Americans from the seventies often had a thermoplastic lacquer layer. If those classics later got a new jacket there or here, it often happened quickly for cost reasons and with an 2K paint. And there the misery began.
Those varnishes do not tolerate each other. And that is not as often said by something scary and chemical, but actually because they have different expansion coefficients. This causes the underlying layer to 'draw' the top layer.
The more serious painters soon realized that an insulation layer between the lacquer layers at least delayed that process for a long time. But in the meantime it has turned out that the people who told us that such a carriage, sprayed in thermoplastic paint, had to be sanded completely bare nevertheless got the same.
So if you think your car needs a new coat of paint, then the advice is: keep sanding until you are on the blank plate. This advice does not apply to Corvette owners.
In the past I also worked on cars with thermoplastic paint, including Rolls Royce from the 30s that were repainted.
Insulating this varnish is not possible, the varnish becomes bloody hot in the summer, in the sun and then the thermoplastic varnish starts to flow again (melting), as a result of which all upper layers also deform.
The only remedy is removal, but a sanding disc becomes full and clogged, the paint melts and remains on the sanding disc like drops.
My method is, with a large roof leather burner (propane burner) the paint is heated considerably (does not burn) so that it melts and then with a putty knife sticking off the paint, this is very easy and fast whole rags go off at once.
yes a disaster the opel cadet that also had something like that or you have to cover it with epoxy layer by layer, not too thick, and not let it dry in the oven for one night,
that was what I had done earlier in the 70 years also had many problems that the paint started to scorch = dissolve with different brands of paint and put filler on top of each other, so it was advisable to work with a factory segment that is close to each other voted and then you have fewer problems, but that is not news that every sprayer knows,