toyota supra mk IV

Sounds like you started with too-gritty (too rough) of sandpaper. Some people wash their bodies in dish soap to remove any mold agents that are on the body from the molding process, but unless there are defects, I usually either A. just prime the body as-is, or B. sand it with some 1500 grit if I think it needs it, then prime it. Some people get crazy and use 2000 grit on up to 15,000 grit polishing pads, but that is how you get super-nice paint. What is on top is only as good as what is underneath. Paint will never "hide" shoddy workmanship underneath. Try a much finer grit next time. Good Luck, Rudy

RudyOnWheels

Some people get crazy and use 2000 grit on up to 15,000 grit polishing pads

Those are for polishing paint, not prep prior to painting. if you used 8000 grit or finer before primer it wouldn't stick, due to no key or teeth and will peel off (especially if using masking tape).

Adreon

after i put the colour on i see it maked like a defect(there is like dust under the paint and a blackish mark). So i would like to know if i sand it down again and wash it off and redo the primer and paint would it work?

Dust usually shows up as a high line under the paint.

you can do touch up like they would do for a small scrape in a body shop. Sand it back, if  you can avoid going through the primer, you don't need to re-primer it and can just add a small touch up of base color.

I'd mask off the panel and repaint the whole panel.