LESSON THREE: Attaching the fins to the fin can 

In this lesson, we will be attaching the styrene fins to the styrene fin canister using Tenax 7R Plastic Welder, a plastic solvent that literally melts the styrene momentarily before the solvent evaporates and the styrene re-dries, leaving a solid plastic joint.

In the last lesson, we attached the styrene fin canister to the motor mount tube and then attached the airframe tube to the motor mount assembly. At this point, the styrene fins are ready to be attached to the styrene fin canister, but before we can do that we should prepare the airframe tube for final finish by priming it and filling the spiral lines in the cardboard tube. Why are we doing this at this point? Because the upper portion of the fins extend beyond the fin canister onto the airframe tubing, and finishing the airframe tubing now will eliminate having to prime and sand around the upper fin area and eliminate excessive primer build-up around the fins, creating sharper, crisper edges.

I seem to always end up questioning which primer and paint is the best when I am relegated to using spray can paints. I never seem to have the problem when I am using automotive spray paints and primers — I use urethane-hardened acrylic enamels and have always had great results from doing so. But since I can't reach into the contents of a spray can and thin the mixture to my desired consistency, I put more thought into my choice of spray can paints.

In the photo above, I went to my local Wally-World and selected five different kinds of paint: one top-coat and the other four primers. Two of the primers, Rustoleum Automobile Primer (#2 & #4), are alike, just different colors. I wanted to see which worked the best for my task ahead, and one of my concerns was a primer that didn't lay on too heavily, filling and obscuring the numerous fine details of the nose cone, fins and fin canister, but I needed a good filling primer that sanded easily for the actual body tube.

I began my airframe tube priming process by masking off the fin canister with masking tape as shown in the photo above. I was careful to closely follow the airframe/fin canister joint so as to get a clean line on the body tube.

While I was at it, I decided to prime the nose cone as well, and carefuly taped up the shoulder of the nose cone, after I had inserted a short length of stiff wire to hang the nose cone from while the primer dried. After I had it taped up, I sanded the nose cone with wet-or-dry 400 grit sandpaper, using it dry, and blowing off the dust created by the sanding process, making sure I got the nose cone well sanded.

I ended up using two different type primers on this rocket in the end, and for the reasons mentioned earlier: I wanted to use a light coating, thin primer on the plastic parts so it wouldn't exccessively fill in the molded in details, but on the airframe tubing I needed a thicker, filling primer. I used the gray Rustoleum Automobile Primer (#2) for the nose cone and styrene fins and fin canister because it went on thin and covered these plastic parts relatively well. I used Dupli-Color High Build Formula Primer (#3) for the airframe tube because it went on well, filled well, and sanded easily. I didn't believe the Krylon Primer (#1) would sand well, so I didn't use it at all. I purchased the two colors of the Rustoleum Automobile Primer because I like to use the lighter primer under light top coats and the darker primer under darker top coats. The white Krylon (#5) was the choice for top coats.

So priming the airframe tube was a process of applying the Dupli-Color primer, letting it dry and sanding it off, then repeating the process. Not one dap of spot putty was used, just prime, sand, prime, sand. Repeat, repeat, and eventually, it's ready. Because of the small size of this rocket, comparatively, I took my time and sanded using 220 grit sandpaper, to make it as smooth as possible.

After the airframe priming and sanding was completed, I finished up by sanding the styrene fin canister with 400 grit wet-or-dry paper, using it dry. I sanded this now because it would be easier to sand than after the fins were attached.

With the airframe primed and sanded, it was then time to attach the styrene fins. The first thing you want to do is remove any excessive plastic flashing from the fin tabs where it mates to the fin canister. Any excess flashing can cause the fin to have a gap between it and the fin canister and any mis-alignment here can cause the fins not to end up mounted straight and cause a weaker joint. So take your time removing the excess plastic flashing using an Exacto or utility knife.

Take the first fin, and snap it into location, checking to see if it is lined up correctly, if there are any gaps and what might be causing it not to fit correctly. Once you have a nice fit, you are ready to apply the Tenax 7R Plastic Welder.

As mentioned before, this liquid solvent is very thin and runs all over the place if not contained, and a plastic melting solvent isn't something you want running all over your nicely molded and detailed plastic parts. But using the Touch-N-Flow applicator with Tenax can be a lesson in futility because it controls the liquid too much, only distributing one tiny drop at the time. That might be nice when building that highly detailed model car or plane, but not when attaching something as large as these fins. Strangely enough, I got my best results putting the Tenax in the Touch-N-Flow applicator's fill bottle and using the short needle on it to apply the liquid.

To get the best results, hold your airframe and fin so that the root edge where you will applying the liquid is horizontally in front of you, and then using the applicator fill bottle, work from your left to your right, applying an even stream of the Tenax liquid. If any runs over either end, just let it go. Don't touch it, let it evaporate. If you get the right amount on, it will run under the fin's root edge following a capillary action, drawing it further into the joint. Set your applicator bottle down and firmly press the fin to the fin canister and hold it approximately 15 seconds. The resulting bond is very, very strong. Flip the airframe over and repeat the proccess, repeating until all four fins are attached.

After you have all four fins attached, the results should look like the photo above. You can see from the photo why I wanted to prime the airframe beforehand, because of the leading edge on the fins. You can also see in the photo, circled in red, the Touch-N-Flow applicator's fill bottle, with its own needle-like tip, which is what I used to attach my fins. It distributed a lot more liquid at one time without having to go drop-by-drop and isn't nearly as finicky to fill as the actual Touch-N-Flow applicator.

At this point, sand the fins with 400 grit wet-or-dry sandpaper, in dry mode, and then apply a few coats of the Rustoleum Automobile Primer, lightly scuffing between each coat. You did tape up the exposed motor mount tube to keep the paint and primer off of it until finished, didn't you? We want our AeroPack retainer to fit and not come off, so make sure you mask that off before hand.