I had my first experience kegging not too long ago.
Got it loaded with OxyClean and hot water and put the top on to give it a shake. Immediately I noticed bubbles coming out of it. Flipped the lid, it still did it. One more time, did it again. Kept at it with sometimes better results and sometimes worse results.
Decided to give it a solid blast of CO2 to see what happens....
Boom....immediate seal. The keg just requires a few psi to seal up.