> What is so difficult about printing the pattern onto clear film
> (overhead transparencies) and make a contact print exposure of the resist?

I don't think anyone said the masking step is difficult. The reason to
avoid this step is not because it's a difficult step, but because it's
an extra step. Every step in a process has the potential for
introducing error.

> It would be FAR simpler and cheapper I would think. (and faster if you need multiple
> boards).

I don't think it's simpler if it involves an extra step. If you can
expose the board directly with a laser, you completely eliminate the
step of making the mask.

Furthermore, if the laser can ablate (i.e., remove) the resist, you
would also eliminate the step of developing the resist image. However,
doing so would probably take more time than its worth, because you
would need to expose a much greater area.

> The sun is a free source of sufficient power UV light.

Not sure how that's relevant.
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