The metal should first be embedded in roof cement then fastened down, then the top flange should be treated with asphalt primer and allowed to dry per the manufacturers recommendations. Dry time will vary based on ambient temperature.
I prefer to install a strip of modified bitumen upside down under a drip edge which was treated with asphalt primer. I mechanically fasten the modified bitumen to the base sheet, prime the top of the metal edge, mechanically fasten the drip edge then torch the new modified bitumen down. It keeps a solid, leak free surface even if the bitumen fishmouths around 2 joints of metal flashing coming together. It also keeps from running nails around the perimeter leading to another penetration but on very flat roofs it can cause ponding because of the double thick layer.
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