Got to mount a dusk to dawn mercury vapor light on a metal building an just wondering how everyone here mounts them an bushings the wire going thru the building. Also what type of wire do you use people around here just use either uf or an so cord. Thanks

I don't know where you are but, I usually use tech screws or toggle bolts to mount a fixture to a metal building and m.c. cable between the fixture and the first box. It never hurts to use a little silicone behind the fixture to seal the hole either.

From your description, I yake it that yours is one of those buildings with sides that are nothing more than sheet metal, with maybe some insulation behind it.

I will usually mount a round "Bell" box on the wall, and attach the light to that. If the ribs in the siding interfere with this, I will mount the box on a pair of 14" long pieces of thin-strut, that 'bridge' between the ribs.

From the back of the box, I will run a piece of EMT or running thread through the wall, into a box on the other side. This other box is not directly attached to the wall, but held against it by the locknuts on the pipe.
(To hold insulation back, I will use a blank cover to a 4-square, with a hole in it, as a giany washer.)
To this inside box I run whatever wiring method I'm using.

I attach the light, or the strut, using toggle bolts. I have not been happy with Teks, or any other fasteners, in this application.

This may sound like a lot of work, a lot of parts, but I have found it quick and direct, without all the fussin & cussin that "easier" methods have.

If you are lucky, have open metal on both sides, the ribs of the metal don't get in the way, and you have a helper to be on the other side.....you can streamline this process by punching a hole in the wall, putting that 4-S cover (described above) on the inside, and using the connector to clamp the box to the wall. In that case, all you need are Teks screws to hold the box from spinning.

You notice I said "use a box?" You need a box. The days of simply poking a wire out a hole and using the fixture as the box are long gone. You need some way to hold the cable in place, and protect it from the sharp metal.

SO is a definite no-no. Suggesting UF is, I take it, a sign you're also concerned about the sharp edges.

Merc Vapor? Seldom used around here, almost everything is MH and sometimes HPS or LPS.

On flimsy metal buildings, I usually cut a 2' x 2' plywood and attach it to the backside of the mounting area. Now you can hole saw right thru both the metal skin and the plywood and attach an appropriate old work box for the type of wiring you are using. The wood is handy for any lag bolts required to mount the fixture to the structure.

If you're high enough then I've seen these fed with a weather head and drip loop as well.

You don't need a box, it is allowed to be done by 410.14(A). If installing over a box, a quick look at 410.14(B) is in order. As you need to cut a large hole in the fixture to allow access to the j-box behind it.

Going through metal I would use a nipple or conduit to go through the metal, once inside any wiring method you see fit, and code compliant.