flat roof

If I use the Britannia Aquashield High Build, https://www.rawlinspaints.com/home/roof-paints/flat-roof-paints/3070-britannia-aquashield-high-build.html, as some gaps/holes could be wider than 3mm, which cheap scrim / alternative could I use under the paint? Could I paint, put scrim then paint...

I need to stop the dripping from a flat roof over my entryway. I am considering fabricating an L-shaped thin aluminum flashing (from the roll material used for making gutters) , 4' long, 1"high. There is an adjacent gutter where excess water would flow into. I could cement this new flashing...