Paint applied to surfaces that are not completely dry or not primed correctly can bubble and flake. To remove the deteriorating paint properly you have to start with the flakes and the surrounding paint that comes off easily. Scrape thoroughly or the surrounding paint can start to flake after repainting. Paint can be reapplied after the area has been sanded, dried and primed properly.
Put on your protective gloves, eyewear and face mask before you start working. Get the easy flakes off. Paint can be peeled off by hand from unsound surfaces. Remove as many flakes as you can.
Use a putty knife to lever up flakes and follow them across the surface to their point of origin, where the paint seems sound and does not come off easily. Work the putty knife under all the edges of the flaked paint to make sure hidden flakes are uncovered.
Use a paint scraper to go over the area, including spots that are not flaked. If the paint is sound, the scraper should not have much effect. Work the scraper across all the edges of the area where flakes came off.
Sand the rough edges. Use heavy grit (50 to 80) sandpaper or a paint removing sanding block to smooth the edges of the flaked-off paint spots. If you find more areas that flake when you are sanding use a putty knife to remove them.
Fill in the edges. You may have to use wood filler to smooth out the flaked spots. Make sure the area is clean and dry. Follow the manufacturer's instructions for the fill product you are using. Some require a primer undercoat before they are applied. Finish with 80-grit sandpaper, followed by 120-grit.
You can use a pressure washer to remove most of the flakes. The problem is that this sometimes injects water under sound paint and can be a cause of flaking after repainting. Heat guns are used to blister and remove paint, but if improperly used they can start a fire.
There are a variety of power drill attachments for removing paint. There are also power sanders and other power tools available at your local hardware shop.
Always dry, sand and prime the area properly before repainting.
Paints, particularly older paints that are likely to flake, include a variety of toxic substances. Lead and asbestos are always potential hazards when working with older painted surfaces. If you that suspect lead or arsenic are present, use a respirator in addition to other protective clothing and eyewear. Collect the flakes and chips and dispose of them properly.