If you are a do-it yourself painter, you may come across one of these paint problems -especially on exterior surfaces - and need to understand why it happened and how to deal with it.
1) Alligatoring - Patterned crackling that looks like alligator skin. It means that the top coat of paint didn't bond to a smooth finish most often caused by applying a top coat over a primer or existing paint that was not the same (oil over latex) or that was still wet. If you apply too much paint per coat, this can happen. Natural aging in extreme climates will also cause this to happen. You MUST scrape or sand to the base surface to overcome the problem. Once the surface is clean and dust-free use a primer and then paint that are meant to work together.
2) Blistering - Lifting of paint from surface in bubbles. Most often caused by painting in sunlight or on a surface that is too hot. Fresh paint exposed to moisture will also blister, as will paint on moist walls (showers, kitchens, laundry). First determine how it occurred (a)if only the newest paint coat is blistered, it was caused by heat or (b) if many layers of paint are affected and the base surface shows through than it was caused by moisture. For heat blisters, scrape or sand to the next coat or primer. Re-paint and be sure the surface does not get hot. For moisture blisters, scrape to the base surface, prime and then paint. Resolve the issue by adding ventilation fans and/or caulking appropriately.
3) Chalking- Formation of a fine powder on the surface of paint. All paints do chalk somewhat but severe chalking is caused by low quality paint, over thinning paint or spreading it too thin and not sealing or priming a porous surface. To repair the problem you need to remove chalk by pressure washing or hand scrubbing. A wire brush or sanding may be needed in some cases. Once clean, if there is no more chalking, and the paint is in good shape, you can paint over it. If moderate chalk remains you should use a penetrating additive to the first coat of water-based paint to help it bond to the surface. If a lot of chalk remains, use an alkyd-based primer and then a high quality top coat.
4) Cracking and flaking - Hairline cracks and paint chips falling from the surface. Generally caused by thinning of paint or spreading it too thin. Unprimed wood surfaces crack easily. If you have painted in windy conditions, the paint may have dried to fast and not bonded to the surface, causing cracks. When painting over many layers of paint, cracking can occur when the older, lower layers lose flexibility and give out with temperature changes. For surface cracks, scrape, sand and re-paint. When cracking is down to the base surface you need to remove all paint and re-prime before using a high quality paint.
5) Peeling - loss of adhesion to the base surface. On wood surfaces, this occurs when wet wood expands and contracts causing the paint to roll at the edges. On metal surfaces it is likely improper surface prep and failure to prime or sand. For wood, you should determine the cause of moisture (water running down from a leaky roof, ground moisture from irrigation or seeping up, too much exposure to moist air)and repair before repainting. Scraping or sanding the loose paint is first, letting the surface dry thoroughly, then using a primer made for that surface. Use a high quality latex paint in two coats, to cover the primed surface.
6) Poor hiding - you can see the underlying surface through your paint. Generally caused by over thinning or overspreading paint, using a low quality paint, and not using poor quality rollers and brushes that don't apply paint well. By using another coat of un-thinned paint, you can resolve this. A roller with the correct nap for the surface will help the paint flow on smoothly.
7) Mildew - mold growing on the paint surface. Causes can be failure to prime bare wood surfaces, painting over existing mold, or excessive humidity and moisture problems. Ventilation fans and dehumidifiers can help resolve interior moisture issues. To be sure its a mildew problem, apply a few drops of bleach to the surface. If it bleaches out..it is likely mildew since stains and dirt will not be affected. Clean the surface with a mildew remover or a mixture of 1 quart liquid bleach + 3 quarts warm water + 1/4 c ammonia free liquid dishwasher detergent. Rinse thoroughly. Allow the surface to dry and prime and then paint. You can purchase paints with mildecides or add one to the primer coat if desired.
8) Efflorescence - White deposits on masonry surfaces. The cause is often failure to remove previous salt deposits and painting before masonry (stucco, concrete, etc..) is fully cured. Removing the efflorescence with a stiff brush or power washer is first. Wash the surface with one part phosphoric acid + seven parts of water adding the ACID TO THE WATER. Rinse and allow to dry, repeating until all efflorescence is gone. Seal the surface with a primer designed to hold back efflorescence and paint with a high quality acrylic finish.
2) Blistering - Lifting of paint from surface in bubbles. Most often caused by painting in sunlight or on a surface that is too hot. Fresh paint exposed to moisture will also blister, as will paint on moist walls (showers, kitchens, laundry). First determine how it occurred (a)if only the newest paint coat is blistered, it was caused by heat or (b) if many layers of paint are affected and the base surface shows through than it was caused by moisture. For heat blisters, scrape or sand to the next coat or primer. Re-paint and be sure the surface does not get hot. For moisture blisters, scrape to the base surface, prime and then paint. Resolve the issue by adding ventilation fans and/or caulking appropriately.3) Chalking- Formation of a fine powder on the surface of paint. All paints do chalk somewhat but severe chalking is caused by low quality paint, over thinning paint or spreading it too thin and not sealing or priming a porous surface. To repair the problem you need to remove chalk by pressure washing or hand scrubbing. A wire brush or sanding may be needed in some cases. Once clean, if there is no more chalking, and the paint is in good shape, you can paint over it. If moderate chalk remains you should use a penetrating additive to the first coat of water-based paint to help it bond to the surface. If a lot of chalk remains, use an alkyd-based primer and then a high quality top coat.4) Cracking and flaking - Hairline cracks and paint chips falling from the surface. Generally caused by thinning of paint or spreading it too thin. Unprimed wood surfaces crack easily. If you have painted in windy conditions, the paint may have dried to fast and not bonded to the surface, causing cracks. When painting over many layers of paint, cracking can occur when the older, lower layers lose flexibility and give out with temperature changes. For surface cracks, scrape, sand and re-paint. When cracking is down to the base surface you need to remove all paint and re-prime before using a high quality paint.
5) Peeling - loss of adhesion to the base surface. On wood surfaces, this occurs when wet wood expands and contracts causing the paint to roll at the edges. On metal surfaces it is likely improper surface prep and failure to prime or sand. For wood, you should determine the cause of moisture (water running down from a leaky roof, ground moisture from irrigation or seeping up, too much exposure to moist air)and repair before repainting. Scraping or sanding the loose paint is first, letting the surface dry thoroughly, then using a primer made for that surface. Use a high quality latex paint in two coats, to cover the primed surface.
6) Poor hiding - you can see the underlying surface through your paint. Generally caused by over thinning or overspreading paint, using a low quality paint, and not using poor quality rollers and brushes that don't apply paint well. By using another coat of un-thinned paint, you can resolve this. A roller with the correct nap for the surface will help the paint flow on smoothly.
7) Mildew - mold growing on the paint surface. Causes can be failure to prime bare wood surfaces, painting over existing mold, or excessive humidity and moisture problems. Ventilation fans and dehumidifiers can help resolve interior moisture issues. To be sure its a mildew problem, apply a few drops of bleach to the surface. If it bleaches out..it is likely mildew since stains and dirt will not be affected. Clean the surface with a mildew remover or a mixture of 1 quart liquid bleach + 3 quarts warm water + 1/4 c ammonia free liquid dishwasher detergent. Rinse thoroughly. Allow the surface to dry and prime and then paint. You can purchase paints with mildecides or add one to the primer coat if desired. 
8) Efflorescence - White deposits on masonry surfaces. The cause is often failure to remove previous salt deposits and painting before masonry (stucco, concrete, etc..) is fully cured. Removing the efflorescence with a stiff brush or power washer is first. Wash the surface with one part phosphoric acid + seven parts of water adding the ACID TO THE WATER. Rinse and allow to dry, repeating until all efflorescence is gone. Seal the surface with a primer designed to hold back efflorescence and paint with a high quality acrylic finish.