# Finishing Aircraft plywood (birch) fins

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#### Fore Check

##### Well-Known Member
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

##### Well-Known Member
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

##### Well-Known Member
Thanks zippy,

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

#### lalligood

##### Well-Known Member
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...

#### Fore Check

##### Well-Known Member
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

##### Well-Known Member
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

##### Well-Known Member
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

##### Well-Known Member
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

##### Well-Known Member
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,

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)

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

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

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

##### Well-Known Member
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-Known Member
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

##### Member
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

##### Well-Known Member
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