Rest the rocket in a solid cradle. Here I'm using a padded PVC stand. Start by cutting off the stump of the old fin using something like an Exacto saw. It doesn't need to be fine-toothed. You just have to get the fin off. Some people have tried that 90 degree cut-off saw used to cut wood dowels off right at the surface. You have to cut through and remove the old epoxy fillets too. After that, you can start the process of removing the fin tab itself. Start by cutting into the airframe next to the fin use a Dremel drill with a cut-off wheel. I'm using that pencil extension that Dremel makes. You can hold this like a dentist's drill with your pinkie finger leaning against the airframe for very good control. Your other hand can easily hold and control the airframe. 

Next, I’m going to work with the replacement fin. Moe was nice enough to fabricate one from the original plans. It's 1/4" birch ply. I’m cutting the fin tab to fit. There is a centering ring at each side of the slot. There will also likely be some residual epoxy where the ring and fin meet the motor tube. I'll try my best to burr that out. I made some cuts at the ends of the replacement fin tab for a perfect fit. I don’t want any gaps between the motor tube, old rings and the new fin. 

 

 

 

 

 

 

    
  

Now, we can get rid of that old fin tab. I need some real control here because I'm going to remove the tab down to the motor surface. The new fin tab will be epoxied to this surface, and I want it as smooth as possible for the best bond. I will go very slowly and complete this step. The resulting slot is definitely not the prettiest, but all the raggedness will disappear after filling and filleting.

     

Here, I finally see how that new fin will fit. I also need to line it up with the airframe at 90 degrees to the surface.

    

 

After epoxying the new fin to the motor tube properly aligned (I use a circular transparent compass at the aft end and site in the fin 120 degrees relative to the other 2). The small gap between the fin and body tube is filled with epoxy one section at a time. I fill a 10cc syringe with mixed West Systems 405/205 epoxy and inject enough for internal fillets. West is thin and watery at this stage. It’s not at all like 30 minute 50:50 that’s very difficult to get through the gap. Now, I can position the rocket horizontally and roll it so that epoxy fills the entire section (formed by one side of the fin, the joint between the fin and motor tube and fin and inner airframe wall). Roll the rocket every 20 minutes so that the fin contact points end up with epoxy fillets. I can’t see that happen, but I know how much epoxy I added and the rocket position is obvious. It might take a few hours. You know you’re done the section when there are no more epoxy leaks out the fin slot gaps. Remember to wipe away any excess epoxy with acetone (West recommendation). I also use 91% isopropyl alcohol. The next day, mix and pour epoxy into the opposite section as before. When dry, the fin is glued solidly inside to the motor tube and body wall. Yes, this does take a couple of days, but it is solid. I make fillets from West 405/205 mixed with 407 Low Density Fairing Filler mixing to a consistency somewhere between chocolate syrup and peanut butter. I use blue masking tape to mask the area I want filleted (usually a few millimeters up the fin and the same distant out on the airframe). Roll the rocket so that the valley created by the fin/airframe is up and level. I create “dams” with the tape at each end of the fin to protect the body from excess epoxy and create the shape and angle of the fillet points. When the epoxy is poured, gravity does the work of leveling the fillet. I run a plastic spoon or short piece of PVC the length of the fillet to get rid of excess epoxy. Do this within just a couple of minutes or the fillet will get too tacky to run the spoon smoothly.  After that, wait only a minute and pull all the tape off quickly. When completely dry, you can use a Dremel burr then a 952 grinding stone to correct the fillets. Go to Vern’s Rocketry website for more details on the filleting procedure.

I went ahead and sanded the rocket and primed with automotive gray. I took the weathered nose cone and primed with automotive sealer/primer and sanded it with 60 grit sand paper. I then sanded progressively with 150, 220 and 400 grit.

I'm sure you can tell from the photo which fin was replaced.

    

It's alive!!