Quick, Easy, and Perfect Epoxy Fillets

When I look back at the kits I built as a kid, I ponder what about them was the most awful; The crooked fins? The badly filled balsa grain? Those were bad, but I have to think it was my lousy fin fillets; icky globs of white glue unevenly smeared along the root of the fin. So it stands to reason that now as an adult rocketeer I like to make sure my fillets look good, and its usually the first thing I admired on another persons model.

Nicely laid epoxy fin fillets add strength and beauty to your model. Conversely, an irregular fillet may hold your fins, but it will detract greatly from the appearance of your completed project. These finishing touches can spell the difference between a rocket you hold proudly while the RSO admires it and a skanky thing that only looks good in silhouette.

The InfoCentral pages at Rocketry Online show an interesting filleting method that entails creating a trough at the fin root for filling with low viscosity epoxy. As much as I like the idea, I see some distinct drawbacks. For one, the method cannot be used if the fin goes over a curve or a transition. And if your epoxy ends up being too stiff to flow well, you could end up with a real mess. With my luck I need very fault tolerant methods.

Shopping at Menard's Cashway Lumber once upon a time, I came across a brand of epoxy that struck me as being very unique. The product is called PowerPoxy, and it comes in a variety of grades. What is very unique is that instead of a squeeze bottle or tube or syringe, it's packaged in tubs (it also comes in syringes). You may not have a Menard's near you, but you can contact visit at www.DoItRight.com for a retail outlet near you (When you get to the page, hit the "More Info" button to type in your zip code).

I've tried a couple of different grades of PowerPoxy, but so far the Wood Grade is my favorite. It sets in 20 minutes so you get plenty of time get it to look right before it sets up on you. The most amazing feature of the product is its viscosity. When properly mixed it is the consistency of cold peanut butter. This is a real advantage over other epoxies, in that it stays where you put it and it will defy gravity and not run. It's also a big improvement over adding micro balloons or other additives to thicken epoxy. Recipe for an excellent fillet:

Tools and Supplies:

• Wood Grade PowerPoxy (no guarantees if you use something different)
• Box of Popsicle sticks (the end radius of the Popsicle stick will determine the fillet radii, so choose wisely). You will consume several with each fillet.
• Scrap piece of cardboard to mix upon
• Paper towels (just in case)
• Wide masking tape
• Denatured alcohol
• Spot putty (also known as glazing putty) found in most auto parts stores.
• Latex gloves

Step-by-Step Instructions

Start by making sure the fins joint is smooth and no clods of epoxy are left from fastening the fins down. I like to use a dremel tool for this.
Next apply the masking tape to either side of the fin root. For best results make sure the tape is an even distance from the root and straight along the tube.
Mix up a batch of power poxy. Scoop equal amounts of hardener and resin (use a Popsicle stick for each) and mix. The stuff is very stiff, so take care not to break a stick by going go at it too hard.
Always use a fresh stick if you go back to the tubs get more material, as a safeguard against cross-contaminating the resin and hardener. Power Poxy is so thick I will often mix it up by rubbing the two craft sticks (laden with resin and hardener)in mid air.
Using another stick, dab the epoxy on the fin joint. Shape the epoxy with end of the Popsicle stick. Hold the edge of a popsicle stick along the fin flat, and jam the rounded end into the fillet, and draw it along the length, do the same again holding the stick tangent to the body tube. Go over the fillet several times until you are satisfied with the shape. If you run out of mixed epoxy, quick make some more and repeat step 3 until you are satisfied.

Keep packing down the epoxy until you have a nice, even fillet

Close Up!

Remove the excess. By now the fillet looks good, but there are wads of epoxy on top of the masking tape. Carefully remove the tape, and be sure not to let any of the excess epoxy fall of or contact the model, discard the tape. Voila! A beautiful fillet!

Dip your finger in the denatured alcohol and wipe it along the fillet to flatten any peaks. Beyond that, leave it be until it's cured. Dampen a paper towel with denatured alcohol and remove any stray epoxy smudges from the model.

Once cured, apply the spot putty liberally to the fillet. Shape with the Popsicle stick and remove most of the excess. This sands quite easily so don't worry too much if it's a little uneven.

Sand with 100 grit paper once the spot putty hardens. All the excess spot putty will come off quickly, while any pits, sags or creases in the epoxy will be nicely filled. You may have to sand off some peaks if you left any in step 6. If you over-sand, reapply more spot putty.

Now you're ready to paint!