messed up when I ordered the ones that came with the the stuff that included my fins) I'll assemble and paint it. Decals are on the way........

I promise pics when its finished. Hopefully before the end of next week or it will have to wait until after the 2 week Christmas vacation. Needless to say, it is my next project. I'm quite excited!

 

[powderburner]

1) Send 'em back (after all, you paid for the stuff, and there is an implicit requirement that fins be straight). Pay extra shipping, and wait a couple weeks.

2) Bend 'em back into shape yourself. Could be risky. If you break them then you are out your $$$$ (the maker can't be blamed if you tear them up). Maybe try reverse-warp-pressing them under some weight, prop up the front & rear 1/8 inch or so, moisten?, wait to see if they come out straight when you take the weight off.

3) Clamp them between strong, straight 'hardback' pieces (like a couple blocks of hardwood) placed near the root on both sides of the fin but with some of the root still sticking out. This should leave you enough to work with to get the fin glued and filleted onto the airframe tube. After you take the hardbacks off, then the airframe joint will hold the fin straight.

4) Use them as is and issue everyone hardhats and kevlar vests.

 

[Fore Check]

Originally posted by powderburner
1) Send 'em back (after all, you paid for the stuff, and there is an implicit requirement that fins be straight). Pay extra shipping, and wait a couple weeks.

2) Bend 'em back into shape yourself. Could be risky. If you break them then you are out your $$$$ (the maker can't be blamed if you tear them up). Maybe try reverse-warp-pressing them under some weight, prop up the front & rear 1/8 inch or so, moisten?, wait to see if they come out straight when you take the weight off.

3) Clamp them between strong, straight 'hardback' pieces (like a couple blocks of hardwood) placed near the root on both sides of the fin but with some of the root still sticking out. This should leave you enough to work with to get the fin glued and filleted onto the airframe tube. After you take the hardbacks off, then the airframe joint will hold the fin straight.

4) Use them as is and issue everyone hardhats and kevlar vests.

Will the method that Jetra described be useful?

I already talked to Balsa Machining about the fins, and he basically said "@$%!/# happens." He blamed it on the size of the fin and made no overture to send me straight fins.

 

[Fore Check]

If they break, well.......... I've wasted more money on stupider stuff. (Nice adjective, eh?)

I guess I can cut them from 4" x 1/8" balsa stock, but I was hoping for fins I can count on not getting chipped, dented, or broken as easily considering their size......... Cutting from balsa would be risky, though, because the grain would not be going the right way.

 

[powderburner]

I hate to see a vendor take an attitude like you report, but on the other hand, if this was any other kind of thin wood product it would probably also warp. Wood really can be a b***h to work with.

Yes, I think Jetra's approach (press it back to flat)should work. I suggest that you be prepared to press it a little beyond flat so that if it springs back a bit it will still end up close to flat. Sometimes moisture/humidity helps wood soften up to take a new shape. And then don't wait around to get the booger glued and filleted, or else it will warp again in the next couple days.

 

[Fore Check]

Will balsa sanding sealer/filler work to smooth out plywood? What would be good to get them smooth? I usually "finish" them prior to application, so it sounds like I need to do sand them and do that part first. My next suggestion would be dry rubbing with carpenter's filler or fill 'n finish.

 

[astrowolf67]

My LOC Graduator had warped fins when I got it. I took the fins, and sandwiched them between damp paper towels, and stacked some heavy books on them. I left them like that for about a week, and it straightened them out.

 

[Stymye]

Forecheck
It could be from the cutting process ,the weather
but should be fixable

keep in mind that all ply is not created equal.and be sure to specify the right ply for the job

a little snip I got from a vendor on rmr(can't remember who ),and I saved it lol

Aircraft Ply is a misnomer, there are at least 4 grades
of aircraft ply,

B grade
which can contain plugs in any of the layers and the layers are
thrown together haphazardly which can mean ALL of your grain is ui-directional, (craft store)

A grade
which can have plugs but only on the inner layers again haphazard grain... (craft store type)

AA grade
which can have plugs on the inner layers only but the sheet are aligned cross grain
(this is what most rocket companies use)

AAA grade
the wood has no plugs and each sheet
is run cross grain,
strong enough to cut it in any direction

Baltic birch is a vague reference
if you order Baltic Birch, depending on the company you ordered it from, you can wind up with
one of a selection of woods from 4 different areas, Finland, Russia (three of thier
ex-satellite countries make Birch Ply)... Finnish Birch is the best since it uses a
waterproof black glue that is much stronger than the other 3.
and doesn't warp near as easy

As for Basswood, as long as you
cut your fins carefully so that as much grain as possible touches a point of the rocket
that it is fastened to you will have a very hard time breaking it if your joint is properly
done. (my choice for mpr)

 

[sandman]

Try this...it can't hurt.

Wet the fins...not a lot, just a little then place them between two flat 2x4's and chuck them into a vise for a day or two.

Or under a bunch of books. The 2x4's will soak up the water and the water will relax the wood fibers a bit and soften them.

When the 2x4's soak up the water and everything is dry, they should be flat.

sandman

 

[Fore Check]

Well, I have somewhat "good" news....

My main panic attack of earlier today was a result of me coming home from work for lunch (I get about an hour and live about 7 min away) to find my BMS order on the porch, so I opened it up to inspect the much anticipated contents over a sandwich. Indeed, the fins are/were warped - and I had to dash back to work.

Now that I'm home and have had an opportunity to mess with them a bit, I seem to be having a degree of success straightening them out simply by bending on them by hand. They seem togh as nails!!! NO hint of cracking or anything with the pressure I'm applying.... I'll keep y'all posted.

As far as which "grade" of ply this is - I have no idea. I ordered the fancy stuff from BMS and this is what they gave me. The surfaces sure are pretty and smooth and free of any irregularities.

 

[Nite Builder]

Forecheck;
Although warpage can be somewhat frustrating I must admit that almost every fin that I cut from birch plywood has at least some degree of warpage. Usually not so bad that I have to take adverse actions to correct it, however I have had situations where they were real bad, and I corrected them by the same means as previously suggested( Little bit of moisture, reverse bending with weights).
Now, as for the finishing part of the fins,lets talk about a 30 min. Epoxy. This is what I use to reinforce "all" of my fins wether they are made of ply or balsa. In some cases(mostly minimum diameter rockets) I have used fiberglass wallboard tape first, then followed by 2-3 coats of 30 min. epoxy. Mounted on a glassed Bt and you have a "Tank". Sanding time is slightly increased( 100 grit,240 grit 400 grit) but you'll also be quite impressed with how well they paint.......
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~>>>====> Good Luck.....FlyHigh

 

[Stymye]

Forecheck

I usually finish ply,and basswood the same as balsa,unless I need to glass the fins for some reason

fill and finish (or some simular type of light filler) or kilz primer

sometimes I use bondo spot putty for smoothing up fillets and filling spirals

something I've learned about bondo
spot putty is great for fixing imperfections but It is extreemily HEAVY!!!!,
way overkill for using exclusivly as a wood filler,plus it shrinks and and cracks very easilly.and is not in any way compatable with epoxy I wouldn't suggest glassing over a completely bondo surface as it's a polyester based material.
although I have glassed over bondo filled spirals without any noticable problems or bubbles