I started with Tamiya's Fine primer in a spray can
I followed up with 2 coats of Humbrol orange acrylic paint from a spray can.
I waited 48 hours and applied 2 coats of future floor finish with a 3/4 inch wide artists brush. I made sure the coats were thin and I waited 12 hours between coats.

The effect became noticeable a day or two later.

I'm baffled. The brush is only used for future and is cleaned with hot water and some dish soap.

What went wrong?

Second problem.
The car is a slot car. It is a 1/24th scale BRM TransAm Camaro.
Normally after the car has about a week to dry, I tape strips of electrical tape on the body in the areas that get scraped if the car rolls over. The tape is the matching color of the paint so you don't notice it when racing. I pull off the tape and replace it every so often. The paint stays like new.

When I pulled the tape off the roof, the future and the top layer of Humbrol pulled off in the exact shape of the strip of tape. This has never occurred in the 15 years of racing.

Baffled as to why beyond the obvious the two layers of Humbrol didn't bond together well.

Any useful thoughts for future consideration/ prevention?

What kind of plastic is the slot car body made of? If it's not styrene plastic like regular model kits that might be your problem. I know that some of the RC clear bodies are lexan and they require a special paint for lexan. Don't know the specifics of why just know that regular hobby paints are incompatible/don't adhere well to the lexan plastic. Hopefully others will chime in with their thoughts and you will find a solution.

The body is styrene, not lexan.

Was the primer oil-based? Maybe there was a reaction with the acrylics/ and or it didn't bond correctly? I have had problems with acrylic, and it usually turns out that I have thinned it too much. Similar "cracking".

Best Guess is the orange paint did not adhere to the primer well enough. The Future then softened the orange paint and as the Future then dried it contracted the orange colour underneath as it was not hard and fast on the primer. Brush painted Future is still a thick layer.

Had a similar issue with Testors Dull coat years ago. Cracked the Tamiya Acryllic OD on an M8 Greyhound a week after it had been painted the OD. It does not matter how long the paint is down, it is still able to be softened somewhat.

Best bet would be to apply the future by airbrush in thin layers.

I'd say the problem is with the Humbrol. I finally gave up on Humbrol after running into about ten containers of paint about five years ago that just would not dry. They felt "cold" and slightly "sticky" to the touch weeks after sitting on the shelf before clear coating. I Finally clear coated with Future, and had almost the same thing happen as you did-- The Humbrol coat beneath the Future started to "crinkle", and although it finally dried, it looked horrible. I used some Ammonia to remove the Future, and the Humbrol came away with it, leaving the primer coat (Alclad Grey) intact. I gave Humbrol one more try in a different shade, and had the same thing happen. I think Humbrol has had some mixing and pigment issues in the past, and those lots are still out there.
VR, Russ

It could have been humidity, but it sounds like the Humbrol just never cured.

Greg,
Did you get your Humbrol at Discount Models several years back?-- That's where I got mine. Hmmm....
VR, Russ

No, I bought mine about a month ago at the place I race slot cars.

Too bad about the Humrol paint. Tamiya's orange looks dull under the florescent lights but the Humbrol looks brighter under the florescent lights. I like the color.