Coating exterior wood surfaces with clear coatings is a somewhat questionable practice. Sunlight penetrates the clear coating, affecting the wood surface. This causes the clear coating to lose adhesion and peel off. If you decide to use a clear finish, it is necessary to set up a strict regular maintenance program. A new clear coating should be applied before deterioration of the old coating begins.
Some clear coatings contain ultra-violet absorbers which help to lengthen their life.
Ask your paint retailer for a recommendation. Latex paints are the usual choice for masonry, but buy one which is designed for use on masonry. Clean all dirt off and use special asphalt roof coatings, many of which have metallic aluminum pigment for heat reflection and durability. Because of moisture problems it is difficult to keep paint intact on rough wood surfaces. However, a water-repellent preservative will give the wood some protection against decay, stain, fungus, swelling, shrinking and warping.