Be sure you are applying pressure against the rivet head (not the tail which forms the shop head) as you squeeze. That's easier to do with a hand squeezer than a pneumatic, but in either case you want to push the rivet head into the skin, which pushes the skin to the rib, while the rivet is squeezed. It doesn't matter which set is moving in the squeezer as long as you pre-load in the proper direction. It helps if the work is not free to move while you squeeze. For the part you are talking about, or stab skins or similar, I lay the part on a table so I can push down against the table. If the bottom side is studded with clecos, it's Ok, they can provide enough support to push against - you don't need to apply tremendous amounts of pressure.
Patrick Kelley - Flagstaff, AZ
RV-6A N156PK - Flying too much to paint
RV-10 14MX(reserved) - Fuselage on gear
EAA Technical Counselor #5357