Buying a patch panel and welding it in saves a whole heck of a lot of time. Usually if you're doing a legit repair you already own a small welder so doing repairs with steel is a no brainer. If the metal is really rusted and you have no patch panel, you would generally take metal and shape it fairly close then smooth with filler. With fiberglass you have to lay it over the rusted to blasted remains to give it some shape to hold it up while it cures.