Hi Bert,
I've built 2 boats using doorskins with good success. A strong light behind the plywood will help you spot voids. Try to select your sheets before buying them. I sometimes cast aside 4 or 5 sheets for each one that I buy. Look for plugs and large knots in the face plys and avoid those sheets. To fix a void, I have used a Dremel tool to grind through the face ply and then fill the void with thickened epoxy. Fiberglass over that repair. Good luck.
Dave