How to paint alpha feathers

Hello,

I'm trying to paint 'robot feathers' onto a 2D wing plane. The feathers consist of a colour map, metalic, roughness, normal, and opacity, all square.

The problem is, whenever I paint on another feather next to the first one, the square alpha map paints over the other feather and makes it invisible (see screenshot).

How do people normally get around this issue?