Sunbleached, old redwood planks suck in a lot of paint — even acrylic paint.
I asked around, the answer: if you want a bright picture, use lots of white paint.

Artist’s vision, composite image, taken from rock paintings at Sego Canyon 1100–1200 AD~ acrylic on redwood fence boards with gel coating, 2012.
The Anasazi and their sometime neighbors such as the Fremont culture, and later the Utes, had no white for rock painting (for pottery yes, rocks no).
What they had was yellow or yellow ochre (from corn and various grains), black (from last night’s fire) and red or red ochre (from the blood of insects).
Where to buy a bucket of insect blood nowadays?
Nowhere. So, I went to the paint store, bought several tubes of acrylic reds and yellows and a black. And then I stepped further out of the indian cultural tradition and got a big tube of acrylic white, too.
About 18 coats of paint later, the photo at left shows what it looks like now.
Peter Neibert at Kentfield
2012