Contributing factors are too-thick finish coat, building up of too many layers, undercoat that was not completely dry, wrong solvent or improperly stirred paint. Repair by sanding smooth and applying proper primer before painting top coat. Chalking or powdering characteristics are designed into some paints in order to keep the surface looking new. Chalking of old paint can interfere with proper bonding of a new coat, so scrub off as much as possible beforehand. In severe cases you might have to apply a bonding primer or sealer.
Mildew manifests itself as patches of black spots.
Though most exterior paints contain mildew-inhibiting ingredients, none works under all circumstances. Remove mildew by washing down the affected area with a bleach solution. Wear long pants, a long-sleeved shirt, gloves, goggles and a respirator to avoid breathing the fumes. Stains that bleed through the paint often are the result of sap from knots in the wood seeping through the surface. But it can also be caused by rust from nails and hardware.