If you select boards with knots, there is a good chance that they will bleed through, even if you use a sealer or shellac based primer. The resins in knots is persistent.
Clear boards are usually used for moldings, including baseboards. You could also use MDF (medium density fiberboard) or solid vinyl composite molding. When painted, they are hard to distinguish from wood in most applications. (I think they look a little too smooth and I prefer wood). For painted applications, there is also fingerjoint molding, which is made of interlocked short pieces and is cheaper than clear boards. Much pre-primed molding is fingerjoint. Occasionally these joints may show if the quality is not high.
For wood or MDF, a light sanding with 120 grit paper is recommended to remove milling glaze and to give the wood a tooth to better take paint. An orbital sander is fine
Then prime and paint. Many prefer oil based primer on raw wood and MDF. Some vinyl says it can be painted directly, but I would also give it a sanding (or avoid it unless necessary, such as in outdoor uses).
Latex paint is fine for a finish coat, but it tends to look less smooth than oil (alkyd) based finishes. Adding a leveling agent like Floetrol helps, but nothing lays as smooth as a good oil based enamel.
(P.S. To me, using a really smooth straight clear board, in furniture or in molding, is appreciating and honoring the wood, the tree, nature. But to each his/her own.)
This is a tough one that will take a bit of work. From personal experience, any time you're going to be applying paint/primer/etc. to any kind of questionable masonry and mixed surfaces, it's going to be a good idea to clean/prep in several stages, then apply a masonry grade primer.
Depending on if there was any kind of debris, mildew, moss growing on the surface, a brush and/or pressure washer could be suggested. If the surfaces are clean and evenly covered, I would use a good stiff deck brush or similar.
I would also suggest a chemical cleaner/stripper containing muriatic acid, which will have a mild etching effect and ensure good adhesion with whatever you decide to paint with.
Then proceed with a good masonry primer, following manufacturers specifications. The same would apply for your paint.
In most of my jobs and personal projects, I would say a good paint job is 90% preparation, 9% good product and 1% skill.
In short, treat all surfaces the same, clean and prep evenly and somewhat indiscriminately. I would suggest a standard latex acrylic. Elastomeric paints tends to be heavier and really suited to new construction and initial application. If the house is older, it's most likely done most of its settling.
This reference is in regards to commercial new construction, but a lot of the premise will be the same.
http://www.masonrymagazine.com/5-08/cleaning.html
Good luck with the project.
Best Answer
You are painting a mural, like Leonardo or Michelangelo.
It is a little easier now since the invention of acrylic artist paints. Modern acrylic/latex wall paint shares many characteristics, but generally is not as rich in color and does not have the durability to resist fading as well.
If you want the best finish and durability, you should coat the wall with a special art primer called gesso. This is a base that holds the finish paint well and is very flexible to resist cracking and other breakdown.
The colors for the design should be artist acrylic paints. There are student level paints that are cheaper than professional grade paints for covering larger areas. If you need to stretch the paint, you can add mediums, but his will give a slightly less intense color. If you want to paint a background color using regular wall paint, you can use a clear gesso. This will save money over having to paint the background with artists acrylics, but the color will probably be slightly less stable than artist colors.
The finished work can be coated with an acrylic varnish, either brushed or sprayed on. This will protect it, but it should not be scrubbed the way you might a conventional flat painted wall.
Information about these materials can be found on the sites of various acrylic artist paint manufacturers.