If indoors, don't worry about them, except for pure appearance, in which case apply a coating system appropriate for the location.
Outdoors, same answer, but different products. Obviously, all coatings should have some degree of lifetime elastomeric properties for the full range of exposure.
If a hairline crack evolves to be a non-hairline crack over time and is revealed through the coating, go to Plan B.
Next time avoid the crack hassle. Use a non-shrink mix design (e. g., a Type K cement or similar via an admixture and non-shrink aggregates), even if your concrete element may be a column or prestressed. (If you are wondering about the column, do a stress/elastic deformation allocation analysis between the rebar and the concrete, then throw in what drying shrinkage stresses may contribute to the steel stress/deformation over the lifetime.)
------------------------------
Robert Bullard P.E., M.ASCE
President
Ahimsa Technic Inc
Ponce Inlet FL
(386)256-3050
------------------------------