I did the Nestucca/Blaine/Cape Lookout trip last Spring. The road is mainly paved, but there is a section between Carlton and Blaine that is hard packed gravel (atleast it was hard packed in May/June).
I road my mountain bike with slicks and had a great time. There was some snow underneath the trees in the higher spots, but the road was totally clear. I did encounter a few logging trucks, but there was very little traffic until I hit Blaine. There are very few stores or even houses in the middle of that ride so I would advise bringing extra food. You can get water at the campgrounds along the road.
I put my bike on the Max and rode to Hillsboro before starting the trip. My wife picked me up at the coast (I can't remember the town, but it is a few miles North of Cape Lookout) and the trip was 108 miles.
There is another road that starts in Nehalem Bay near the town of Nehalem and connects with the highway that goes to Vernonia, but it has about 15 miles of gravel. It follows the river and is truly a spectacular ride, but my hands were numb from the bumps by the time we got back on pavement.
Lots of options! We talked about organizing a BF ride to the coast a few months ago, but never got past the planning phase.