Paint thickness for sanding

Just a quick question in regards to wet sanding paint.  When ever i apply paint to car bodies, then go to wet sand, I seem to always sand through the coats to the plastic.  I usually apply two the three coats and sand starting from 2400 up to 8000.  Should I be applying more coats of paint, and start with a finer grit of paper?.  What is the best amount of coats to apply?.  I paint with both laquer and acrylic.

Thanks.

 

 

Pete Lovely Roger Loyer Jean Lucas Jean Lucienbonnet