I usually use these clamp connectors to connect flexible metal conduit to a box:
However, I am now installing a weather proof light fixture box outside of a shed (just surface mounting it, poking the FMC through the wall straight into the back of the box – basically like this but surface mounted). So I didn't think that warranted fully water proof conduit, but I can't seem to find any connectors that will keep water out of the box at the connection point. Do I not need to be worried about this?
Best Answer
In my opinion FMC is no good for this application, even though it's only barely outside, it's not rated for outdoor use. (I realize lots of people would either disagree or figure it's close enough and run with an indoor rated wiring method.)
The easiest thing to do would probably be to set a box inside and transition from FMC to UF cable - run the UF through the wall to the back of the box with a suitable waterproof connector such as this one from Arlington:
http://www.aifittings.com/catalog/non-metallic-cable-fittings-and-supports/uf-connectors/UF50
To connect the UF connector to the inside box, you could use an indoor-rated clamp as long as it's listing / instructions / labelling says it's for use with UF. It might be easier to just use the outdoor connection indoors as well.
You might be able to find what's called a "transition fitting" or a "this-to-that" that will connect UF directly to FMC but again it's probably simpler and easier to just transition in a box.