These surfaces should be primed with exterior alkyd or acrylic primer, before applying a finish coat. If you must paint over stain or varnish, sand first, and prime with alkyd before painting with alkyd or exterior latex. Concrete It takes a month for concrete floors to cure, and another month to finally dry out. Floors can be difficult to paint, even with appropriate products, if chemical hardeners were used. Abrasive blasting is sometimes required.
Muriatic acid is also effective for removing laitence.
Alternatively, tape a piece of vapour barrier to the floor, and look for condensation under the plastic, after 2-3 days. Basement floors may remain perpetually damp, in the absence of adequate vapour barrier underneath. Abraisive blasting is the best way to remove mill scale. Very heavily blasted steel will rust at the peaks of its texture, before the second coat can be applied. The texture is determined by the grit of the sand used.