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What you said will work. Boards I've left under flourescent lights have eventually exposed but that was more by accident than design.
It's possible to use exposure to sunlight to make a PCB. You just need to make sure that the PCB face at right angles to the sun.
You want to take a strip of PCB, mask it with a piece of thick card and
leave a little bit of board exposed. Exposed 5 minutes then move it by
another 1cm. Keep going until you've exposed the last part of the board.
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