I have a built-in gutter that is leaking at the point where the bottom of a valley flashing, which is metal, meets the gutter lining which is some sort of rubber. The lining near this spot is in a bad shape, causing a leak (I've done some temporary patching using cement). The rubber is just tucked under the flashing without any sealant, and this opening may also need to be closed.
I wish to cover this area so water can flow smoothly into the gutter without getting under the flashing. How should I do this? I'm thinking of using seam tape of some type and then covering the metal, the tape, and the rubber with liquid EPDM. Is this ok, and do I need a primer?