Paint preparation is one of the ugly sides of home improvement projects. All that sanding and scraping and cleaning -- no one really likes to do it, but it's essential to a good paint job.
So, like it or not, it's a step that you don't want to skip or do poorly, since the quality of your finished product pretty much depends on it.
There are three basic things to remember for properly preparing any surface that you want to paint - - get it clean, smooth and dry.
Clean it up
Paint will stick to most of the things it comes in contact with, which is not necessarily a good thing. If the surface being painted is dusty, for example, the paint will stick to the dust. Since the dust is not well-adhered to the surface below, you have an obvious recipe for paint failure.
It's important that the surface being painted has been cleaned to remove dust, dirt, grease and anything else that could get under the paint film and prevent it from forming a solid bond.
For previously painted exterior surfaces, such as siding, dust everything off with a soft brush attached to a painter's extension pole. If the old paint is in generally good condition, you can also spray off the dust and dirt with a pressure washer that is set on low pressure and wide spray. Do not use high pressure or a concentrated spray nozzle, which can drive water into the siding.
For interior surfaces such as cabinets or moldings, dust off the surface with a brush (use an inexpensive chip brush - - not your good paint brushes), then clean the surface with a degreaser to remove any grease residue. If the surface is glossy, lightly sand it to roughen it up slightly, then wipe or vacuum it off to remove any dust.
For new wood, such as new cabinets or furniture, your best bet is to use a tack cloth, available from any paint store or home center. A tack cloth is simply a piece of lint-free fabric that has been treated with a material to make it slightly sticky so that it will pick up fine dirt and dust particles.
A solid start
The same holds true for painting over old paint that is not well-adhered. The new paint film will stick to the old paint, and again, since that old paint is not attached to the surface below, the new paint job will eventually fail. There's simply no easy way to remove old paint, and you have four basic choices.
Scraping: The tried and true method is to use a hand paint scraper and scrape off any loose paint. A pull-type scraper works best for most situations, and helps prevent gouging into the wood below. Scrape with the grain in both directions until all that remains is paint that is well-adhered to the underlying surface. For small areas, a push-type scraper, such as a putty knife, will work as well.
Sanding: This is the other most common paint removal method, used alone or in combination with scraping. A pad sander works best for most situations, using an open-coat, coarse-grit paper for the initial paint removal. Avoid rotary sanders, which can gouge and raise the grain on some older siding, and belt sanders, which remove too much underlying material.
Chemicals: For small areas, such as a piece of furniture, you can use a chemical stripper to remove paint, varnish and other finishes. Follow the manufacturer's instructions carefully, and make sure you have adequate ventilation.
Heat: Using a heat gun will work to remove paint in smaller areas, but it can be dangerous. You can damage the surface you're removing the paint from, and even worse, the heated paint film or the wood can catch fire. Skip this method.
Remember that a paint film is very thin, so whatever you've left on the surface is going to show through the new paint. Once you're done with the scraping and other paint removal work, sand the stripped areas with a medium to medium-fine grit of sandpaper to smooth out the wood and feather down the edges of any remaining paint.
Dry it out
Rule No. 3 is that the surface to be painted needs to be dry. This applies to actual water that is standing on the surface being painted, as well as to material that is damp, such as wood that's been out in the weather. Any type of excessive moisture, either on or in the wood, can cause the paint to fail.