I used the four step process and, so far..it's held up just like new..
It depends on what you're shooting for..On one of mine (64GT) it's silver on the top and, the other one (84 Sovereign) it a dark grey to match the under carriage.
The main thing is to put a top coating to protect the POR15 from the UV's. Otherwise, if you don't..it will turn slightly yellow.
As far as underneath, I really don't think you need to worry abt a second coating for the POR15..Unless you're planning on going belly-up for sunning...