Fin papering techniques, methods - documenting pro's and con's

[afadeev]

I've been papering my balsa and basswood fins for years, exploring and improving on various techniques along the way.
Adhesive labels, or gluing paper skins with wood glue? Or just sealing the wood grain with wood filler (CWF) or primer? Sealing the edges with CA, or not? Sealing the entire surface of balsa fin with CA?

I've developed my preferred approach (paper skins glued with TBII, then edges sealed with CA), but was mostly out of convenience and guesswork. I've never documented or run side-by-side comparisons for benefits and weight penalties from various approaches. Searching this and other forums, hasn't turned up documented comparisons on this subject either.
Thus, I've decided to setup and document my own set of experiments.
I will share the findings in this thread.
Feel free to send suggestions my way for other techniques I should evaluate.

Here is the list of variables that I intend to measure and document:

• Starting weight.
• Finished weight
• Finished surface smoothness
• Ease and total time of prep
• Strength ?

Subject of the experiment - balsa fins from BMS School rocket: https://www.balsamachining.com/


Fin preparation techniques that will be evaluated:
1). Balsa fin grains filled with Elmer's carpenter's wood filler (CWF).
2). Balsa fin grains filled with Elmer's carpenter's wood filler (CWF). Then primed, painted, and polished to "ready to fly" state.
3). Balsa fin grains filled with few coats of primer paint? Then primed, painted, and polished to "ready to fly" state.
4). Balsa fins papered with glued-on (TBII) skins.
5). Balsa fins papered with glued-on (TBII) skins. Then primed, painted, and polished to "ready to fly" state.
6). Balsa fins papered with stick-on adhesive labels skins.
7). Balsa fins papered with glued-on epoxy (BSI 30 mins?) skins.
8). Balsa fins soaked in CA, tip-to-tip. Then sanded, painted, and polished to "ready to fly" state.

Target output data matrix:

Weight before (in grams)

Weight after (in grams)

Finished smoothness (1-5 star subjective eval)

East and time of total prep (in minutes & 1-5 star subjective eval)

Strength (destructive test? 1-5 star subjective eval?)

Feel free to comment, make recommendations on what other variables I should track.
Preferably before I get too far along in the experiment.

 

[afadeev]

Step 1: measuring the "before" weights.
Surprisingly, the 4 (four) fins that came out of the same balsa sheet measured significantly different weights. Evidently, the density of balsa varies a LOT even within the same sheet.

4 fins, together, weighted at 3.5 grams.


However, 2 (two) of them individually weighted in at 1.2 and 1.3 grams, while the other two weight 0.6 grams. With rounding errors, that adds up to 3.5g.

Discovery #1 - balsa fin weights vary by a factor of 2x, right off the sheet:



I've rounded the edges with NCR sanding block (to achieve even shape, and also prep the best four fins for use on the rocket down th eline), then color coded the root of each fin, and will track their weight gains throughout the process.

Visually, you can't tell which fin is denser and heavier.
In the picture below they are arranged with the heavier fins on the left, lightest on the right:


NCR sanding blocks link is below, just in case someone is tempted to give them a try (I find them really handy for achieving consistent fin edge curvature): https://northcoastrocketry.com/collections/accessories/products/airfoil-assistants

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[rharshberger]

I have had self adhesive label paper bubble in the center of fins usually when the cheap WalMart spray paints are used that are more solvent than pigment just something to watch out for, not an issue with glued on skins (Elmers Glue-All or TB products).

 

[neil_w]

Discovery #1 - balsa fin weights vary by a factor of 2x, right off the sheet.

Now that I would not have ever guessed. When I've weighed my fins individually (which is to say, only a few times ever at most), they've been pretty consistent. Now I'm gonna start weighing them in the future just out of curiosity.

 

[rharshberger]

Now that I would not have ever guessed. When I've weighed my fins individually (which is to say, only a few times ever at most), they've been pretty consistent. Now I'm gonna start weighing them in the future just out of curiosity.

I can believe that balsa varies that much, trees that grow very rapidly dont seem to be as consistent in density as slower growing trees like oak or walnut. We have a hybrid poplar around here used to pulp wood farms, it grows like a bat out of hell (larger than 6" in less than 10 years) and its some of the most variable density wood I have ever dealt with.

 

[afadeev]

Now that I would not have ever guessed. When I've weighed my fins individually (which is to say, only a few times ever at most), they've been pretty consistent.

Same here!

This got me wondering, so I unpacked a second set of fins from another BMS School rocket kit, and could immediately tell that these were lighter and flimsier.
The lightest of the fins in the new pack, weighting just 0.4g, was physically distorting around my fingers as I was holding it to sand the edges.

No surprise that I had broken many a balsa fin tip while sanding them in the past. I used to blame it all on my clumsiness, which still is a reasonable hypothesis, but some of the balsa areas are remarkably more pliable and fragile than others!

All 4 (four) new fins weight 2.2g in total (vs. 3.5g for the first set), with individual weights as follows: 0.4, 0.5, 0.6, 0.7g.


Though the specific weights differ from the first set (there is some overlap), the appreciable wood density, strength, and weight variability is still evident.
This will complicate my experiment.

The lightest and the most porous of the fins are significantly flimsier than the more dense fin samples. Thus, the final rigidity and strength comparisons will be biased by the variable starting values.

No matter. We can still record and evaluate delta weight gains from different finishing techniques, and the subjective strength improvement they bring.
I also do have 4 similarly weighted .06-.07g fins, and I can still compare strength variability across that sub-set!

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