UpdateMy loosing battle with the Ferrari engine continues...
The larger gaps were filled with acrylic putty (white). The hairline gaps were filled with
Tamiya Surface Primer Grey harder to see in the picture, because the primer it's also grey. I had to "paint" with liquid putty almost every single join line. The proud standing pin marks were cut off or sanded, the recessed ones were filled. I removed as many seam lines I could reach to. I corrected the bends in the exhaust pipes and I filled up the gaps where the pipes join the mufflers. Now has been primed again and the witch hunt continues...

On the other battlefront I am slowly winning. The body parts primed with
Mr. Surfacer 1500 Grey, thru
H&S Evolution 0.4mm @20psi. I used a thick mix (approx 50%), because I wanted the primer to act as a micro-filler as well.
If you noticed it, I primed the body from the interior too, because I wanted to block the translucence of the plastic. Three coats were requited! Many modelers that had the bad luck to work with red or yellow plastic, complained about how hard is to cover these colors. The problem is that the light travels thru the plastic, the same way a LED or the light stripes work. Covering only the exterior surface of the model doesn't help. Applying many coats helps for opacity, but leads to other problems. The solution is to apply the primer or paint from inside - as many coats as necessary, and only the strictly needed coats on the outside.

As anticipated, a shadow of the gap reappeared on the quarter panel. Also more filler needed at the ends:

The interior painted. Yes, I have used the primer as a base coat, sprayed thru the same airbrush, but thinned approx 2 parts primer to 3 parts solvent, because I wanted a more "wet" and more smooth application.

On both the body and the interior parts I've got good results. The body needs more work, especially the rear quarter, but the interior parts are OK and ready for clear coat before decals.
Cheers!
Gabriel