This page illustrates how the Coldfire fin fillets were created and finished.

The first step was to rough-up the corner where the fin meets the body tube.  A small length of 1/4" square dowel was used with some 100 grit sandpaper wrapped around it. This helps give the epoxy something to "bite into" on the otherwise smooth fiberglass surfaces.

After sanding the fin joint, the dust was brushed away and a cotton rag saturated with isopropyl alcohol was used to wipe the surfaces nice and clean. 

Next step was to mark the fin and the body tube with a guide line for masking tape.  A 3/8" square dowel was used as a straight edge.  Setting it into the joint made it easy to mark a line on both surfaces that was an equal distance from the joint.

3M Scotch-Blue painters tape (1.5" wide) was applied to the line just marked. 

The fin joint was then wetted with plain West System epoxy (no fillers.)  I used a small disposable brush to apply a generous amount to wet out the surface.

Next step was to mix up a batch of West System epoxy and thicken it with some #406 Colloidal Silica filler.  It makes a nice creamy mixture. I made it thicker than mayonnaise but not quite as thick as peanut butter.  It was thick enough to hold a shape without running.  No need to be too careful here, the masking tape prevents the epoxy from getting into places we don't want it.

A short section of 1" PVC pipe was used to shape the epoxy and push the excess onto the blue tape.   It was helpful to dunk the piece of PVC pipe in a small bowel of isopropyl alcohol to wet the surface of it in order to help shape the epoxy without too much smearing and pulling. 

The small length of PVC pipe can also be used to shape the leading and trailing ends of the fillet at the front and aft edges of the fin. Just rotate it around from one side of the fin to the other.

The blue masking tape was removed once both sides and each end of the fillet were completed.  This was done while the epoxy was still wet.  The excess epoxy is taken away with the tape.  Carefully pulling the tape away at the angle shown here leaves a nice perfect straight edge for the fillet.

First of three fillets is now done and the rocket is set aside for the epoxy to cure.

This method ensures that all three fin joints will have exactly the same amount of epoxy and exactly the same shape.  The results are much more consistent than trying to shape the fillets by free hand.

Once all three fins had been completed, it was time to do some wet sanding.  The photo below shows my setup for wet sanding.  The table is covered with a large sheet of 6-mil clear plastic like that available at most any hardware store.  (Home Depot for example.) 

A clear plastic container has an inch or two of water in it to soak the sandpaper before use.  I use 320 grit "wet-or-dry" sandpaper.  A water spray bottle is used to wet down the airframe for sanding.  Wet sanding works really well for getting a nice smooth finish.  The sandpaper does not "load up" with debris and the epoxy mixture sands reasonably easily.  The edges of the fin fillets were blended nice and smooth.

After the initial wet sanding the epoxy fillets will usually still have some small pin holes and other voids in the surface. These can be filled with Icing. It was available at my local PPG automotive paint store.  Squeeze out only enough that can be used in 5-10 minutes and mix in a very small dab of the blue hardener.  It will cure in about 3-15 minutes depending on how much hardener you add. 

All three fins were completed and then more wet sanding was done. Icing sands very easily.

Each fin joint was wet sanded after adding the Icing.  They should be looking very nice at this point.

The entire booster was wet sanded to get the surface ready for primer and paint.

After wet sanding the whole airframe it was time to spray it with  Kilz sealer-primer.  This primer is relatively inexpensive and very high in solids.  It does a great job filling any remaining pin holes or hiding other surface defects.

The final result made all this work worthwhile!  

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