[StuBeeDoo]

Use flat primer right over the top?

Now I am worried, I have a couple F-40s that I need to respray....

I will probably give that ^^^ suggestion a go for my final attempt before I give-up.  I might have already given-up, but I got a dose of bidding fever.

In respect of your F40s, I did have a similar, but not quite as bad, issue with a BMW M3 of similar vintage.  Fortunately I already had a spare.