Aerodynamic bike helmet!

Step 18 Apply a thin layer of metal bonding compound. Let dry (20 min).

Use _much less_ catalyst than the manufacturer specifies (you can do a test piece to make sure it hardens). It makes your life easiest if this compound is pretty liquidy and goes on like paint. Too much catalyst makes it more like bondo and just means it will take longer to sand it smooth. After it dries, you should sand it in the same way you sanded the spackle earlier. Perfectionists may choose to wetsand the surface.