The method we use for a large panel like a roof insert is to first fit it into place with no adhesive using #8 self drilling screws about 2" apart. Any gaps are worked till the fit is close. Next the area around the opening is media blasted and then roughed up for two inches back from the edges with #36 grinding disc. Same is done around the edge of the metal insert. Enough bond is applied to where it will squeeze out on both sides of the seam and using ice picks as guides the panel is lowered into place. The self drilling screws are temporarily installed till bonds tacks good. Last, the screws are removed and replaced with ones that have been notched halfway through just under the head and tightened till the head snaps off. This give the insert both a mechanical and adhesive bond. When 24 grit grinding discs were available for lead work they created so much less heat than finer discs one could just grind the screw heads off along with the squished out bonding agent. Any bonded panel that shows shadows was not done right.

Evan