My last has was built in the 60s. All sheetrock on exterior walls had a tin-foil-like coating in the backside. I think it was fire-retardant. No stud finders worked on my exterior walls. Try the magnet method to find nails. Try and find the faint indications of nail heads on the contours of the sheet rocks finish. Use outlets as a starting reference since they are usually mounted to studs. Use a pin nail to locate studs. Several ways but all of them take patience and mapping out.