I know this is a post to an older thread, but thought I would share.
I have come up with a clean look with a minimal size hole needed for exterior lights on a pole building with steel sides.
Items needed: (1) Plastic Round Junction Box Wet Location, (1) 4” PVC Union, (1) ¾” plastic threaded adapter , about 6” of ¾” pvc conduit and clear caulk.
First thing is to cut the 4” PVC union. This will become the collar around the junction box. The trick is to cut it to a thickness of 1-1/2” while leaving the full stop in the center of the union. This stop fits snug around the outside of the junction box. I used my table saw to make the two cuts. The first was to cut the center to one side of the stop. The second cut was to then trim the remaining down to the 1 ½” size.
Next, cut the ears off of the Plastic Round Junction Box Wet Location. This will allow for a tighter fit. Now the junction box should have a nice fit inside the trimmed down pvc union.
Now locate where you want the lights. I have bridged the small ribs at various positions. I tried to place them with in 3” from them top of a purlin. I drilled a pilot hole from the inside. Then take a 1-1/2” hole saw and drill from the outside of the steel. It’s helpful to have someone on the inside holding a scrap piece of 2x4 to drill against. Ultimately you want to drill all the way through the 2x4, but this can be done ‘on the ground’ after you get through the steel.
At this point you should have a hole in your steel and a 2x4 with a hole as well. Now cut a piece of ¾” conduit to length. I think I cut mine to 3”. Glue this up to the ¾” plastic threaded adapter and thread into the back side of the junction box. Us the supplied plugs to close the 4 holes side hole on the junction box.
Apply a generous amount of clear caulk to the back of the junction box. I laid down about ¼” of caulk to the entire back except for the outermost ½” area. This will allow the caulk to spread out and not have any squeeze out. Now predrill two holes through the junction box, steel and 2x4 with the hole. Screw everything up. I added two short pieces of 2x4 vertically on the inside to attach the purlin to the 2x4 which the junction box is screwed into.
Slide the piece of PVC union on top of the junction box. Now when you attach a light fixture, it will tighten up against the junction box as well as the ‘collar’ and give a nice clean finish. I painted my collar with rustoleum krylon paint that closely matches my lights.
