AIRFOIL FINS
Parts Involved:
- 3x 1/4" Plywood Fins (Kit)
Other Supplies:
- 1" Masking Tape (Blue)
- Ruler
- Pencil
- Hobby Knife
- Medium Grit Sandpaper
- Palm Vibrating Sander
- Sanding Block
Summary: I wanted a nice airfoil on the fins, but I didn't like the look of a rounded leading edge and tapered trailing edge on this shape fin, so I decided to go with a chiseled diamond pattern on all 3 sides. I don't have a table saw blade that is good enough for fine detail, so this is the method I came up with.
Step 1 - Mark 3/8" Border on Fin Edges: I took a high quality steel ruler and marked off a 3/8" border on all sides (except for root) of the fins with a pencil.
Step 2 - Tape off Border: Next I took some 1" wide masking tape and applied it on the inside of the fin borders. I only had regular tan tape at the time. Blue would make it much easier to see during this process. Use an Xacto knife to cut off excess.
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Step 3 - Sand Edges: At this point, I took my palm sander and carefully started removing fin material. This process ended up being easier than I thought it was going to be because of the layers in the plywood. When sanding, you can see lines created as you pass through them. They make for handy guides to keep things straight. Once I had the sanding close with my palm sander, I switched to a sanding block to fine tune the look.
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I loved your idea of the chiseled diamond pattern but didn't trust myself to hand sanding them, so I built a sanding jig.
I used two pieces of scrap oak 2x2's. For the first piece I set the table saw to cut a 19.5 degree angle (I had to do trigonometry - argh!) and did the cut. You flip the 2x2 180 degrees end-over-end and you have your sanding angle. Next I cut that piece in two and glued the angled cut piece to the 1x2 remainder to provide a flat surface to sand with. That iteration was hard to work with because of insufficient flat service to maintain angle integrity, so I screwed another 2x2 piece to the first. After doing one edge, I would make one modification. I would glue the angled cut piece up the side face of the (now) 1x2 by half the width of the fin. That would then provide a natural stop for the flat edge of the fin once you reach the edge of the masking tape one the first side. When doing the other side you'll have to go slow to not over sand it. Here's the end result (without said modification):
Using full sheets of 80-100 grit sandpaper, cut a long length about an inch wide, fold the edges over the end of the angled section and affix with good duct tape (I use Gorilla). When that side of the sandpaper gets worn flip it over end-to-end.
Place the fin at the edge of a table and clamp down tight. Place the sanding jig next to the edge and start sanding.
I've done one edge so far, here's the result.
When all edges are done, do a final light hand sanding with 220 grit or finer.
The beauty of this is that this is that without the mod I mentioned above, it can be used for any fin width, just set the masking tape farther back based on the width of the fin (a little trig required to do the calculation - Full disclosure, I didn't remember all of my high school trig off the bat after not using it for 50 years. Wikipedia to the rescue...). If you do the mod then you would need to make a jig for each different fin width. Either way, you should be able to sand perfect edges every time for any fin with straight edges.
