Chalking: formation of fine powder on the surface of the paint film during weathering. Although some degree of chalking is normal, excessive is not good.

This is usually caused by using a low grade, highly pigmented paint, or using an interior paint for outdoor decorating.

To solve the problem you would remove as much of the residue as possible using a stiff wire brush, then rinse the area with a hose or power washer. If after this there is still chalk present then you would apply an oil or latex primer, and then repaint with a good exterior coating..

Cracking/Flaking: the splitting of a dry paint film as a result of aging. It starts as cracks, and turns into flakes.
This is caused by using low quality paint, or a paint that has inadequate adhesion and flexibility; overthinning or overspreading paint; not prepping the surface well, or not using primer on bare walls; excessive hardening or alkyd paint; exposure to moisture or humidity during the curing of the paint.

To fix this problem you would remove flakes with a scraper or wire brush, sand the surface and feather the edges, then prime if the wood is bare, and repaint. Use a top quality primer and top coat and you should prevent a reoccurance of the problem.

Mildew: black, gray, or brown spots on the surface of paint.

This usually forms in areas that tend to be damp and receive little or no sunlight (bathrooms), or from painting over an area where mildew was previously without first removing it.

To solve this problem you will want to test for mildew by applying a few drops of bleach to the area, if it is bleached away then it is probably mildew. In order to remove the mildew and its spores you want to scrub the area with household bleach.

Rust Stains From Nail Heads

This is not talking about nails you put in the wall to hang art, pictures, etc. These are spots from nails used in the construction of your house. Sometimes, nail heads can rust and create spots on painted surfaces in your home. Often time this problem cannot be avoided as it is caused by using uncoated steel nails in areas of excessive moisture under the paint. So, while you can't remove the uncoated nails, you can correct the moisture problem.
Start by trying to locate the source of excessive moisture, and try eliminating the problem by shutting off the condensation that causes the moisture. Then, remove the stained paint around all nail heads by sanding the area. You do not just want to sand off the showing rust, you want to sand clear down to the nail head, then sand the nail head itself to remove the built-up rust so the problem does not persist. Use a nail punch to countersink all nail heads approximately 1/8" below the wood surface. Then apply an even layer of undercoat over the countersunk nail and the area around it. Prime the area, and fill the countersunk hole with a caulking compound. Apply one coat of a good grade of outside house paint. After adequate drying time, apply a second coat.