Weatherproofing a sheet metal duct

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I've done a fair bit of research today and find conflicting answers. Is it possible to "spot" weld a sheet metal angle collar around an air duct without an expensive clamshell setup? I have a square air duct that will be penetrating an exterior wall and I'd like to tack a collar around it to provide a vertical "t" surface to it that I can then attach to the rough opening in the exterior wall in much the same way a prefab window is installed creating a weatherproof pass through.
Reply:probably just as easy to metal screw it to the duct..... (after you lay down some rolled caulking under it),  then caulk tthe flange to the wall the same way.Miller 211Miller T-bolt ac/dcVictor o/aMilwaukee ChopsawDewalt, Milwaukee minigrinderDelta drill pressDelta bench grinderair grinder, die grinderHF pipe benderHF horizontal bandsaw
Reply:There's something I hadn't thought of.Thanks.
Reply:Originally Posted by tekstarprobably just as easy to metal screw it to the duct..... (after you lay down some rolled caulking under it),  then caulk tthe flange to the wall the same way.
Reply:Well, metal screws won the contest with caulking because when I want to remove the unit from the thru-wall space, screws allow me to release the duct easily through the front by removing the face plate. So thanks. Knocking the electric bill down this summer will make it all worth it when we were used to having the central air working day and night in over-100 degree days. We've never had an evap cooler before and today, with the outside air temp at 90 and humidity at 12%, it's amazing what it and a little metal working and stucco/framing work will make. The inside temp went down to 68 in just a few minutes with little power consumption.
Reply:And welding will destroy the Galvy.....  Miller Dynasty 350Twenty Six HammersThree Crow BarsBig Rock
Reply:If its a lap joint drill a hole in the outer sheet then tack it to the inner one by plug welding the hole.Welding/Fab Pics: www.UtahWeld.com