Aerodynamic bike helmet!

Step 11 Cover the foam with a thin layer of spackle. Let dry overnight.

If you put the spackle on too thickly like I did small cracks can develop. I would suggest a layer that is less than 0.5cm thick if possible. Mine was 0.5cm thick in the thickest regions -- I could live with the cracks, but much thicker and you probably would have to fill them in at this step instead of moving on.