Chris,

You may be clamping too tight. Epoxy has no strength when it is too thin, it needs a bit of body for strength. If you clamp too tight you actually squeeze the epoxy out from between the materials and it will be very weak at that point.

Also, the grit size for sanding may make a difference, if it is too fine then the scratch marks are not deep enough. Try using 120 or rougher and don't sand in one direction. Try to use circular motions to create more random scratch marks.

The screw-ups are part of the learning experience which does not end, at least for me it has not ended yet.