I do not like shaping external fin fillets.
As I talked about above, I measured the root chord of these fins to 30 cm. A 6% radius fillet (18 mm) looked ridiculously big to me, so I decided to use a 4% radius fillet (12 mm). So I started out by removing all my existing fillet masks, and re-masking everything off to 12.5mm (1/2"):
Note that there is a small fillet of structural epoxy against the external joint. I shaped this earlier on, when I bonded the fin into the motor mount tube. My rationale is that cutting a slot in the body tube weakens the tube, so I want a little extra strength at this joint, even though it's reinforced with another structural fillet from the other (internal) side.
For a tool, I dug through my scrap pile until I found a piece of 3/4" ID Schedule 40 PVC pipe that was about 1.5" long. This is (just barely over) 1" OD, so just about perfect. I polished down the ends with some sandpaper to smooth off the burrs.
Then I mixed up a generous batch of Rocketpoxy and started adding microballoons to it. Since this fillet is mostly for aerodynamics rather than strength (see above comment), I didn't see a point in using unadulterated structural epoxy. It would add a ton of unnecessary weight on the wrong side of the rocket. I stopped when the epoxy mixture got to the consistency of airy bread or cookie dough (about the same as wood filler, actually).
Then I started to pack the fillet joint with epoxy... first mistake.
This is
way too much epoxy. :eyepop: Something like half that amount would have worked. Instead, I made a mess.... I ended up wetting the entire surface of the fillet and my shaping tool with isopropyl alcohol. Using
only one edge of the tool, I scraped away epoxy in successively deeper passes. If the tool felt like it was "catching" I was pulling off too much, and starting to disturb the underlying epoxy time to stop, clean off the tool, re-wet with more isopropyl alcohol, and start again. I was also not at all shy about getting my (gloved) hands with alcohol and getting in there to smooth things out with my fingers.
After about ten minutes of frantic labor, I had the long sections mostly shaped. I kept at it, making the angle of the tool more and more acute, until I could pass it along the fillet while pressed into the joint (that is, in contact with the body tube and fin masking tapes). That left something like this:
Then I tackled the corners... basically I tried to reverse the direction of the tool around the fin, so that at the tip of the fin the tool was perpendicular. That is a bad explanation, so here is a picture:
Whew. After that was done, and boy did it generate a lot of mess, the epoxy was starting to set up pretty firmly. I pulled the masking tape off and used Q-tips soaked in isopropyl alcohol to clean up the seams (and other stray blobs). I'm reasonably happy with the end result, although it's definitely going to require some sanding and a few coats of wood filler to get the little imperfections smoothed out:
One down, two to go....