When the ball mount on your trailer hitch is rusted in place, don't whack at it with a sledgehammer. Here's what to do instead. Buy a can of penetrating oil ($6) and an air chisel fitted with a "hammer" bit. The air chisel will run about $15 and the extra bit will be about $8.
Spray the opening to the hitch receiver. This newest style of penetrating fluid chills the metal, causing it to contract to help break the rust seal. Then use an air chisel fitted with a hammer bit to knock everything loose. The air hammer will break up the rust, spread the penetrant and free up the ball mount. Once it's free, just tap out the rusted ball mount with a regular hammer. If you plan to reuse the rusty mount, coat it with a rust converter or waterproof marine grease. Check out these great trailer upgrades