Painting seems like an easy DIY job -- and it can be if you take time to prep and make sure you're using the right products. Take heed of these precautions to achieve professional results.
Take Time to Prep
Before painting, you'll need to patch holes in the wall with joint compound or spackle and sand it down to get an even surface ready for paint. This takes time, especially for the products to dry, so attempting to patch, spackle, and sand holes the day you intend to paint could be discouraging. Spackle should be applied in two or three thin coats, with sanding after each coat. Apply fabric tape over cracks. Use fans to speed drying times. Don't spackle and patch cracks and holes in a thick coat and wait a week to sand it -- the longer the spackle or joint compound sits, the harder it is to sand.
Don't try to paint around switch plates and electrical covers. Remove them and put the screws in a safe place so that they aren't lost when you reattach the plates. Cover switches and outlets with masking tape. Do all these things ahead of time, and you will find your interior painting projects more enjoyable and effective.
Use the Proper Type of Paint
The most common mistake is using the wrong type of paint. If the existing paint is oil-based, you must paint over it with an oil-based paint. If it's latex paint, you must paint over it with latex paint. Otherwise, the paint will peel off. To determine what type of paint it is, moisten a rag with denatured alcohol and rub it over the paint. If some of the paint comes off on the cloth, it's latex. If nothing comes off, it's oil-based. You can paint over oil-based paint with latex paint, but it must first be painted with an oil-based primer/sealer.
Give Attention to Trim
Paint the walls first and then the trim, not vice versa. You can get a little wall paint on the baseboards and window frames because it will be covered when you paint the trim. To keep things neat, put two-inch masking tape on the carpet before painting baseboards. Pull the tape off when the paint on it dries. Leaving the tape on overnight is OK, but it's a common mistake to let it sit for a week and then try to remove it, which can be difficult and cause damage.
When taping off windows, don't put masking tape on the glass. Instead, paint about 1/16 of an inch onto the glass and remove it with a single-edged razor blade when it's dry. This seals the windows. Again, it's a common mistake to let the paint sit on the windows for a week, making it very hard to remove after the fact. Razor it off right after it's dry.
Check for Lead-Based Paint
The Environmental Protection Agency has special rules and laws for working around and disposing of lead-based paint, which is very dangerous to your health. If the house you're working on was constructed before 1979, it will likely have lead-based paint in it.
The area you're working in needs to be sealed off from the rest of the structure, and special safety gear and respirators must be worn. In most cases, you will be required to take a special course before legally working on houses with lead-based paint. Check with your local paint store for availability, cost, and hours required for the lead-based work course.
Seal Smoke-Stained Walls
One last common mistake is to paint over smoke-stained walls without first sealing them with an oil-based primer/sealer. The smoke stains will not be covered with an extra coat of paint -- you must use a sealant. Otherwise, the stain will bleed through, and often in streaks. Please note that you must wear a respirator for this step, as this type of sealer is volatile and you may become sick without protecting your respiratory system. For this reason, also use plenty of ventilation, opening windows and doors. Read the warning label printed on the paint can for more information on this.