Fill Pattern on a complex shape

I'm designing a sort of exoskeleton for a hand, and I neeed to apply a pattern on it.

The piece will be 3d printed, and the pattern consists of a 3-tip-star shape, repeated many times all around the hand.

I created a mock surface that mimicks the actual surface (I used a boundary blend, the resulting surface is actually made up of 2 parts), then I sketched the pattern shape and used Fill Pattern.

On one half of the boundary blend everything works fine, but when I pattern on the other half I get what you see in the screenshot.

Any ideas on how to make it wrap around the hand and not around the air?

Thank you,

Guido