I feel for you having had similar issues on my mk2.5.
It was great until it went for MOT…
I ended up buying a mig welder and doing it myself very slowly as it was not cost effective to pay someone.
It was not ever a ‘quality job’ but was solid and bought another couple of years life.
I got away with the arch lip, there were a few bubbles. Rubbed it down, knocked it in, sealed with epoxy, filled and sprayed and it looked pretty good but it won’t last forever.
My biggest problem was the inner arches, lower sill, and front chassis legs, check out everything before you start as I found so many dodgy places once I started.
If you just put the wheels on full lock you can poke the front chassis rails with a long screwdriver, any blistering will probably have come from the inside.
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