Tuesday, July 9, 2013

The Secret to Perfect Fins

It's shaping up to be another skimpy summer for rocket launching in Colorado. Fire bans have been declared almost everywhere in the state, and even in places where there are none (including Denver) you can't help but feel guilty just thinking about launching with all the wildfire destruction going on.

But there is always building. I've just finished two kits: Pemberton's Little Bucky Jones and FlisKit's Buck Shot.

The Buck Shot is my first MicroMaxx kit, so I'm excited to venture into that territory. I know my limitations, and my build skills aren't up to the challenge of making a high quality small model, so I just put it together and painted it as best I could without sinking in time going for perfection.

The LBJ, on the other hand, is such a unique and gorgeous kit that I found myself obsessing over it for weeks. Which brings me, sort of, to the topic of this post.

For several months I'd been vaguely planning on writing my tract on the ultimate solution to perfect fin finishing. Of course, most of the delay involved figuring out how to perfectly finish fins. There is no shortage of information out there on blogs, vendor videos, and message boards, and everyone has their favorite techniques. But something must be the best, right?

I have, I'm proud to say, discovered the true, absolute best solution. Are you ready?


There's sanding sealer, Elmer's Fill 'n' Finish, microballoons, CA glue, Kilz, urethane, on and on. But ultimately, no matter what materials you use, it comes down to patience. Lots of layers, lots of sanding, until you have it just right.

Sanding sealer stinks, FnF can be a little delicate, watered down Elmers can warp fins. But for me, the patience thing is harder to manage than any of those. Though if I stick to a willingness to always do another layer of fill/sand, or primer/sand, the imperfections that I see (and there are always some) get smaller and smaller.

That said, here is my current process when I'm tying to do a really nice job on a rocket. I'm not going to go into extreme detail on any particular technique; there is so much info on the web, and really you just have to try things out and experiment. But here are my favorite methods. And I couldn't help myself - I've gone beyond fins to describe all of my "best practices."

Construction Adhesive

I've barely dabbled in mid power rockets at this point, so for me the go-to glue is wood glue. I've used a lot of Elmer's, but lately I've been using Titebond. Used properly on balsa and cardboard/paper, this will make a bond stronger than the materials themselves. I do, however, sometimes use 5 minute epoxy in two cases. First, because it doesn't grab as quickly as wood glue, it is handy for doing things with a tight fit, like engine mounts, centering rings, and couplers. Second, it is nice to use if I have to fill a nose cone with some lead shot to weigh it down. Stinky stuff though. I avoid it when I can, and wear nitrile gloves when I do use it (but even here, the web has lots of conflicting information on what gloves are best protection from epoxy).


Light plywood and basswood are great because the grain is typically a lot tighter than balsa. I've gotten some terrific, smooth finishes on these with just one thin layer of FnF, sand, and then a couple primer/sand cycles.

I find balsa tricker. I've gotten decent results with one or two FnF/sand cycles, followed by a couple of layers of sanding sealer (sanding in between), then primer/sand/primer/sand. But lately, I've become a fan of this:

  1. Before mounting, laminate the fins with ordinary copier paper, glued on with a thin layer of wood glue. It is very important to make sure you smear the entire fin surface with the glue before sticking on the paper. I smooth out the paper as best I can - I use a small section of PVC pipe. The fins may warp like crazy with the wet glue on them. Put them between wax paper sheets and press them between two very flat surfaces, weighted down amply, for a day or so to dry.
  2. Smear the fins with a thin layer of CA glue (crazy glue) and let them dry. This will turn the white paper covering translucent as the CA glue penetrates and hardens the paper and wood fibers.
  3. Sand sand sand until smooth. The CA glue will leave a bit of texture, but with a lot of sanding you can get them glassy smooth.
  4. Mount the fins on the rocket.

This process adds a ton of strength, and you don't have to worry about grain showing. I recently did a test where I weighed fins before and after this process, and also compared the weight to lite ply fins of the same size. The laminated fins were still a lot lighter (but I lost my data!).

Body Tube Spirals

This is something that I never considered early in my rocket career, but now I can't stop myself from doing it - I'm compulsively bothered by visible spirals on a painted rocket. I use some slightly watered down Elmer's FnF (now just called Wood Filler) and paint it along the spirals. Then after it dries, I sand the whole tube. It's not a lot of effort, so it is totally worth it (though I might not say the same thing if I built an Estes Mean Machine). Some people claim to get rid of spirals with several coats of primer and sanding, and that might work with a thick primer like Kilz, but I think this is a good approach. Sanding the tube also helps with fin and paint adhesion.


I have two favorites here. For small, light rockets, I like Titebond Molding and Trim glue. It doesn't run, and shrinks and bubbles less than other wood glues. (I had a hard time finding it. Regular hardware stores don't seem to carry it. I had to go to a specialty woodworking store.)

For bigger and higher power rockets, I like Fixit epoxy clay. You can find some good directions for its use on the Apogee Components website. It's not quite as easy as it looks. When mixed, it's just a little too firm to just smooth out with a swipe of your finger. You have to work at it. In fact, you need to work at it. When dry, it sands nicely, but of course is much harder than the body tube and fins, so if you need to do a lot of sanding to get the fillets nice and smooth, you might end up digging in to the other components. It takes practice - my first rocket using this had OK-looking fillets. My second one had a few that were perfect, and a couple with minor imperfections. But in the end, you can get super solid joints, with fins that look like they grew right out of the body tube.

Nose Cones

For balsa, I've gotten amazing looking results with a layer or two of Fnf/sanding, then the same with sanding sealer, then a few coats of primer and sanding, finishing with wet sanding with a really fine grit. Totally worth the effort, though it still results in a nose cone that will get dinged the first time you chuck it in the back of the car with a bunch of other stuff. I've seen recommendations to add a couple layers of CA glue to harden it up and give it a sort of shell, so I'm still experimenting with that.
For plastic nose cones, like BT spirals, I'd never considered anything to do in my early days. Now, I always sand the seams *completely* down so they cannot be seen or felt at all. Then I need to smooth it out with successively fine layers of sandpaper. Ten more minutes of effort, but a way way nicer looking nose cone.


Lots and lots of people swear by Kilz because it goes on so thick. And it is great for hiding all the little blemishes - just like the "orange peel" texture applied to drywall in all modern residential work. So there is typically a lot of sanding needed to get back to a really smooth finish. If you're willing to do that, Kilz is great. However, I prefer not to use it on small models, or ones with lots of detailed pieces and angles that are hard to sand. In that case, I'll use a "regular" sandable primer (I use Rustoleum).


I don't have experience with enough brands to give a strong recommendation here. I can only say that Valspar is awful, and that I use Rustoleum Ultra Cover 2x glossy pretty exclusively and am happy with it. It goes on pretty well and there is a good color selection.

Clear Coat?

Question mark here. Do you need a clear coat? It is often recommended over decals (though I'm not a huge decal user), but many say that it will eventually dry and crack/peel the decals. I found this process on the web, using Pledge and Simple Green. It works, but it is not magically transforming. I use it occasionally and am still undecided if it is worth it.

So there we are - that's the state of my technique as of mid-2013. Perhaps, even hopefully, I'll want to totally revise this in years to come. I like to try new materials and methods, and I'm sure I'll get new favorites.

But it all comes back down to the P-word. Without lots of patience, nothing is going to look good.

Of course, despite all this, I look at every rocket I build, especially the ones I really really tried on, and just see the little flaws. And the parts, that were perfect? Well, they get bashed up in flight anyway. So at some point in a build, I always end up saying "enough is enough" and I just call it done so I can get the bird in the air. Patience has its limits, at least with me.

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