One side of my home has been quite weathered and the wood is quite rough. I wonder if anyone has mixed primer paint with a portland cement product to produce a thick and durable product that would fill the roughness and prime the wood for followup painting?
Sand the surface lightly with some 120 grit sand paper. Mix a solution 50/50 turpentine and boiled linseed oil. Apply the mixture liberally to the surface. Do not apply a second coat or you will get an unnecessary build up. (Make sure the rags you use are laid flat on a non flammable surface and allowed to completely dry before discarding. These can cause spontaneous combustion!) Allow the mixture to dry for at least 48 hours. Apply a coat of oil-based primer. I would use a 3/4 nap roller cover and then use a brush to spread it evenly with the grain.
Personally, I thing the weathered wood look adds a lot of character to a home. However, if you are dead set on filling in the remaining cracks it will be a lot of work. You will need to apply a coat of some type of filler and smooth it out with a wide drywall knife. MH Ready Patch is pre-mixed and is reasonably priced. It's also an oil-based product. Perfect for exterior conditions. Sands out clean after about 45 minutes of drying. If you do use a filler, you will have to apply a second coat of primer before applying the top coat.
You can use any top coat you like. However, if you want something that is the top of the line in these instances, use some Allback Linseed Oil Paint..
I doubt that will go well. Wood expands and contracts a great deal due to changes in temperature and humidity. Cement doesn't, especially portland cement. In addition cement doesn't really bond all that well to wood: that's why it's commonly used to make forms. My guess is that the cement would pop out within a year, if not less and make it look way worse than now.
There are products specifically made for this purpose. You should use one of those instead if you want to do this. Use one made for outdoor applications.