Expando. When you absolutely don't want it to leak. Expando.
Clean your pipe threads dry, brake clean is best here, put powder in a cup, use a water bottle cap to add a few drops of water at a time. Do not make it too thin. Mix until its around as thick as a smoothie or soft serve ice cream. Apply (I apply to both female and male sides).
Now, you MUST, and I mean must, wait 24 hours before pressurizing the system. Or it won't have time to setup and harden. If you use it correctly it will not leak. I've ran pipes on closed in walls with it and zero issues.