Hello Jev, apologies for the belated response, what is spin casting? I use heavy duty foil, cut into squares, a little longer than the weed guard, and about 2 inches long, than roll these one at a time onto a 16d nail slide off than slip over weed guard, pry hook out a fraction and insert into foil, actually just kind of catch the hook into the foil, this keeps it on, and bake away, even at 350 for 15 minutes the weed guards are good to go. Maybe when the led hits the guard this is the problem? Also these fiber guards are not a perfect science, I feel that some are short before you pour, you just do not notice it. Thank you Brian