Painting - Primer ?

Going to be using Benjamin Moore oil-based solid color stain on an addition and the remainder of the house. The cedar clapboards on the house were replaced about 15 years ago and were never stained or painted. The addition will be new cedar clapboards. There are exposed square head nails on the house that appear not to have rusted. I plan on using a siding cleaner, then lightly pressure washing the old siding.

I have several questions for which i've gotten conflicting info:

1) Do i need primer on the new clapboards
2) Do I need primer on the old clapboards
3) What is best to clean the old clapboards
4) Can I stain right over the nail heads

Thanks

Usually primer is not need when staining. The only exceptions are when a light color stain is applied to wood with tannin bleeding tendancies. I usually use bleach water and pressure washer for cleaning although there are many siding/deck cleaning products available. Siding is usaully installed with galvanized nails, so unless there are rusty nail heads there should be no problems staining over them. Because of the way old siding has weathered the new siding may not look just like the old after they both are stained.

I use the Sikkens Oil-based Solid Color Stain, but I can't imagine the procees being that much different
1) Nope
2) Nope
3) What you said ^
4) Yup

There are some "ifs" based on some conditions (old paint, heavilty weathered siding...), but that's basically it
I'm assuming it's an old saltbox and the square heads are supposed to stick out a little?

Regardless of whether the nails stick out or are set in:
Siding is usaully installed with galvanized nails, so unless there are rusty nail heads there should be no problems staining over them.