Cracked splitting cedar siding
Dunno if you can paint over a solid stain. Hopefully someone else will know.
The splitting at the ends of boards that are exposed to the elements is due to the non-uniform absorbtion of moisture into wood. Basically, the end grain of wood both absorbs water (and other liquids, like wood stains) a full 15 times faster (and I think, 15 times as much) as through any other part of the exposed wood.
When a board gets rained on, the moisture is most rapidly absorbed at the end grains of your boards, and quickly disperses into the wood from there. Then, the wood starts to dry, and it loses moisture 15 times faster at the end grain, too. Since wood cell walls shrink in thickness as the wood dries, you can get tremendous tension in the end grain of a board because the wood cells there want to shrink in thickness, whereas the wood further from the end of the board doesn't want to shrink. The result is that the board splits at it's ends. This is precisely the reason why every 2X12 in the lumber yard will be split for about a foot to 18 inches from it's end.
If you have any exposed end grain on your cedar siding, I would paint that end grain with an exterior high gloss oil based paint to prevent the swelling and shrinking.
Also, I would take a tube of any latex or "acrylic" caulking and fill those splits with that caulk before priming and painting (or staining). Latex, or "acrylic" caulks are the same as latex paints in that they will allow individual water molecules to evaporate out through the paint, but won't allow liquid water to get into the wall through the paint. (Post again if you want to know why latexes/acrylics can do this.)
I'm thinking the problem may be warm moist air escaping into your exterior walls during the winter to form frost. When that frost melts in sprink, it may be causing your cedar siding to get wet, thus causing the splits in your boards. The solution MAY be to simply use some sort of foam gasket or whatever around switches and duplex electrical receptacles in your exterior walls.