Another option instead of welding is to use body adhesive. If you can cut all the rust out or the worst areas around the holes then you get all the loose or flaking rust off and treat it with phosphoric acid which turns the rust into iron phosphate. Scuff it down and paint it with a DTM rust inhibiting paint . I have been using a product called zero rust which has held up really well and is less expensive than some of the other coatings like POR15. Probably not as durable but in areas like a tail light bucket or the inside of body panels it holds up well. Then hammer out a patch panel to fit and paint it up. I bought a cheap 1/2" x 18" little electric belt sander on amazon that has been holding up well and I grind the areas back to bare metal that are going to be glued. The body adhesive acts as both a glue and a seam sealer so depending on the location can be better than welding because it also seals all the bare metal.
After writing all this I remembered you are in Winnipeg without much heat in your workspace so this may be of no help at all to you as I think the area needs to stay warm until fully cured.
After writing all this I remembered you are in Winnipeg without much heat in your workspace so this may be of no help at all to you as I think the area needs to stay warm until fully cured.