Preparation is arguably the most important part of any paint job, whether inside or out, if you apply new paint over peeling surfaces, stains or dirt your new work won't look good and it won't last. This page contains tips and techniques for preparing surfaces including drywall and plaster, wood trim and exterior siding. Links are included to more detailed instruction for extreme situations like water-damaged walls and ceilings and repairing rotted exterior wood.
Before preparing walls, ceilings and trim for painting an interior spaces, first move all the furniture away from the walls and into the center of the room. You may also want to remove some smaller items such as chairs and the like to make more room to work. Cover the remaining furniture with a drop cloth to help contain dust and paint chips. Also cover the floors around the perimeter of the room with canvas drop cloths. You can use plastic to cover the furniture but use canvas on the floors to prevent slipping when you walk over them.
With everything removed or covered, the repairs can begin. First scrape off all damaged or peeling paint and wall material from the walls and ceiling. Use a putty knife to dig out everything that appears to be loose. Go around the edges of these damaged areas, pushing out with the putty knife to be sure you've removed all that will come off. The goal is to leave only solid, undamaged surfaces on which to build the repairs. While you're at it, remove any nails or hardware that you won't be using again and clear the debris and dust from the remaining holes.
Prep trim molding by going around the edges where it meets the walls with the point of the putty knife to open any cracks there, so they can be re-caulked. Also use the putty knife to scrape off any loose, peeling paint on the flat surfaces of the moldings and clear cracks between the individual profiles by dragging the blade through them several times. If you're working around windows or french doors with glass panels, use a razor blade scraper to remove any old paint from the glass.
Sand all the trim with medium grit sandpaper to slightly rough up the old paint and smooth out the edges where you scraped the peeling paint. If you're dealing with old latex paint, avoid sanding too hard or it will cut the coating and cause the edges to curl and peel. As a general rule you can only really sand oil-based coatings to remove the surface, on latex use only light pressure with the sandpaper to scuff the shine a bit.
To repair wall and ceiling cracks use the method at this link. Repair any surface bubbling, damaged drywall paper, nail pops, water damage or other wallboard problems and then sand all dry joint compound patches using a medium sandpaper. Vacuum the dust off and then wipe the patches lightly with a damp sponge or rag to remove the finest particles. Pick up the drop cloths from the floor, folding them in toward the center to contain the dust and carry them outside to shake them clean. Vacuum the dust from the floors and moldings and then lay the drop cloths back down around the room.
Seal any stains from water, smoke, ink, etc. on the walls and trim moldings using a primer-sealer to liberally coat them. Also prime all the joint compound on the wall repairs using a latex primer or flat latex paint. If you're using flat latex for your finish you can use it to prime the patches and save yourself a bit of work. If you're using semi-gloss or another shiny paint you'll need to stick with a flat primer because joint compound will burn through shiny coatings causing flashing in the finish.
You can repair nail or screw holes in the trim molding in one of two ways. If the holes are large (more than about a quarter inch), repair them using the nail pop repair procedure and vinyl spackle. When the spackle dries, sand it and check the holes to see if the spackle shrank and sunk a bit, if necessary, apply another coat to level the depression. When the final coat is dry, sand it and prime with flat latex paint or primer.
If the holes in the trim are smaller than a quarter inch, which is usually the case with nail holes, you can use painter's putty to quickly fill them. The putty requires just one application and doesn't need priming making it a much easier solution for holes in wood trim.
To use this method roll a small, one inch ball of putty back and forth in your hand until it is soft and no longer sticky. You may need to add a bit of whiting to it to absorb the excess oil and make the putty more manageable.
When you're ready, use your thumb to press the putty ball into a nail hole and hold it there. Slide a putty knife blade, in an upward motion, between your thumb and the molding to cut the putty off flush. Press hard while doing this to get the best finish.
If the filler edges curl up, gently rub in an outward direction away from the hole to even it out. Be careful not to press over the hole to avoid creating an unwanted depression. If you mess up the putty in the hole or if it doesn't fill the space evenly, press the ball into it again and cut it off with the putty knife. Repeat this until you get the fill you want. Check the bottom of this page for a video demonstrating how to fill nail holes in trim using this method.
When the wall, ceiling and trim repairs have been finished and primed, the caulking can be done. Use acrylic latex caulk to fill the gaps between wallboard and trim as well as the seams between the molding profiles. Most cracks will be pencil thin but if you have cracks wider than about 1|4 inch, you will need to fill them with something else first and then apply the caulk to finish. You can use thin pieces of wood like MDF in some cases or spray foam insulation works well to fill gaps and it will remain flexible to reduce the chance of future cracks developing.
Some cracks may require two applications of caulk to completely fill them. With wide gaps like this build up the caulk on one side of the crack first, let it set, and then fill the remaining space with a second application. Let this extra thick caulk set overnight before trying to paint over it to avoid ruining the finish.
Exterior painting is more complicated and labor intensive than interior painting. Exposure to sunlight, rain, cold and heat is very stressing on the paint coating. Because of the extreme conditions, exterior surfaces require more extensive preparation, priming and a durable paint coating to ensure a lasting finish.
In addition to proper preparation, exterior surface will have to be in good repair before they are painted. If there are any breaks in exterior surfaces such as peeling paint, cracks or rotting wood, water will certainly penetrate these and ruin the new paint finish very quickly.
Peeling paint is the biggest problem with most exterior paint jobs. Paint will always peel if there are any contaminants between the house surface and the paint film. Contamination on exterior surfaces comes from a number of sources, probably the most common is water, but dust, rust, algae, mildew, smoke and air pollutants can also cause exterior paint to peel if they're not removed or sealed first.
Moisture affecting exterior painted surfaces can come from within or it may be saturating the surface from the outside as with rain or an overflowing gutter. In either case the water source must be remedied before any prep and painting can begin. Water from within a wall usually will be coming from a leaky pipe or rain water may backup in the gutter and run down the inside of the house siding.
Gutters are usually easily restored by removing leaves and other debris from them and the downspouts running to the ground. The downspouts will usually be harder to clean than the gutter, requiring some disassembly at the bottom to clear them. After you've done this once you will likely want to avoid it again by adding gutter covers that will block more debris from collecting. Some gutter covers work better than others so if you can afford it, you should go the extra mile and get a high quality system that works well and will do so for years to come.
Start your exterior painting preparation by scraping all loose and peeling paint, putty and caulk from the surface. Slip a putty knife under to lift off loose material and use a pull-type paint scraper to remove paint, etc. that looks loose but doesn't come free easily.
Lightly sand the edges of any bare spots created by the scraping with 120 grit sandpaper for latex paint and 80 grit for harder, oil and alkyd-based paints. To determine the type of paint you're working with try sanding it lightly with 120 paper. If you create dust by sanding the paint, it's oil or alkyd. If the paint balls up or flakes come away from the surface, it's latex paint. Because of this tearing away of the paint coat, don't sand too hard on latex paint, it will create a rough edge around bare spots which will encourage future peeling.
In addition to sanding to remove debris, you should also rough up any shiny paint like semi or high-gloss with a quick pass of the sandpaper. This will help the new paint bond better than it otherwise would to a slick finish.
If you have old oil-based paint on exterior wood you may have some areas where the paint has come off completely and adjacent spots with lumps of old, thick paint remaining. In these cases you can use a heat gun to force this stubborn paint off. Start with a low setting on the heat gun and slowly heat the paint by passing the tip back and forth over it. When the paint is soft, slip a putty knife under it remove it completely. Sand these areas well when you're done. Check the bottom of this page for a video demonstrating how to use a heat gun to strip paint.
After all the scraping and sanding has been done, wash the surface to be painted. Use a garden hose to rinse all the dust off the siding and then scrub it with a nylon brush or broom dipped in a cleaning solution of laundry detergent and warm water. If you have a mildew problem, add a cup or two of household bleach to the solution before scrubbing. Rinse the siding and let it dry completely before proceeding.
With your clean, dry exterior surface you're ready to do any necessary priming, caulking and repairs. Prime before you caulk or patch to ensure a good bond with the fillers. Use a top grade oil or alkyd-based exterior primer to be sure your new paint won't peel, latex primers simply don't stand up well outside. There's no need to prime existing paint that is still in good condition. Primer is only needed to bond to bare spots and block stains or water damage. Let the primer dry overnight before proceeding but don't let it stay exposed for more than a week before finish coating.
Use an exterior grade latex caulk for cracks and let it cure overnight before applying your new paint coat. Be sure to caulk all cracks due to splitting wood and joints around all trim like door and window casings. If you're painting the trim too, caulk any cracks between the molding profiles such as where a door or window jamb meets the casing and along the sill apron.
Use an exterior spackle for minor repairs to wood trim and siding, and an epoxy filler for severely damaged and rotted wood. Let the patching material set according to the label directions and then sand it smooth. A sanding block or power sander will be needed to sand most exterior patching compounds because they become quite hard. When you're done, remove the sanding dust with a duster or vacuum and spot prime the patches.