In article , "The
Natural Philosopher" says...
JoeJoe wrote:

What is the difference between a primer and an undercoat?

Primer is there to stick to the ioriginal surface and seal it and
possibly to create a smooth surface - i.e a good primer sticks well to
teh (wood) is thick and can be sanded to give a toally grain free finish.
The unedrcaot is there to basically provide MOST of tghe coulr - its teh
bit that reflects most of te light.

Primer doesn't give much build, it's the undercoat that you sand for
a smooth finish.