Answer: I think that most people would say that it’s best to replace the entire trunk floor, but those are usually the people who haven’t actually done it themselves. All things being relative, replacing a trunk floor is not that difficult. It’s getting it to look like it was never replaced that’s difficult. I’ve seen trunk floors patched with fiberglass, coffee cans, sheet metal, stop signs and even cardboard. I’m guessing that you want it to look a little better than that.
My recommendation is that you put a patch in the trunk floor. This is not particularly difficult, but it will be time consuming. The fuel tank will need to be removed to allow access to the bottom of the trunk floor as well as to prevent your car from going boom. With the tank removed you’ll be able to see if the rust has extended to any other area such as the floor brace/gas tank strap mount, or the body mount brace. These parts are all readily available and should be replaced if rusted.
Any qualified shop can make a patch out of sheet metal, but I would recommend buying a high quality reproduction drivers side trunk pan section, and cutting the patch out of that. It will fit better and it will have the ridges that will match the ones in your trunk floor. By patching the rusted area you will be able to keep the work localized and a talented welder can make it practically invisible. The main reason that I don’t recommend replacing the entire trunk floor is because it would require welding very close to the quarter panel which can easily damage it. Replacing an entire pan in one piece would require removal of a quarter panel in order to get the pan in the trunk! Replacement of the entire trunk floor would also require duplicating factory spot welds as well as sealing the seams.
Keep it as simple as possible and the results should be great.
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