Here's a mistake I made, how I discovered it, and how I fixed it.
Fabricators are always on a quest to eliminating pinholes of light in shades with blackout lining.
I've been refining my blackout shade method to ensure that it is 100% guaranteed foolproof, is streamlined to the max, and yields a beautiful product. I now have a method I love to use.
Recently I made 5 flat romans with blackout lining for 3 bedrooms. These were reverse mounted on velcro, with toppers to conceal the grommets that guide the cords to the RBS lift system.
Back at the studio, I fused face fabric to a Dofix fusible blackout lining, and using a rotary cutter, sliced it into strips a little shorter than the topper.
Returning to the house, I used double-sided adhesive tape to attach the strips to the board under the toppers to hold them in place, then stapled them to the board face. They are an inch shorter than the toppers so they can't be seen, but in case someone lifts up the toppers, they see face fabric.
They totally black out that awful light bleed!