There really isn't much that "sticks" to those plastic tanks. I'm pretty confident a pressure washer is going to release anything that might be inside (or outside) of that tank.

As a word of caution, be very careful handling the plastic tank, and everything attached to it. be vary careful removing anything from that tank, and even removing and replacing the tank itself. As said before, pretty much nothing sticks to that plastic, that would include any kind of tank patch kit. A cracked plastic tank is a junk plastic tank.