I bought some UHMWPE sticky tape and put about an 1" along the joining pieces over each of the holes and then trim to suit, so that when I tighten the screws they just bear on the tape and do not mark the rails. I have 3 sets of joiners and 4 rails and have not managed to damage any of them yet. I also have the Betterly connector although Phil Beckley (Festool UK) advocates using the saw itself to align the rails prior to tightening the joining pieces.