Crank bolts are normally fine with grease. The torque required to tigten them proberly can be a more than people realise.

If the bolt is loosening because the crank arm is moving on the spindle then loctite often doesn't help. Your weight on a ~6 inch leaver is often more than loctite can hold. I would take it to a decent shop or get a torque wrench to check it is tight enough before resorting to loctite.