Bingo!!

The actual areas of blistering are magnetic but on further inspection part of the wheel arch is not so a bodged repair is the conclusion

Verified by comparing it to the other side as well.

Thanks you guys for all your inputs .Now to see what the best cause of action is !

So it’s probably a wet sanded piece of body filler that the moisture is leeching out of in the heat. Probably going to be a case of removing the paint, the filler and re-applying. 2019 5.0 SC SVA (Current)
2012 5.0 SC AB. Sadly written off by a campervan
23 plate Focus ST (Current)
1942 VEP Ford GPW Jeep (Willys) (Current)
1943 Whites M16 Halftrack (Current)
16 plate Eurofighter Typhoon, 2 x Eurojet engines with 20,000lbs thrust each. 1380mph

Those blistered areas are magnetic so oxidisation of the zinc plating caused by its partial removal by sanding/blending in in the repair shop is more likely rather than filler buddy.
I know a local lad whose mustard on panel repair work so I am going to run this by him for a opinion of whats the best cause of action