Something I have always used with great success, but not on an uncovered motor yet...

Start with transmission fluid in the cylinders fill them up and let them set for at least a week. When the fluid has started to drain down past the rings, try to break the cylinders free with an extension bar on the crank. DO NOT BE TO AGGRESSIVE ON THIS - IF YOU ARE PATIENT IT WILL BREAK FREE FAIRLY EASY IF YOU GET THEM FREE, DO NOT CONTINUE TO TURN MOTOR OVER. If unsuccessful top off with more tranny fluid and wait another week, then repeat process.

When able to "break" initial rust set loose- do not continue to rotate pistons- fill cylinders up with tranny fluid again and let them set another week to drain down again. When they drain down, slightly move the pistons once more, but do not rotate them.

Fill cylinders with diesel fuel, and let them drain down at least a week. If they drain sooner GREAT fill again and wait the remainder of the week. When drained, "bump" pistons again but do not rotate.

As a final stage, fill cylinders with PB blaster or WD-40 and let it drain down for another week, if it drains down sooner- GREAT! Bump pistons again Fill and let them drain down for the remainder of the week. When pistons are free, start to rotate motor slowly back and forth with PLENTY of WD-40 as a lubricant.

Yes this takes about 3 weeks to a month- but I have always had awesome results doing this. You take the heavy fluids that are most reactive and loosen the big stuff, then go progressively thinner to remove the smaller stuff. If you rotate the pistons with the big stuff in there still, you are grinding all that rust into the cylinder walls and defeating the purpose.

Make sure you flush the oil pan out if the motor is in the car and you are going to try to start it without tearing it apart. It will smoke like crazy for a while until you burn off the residual chemicals, and the compression comes back up- its normal.