How do I prep a house for exterior painting?

Prep is most of an exterior paint job, and it decides how long the paint lasts. The order matters: stop the water and fix rot, wash, scrape, sand, prime, then caulk and fill. Do it on dry days, and if the house is old, plan for lead paint before you pick up a scraper.

Start with the year the house was built

The EPA's rule of thumb: If your home was built before 1978, it may have lead-based paint; the older your home, the more likely it has lead-based paint.^[3] If you cannot find out, the agency says to assume the area you are renovating has lead-based paint.^[3] A certified lead inspector or risk assessor can test it if you would rather know.

That choice changes your methods. The USDA Forest Products Laboratory is blunt: "Do not sand lead-based paint."^[1] Scrape wet, use wet-dry sandpaper or a damp sanding sponge, and if you strip paint with heat, use a low-temperature heat gun under 1,100 degrees Fahrenheit,^[3] never a torch. The setup federal rules require of paid crews on pre-1978 houses is a good model for your own: cover the ground with plastic sheeting extending 10 feet beyond the surfaces being worked on, and close all doors and windows within 20 feet.^[4]

Fix water and rot before anything else

Most peeling paint is a moisture problem, and painting over it wastes the paint. In the National Park Service's words, "There is no sense in repainting before dealing with the moisture problems because new paint will simply fail."^[2] The usual culprits are faulty flashing, leaking gutters, defective roof shingles, cracks and holes in siding and trim, deteriorated caulking in joints and seams, and shrubbery growing too close to painted wood.^[2]

Push a screwdriver into sills, the bottoms of door and window casings, and board ends. Soft wood has to be repaired or replaced, since paint is not a preservative. It will not prevent decay if the wood is moist.^[1] The Park Service notes that a window sill that has sat under standing water is a strong candidate for rot, and repair or replacement should come before repainting.^[2]

Wash the house

Dirt, chalk and mildew all stop new paint from sticking. For ordinary grime, the Park Service suggests half a cup of household detergent in a gallon of water, scrubbed on with a medium-soft bristle brush, then rinsed thoroughly and left to dry.^[2] Clean chalky old paint well: latex paint cannot penetrate a chalky surface and adhere.^[2]

To tell mildew from dirt, put a drop of household bleach on a dark spot: mildew usually bleaches out in a few seconds; dirt does not.^[1] For mildew, One part household bleach to five parts water should be adequate, and a mix stronger than one part bleach to three parts water should never be needed.^[1] Use powdered detergent, not liquid, which may contain ingredients that react with bleach to give toxic fumes, and never mix bleach with ammonia.^[1]

Rinse with a garden hose and keep the water stream pointed down so you do not flood the back of the siding.^[1] Be careful with a pressure washer: the Park Service does not recommend waterblasting above 600 psi to remove paint, because it can force water into the woodwork.^[2] Let the house dry completely before the next step.

Scrape and sand

Scrape away paint that is loose, blistered or flaking, pushing from the loose area toward the edge where paint is still tight. Paint that is firmly bonded can stay. Mist the surface first; the EPA lists a spray mister or pump sprayer to mist areas before sanding, scraping^[3] as a basic dust-control tool.

On paint you know is lead-free, sand areas of exposed wood with 50- to 80-grit sandpaper to remove the weathered surface and to feather the abrupt paint edge.^[1] Bare wood degrades in the sun faster than it looks: Bare wood that sees even a week of south-facing exposure to sunlight can have significantly lower paint performance than fresh wood.^[1] If wood looks weathered and silver or grey, it is severely sunburned and will likely need much more extensive sanding^[1] before paint will hold.

Choose tools that do not tear up the wood. Rotary sanding discs leave circular dents and rotary wire strippers can shred wood,^[2] and on pre-1978 houses, high-speed sanders, grinders, needle guns and abrasive blasting are banned on painted surfaces unless the machine has a shroud and a HEPA vacuum attachment^[4] when a contractor does the work.

Prime, fill and caulk

Bare wood should be primed within 48 hours,^[2] before it weathers again. On cedar, redwood and knotty wood, stain-blocking primers are advised to block extractives bleed into the topcoat.^[1] Where nail heads rust through, countersink the fastener, spot prime, caulk, spot prime the caulk, and topcoat.^[1] Caulk open joints where trim meets siding, but leave the bottom edges of lap siding open so water that gets behind it can drain.

If you are unsure the old paint is sound, test it. Prime a spot, let it dry overnight, press an adhesive bandage on and snap it off. If the new primer adheres to the tape, the primer was not well bonded to the wood, and premature peeling is likely.^[1]

Pick the painting days

Apply latex-based waterborne paints when the temperature is at least 10 °C (50 °F) and expected to remain above this temperature for 24 h.^[1] Avoid painting late in the afternoon if heavy dew is expected during the night.^[1] On smooth wood, the best service life comes from a primer and two topcoats.^[1]

Work safely on a ladder

Virginia Commonwealth University's ladder rules are a sound checklist: pull the base out to one-fourth of the height to the point of support, extend the ladder at least 3 feet above the eave or gutter it rests against, keep three points of contact while climbing, and never stand on the top two rungs.^[5] Metal ladders must not be used on or near electrical wires.^[5] Virginia law bars anyone doing the work as a trade or business from placing a person or a ladder within 10 feet of an overhead high-voltage line,^[6] and that distance is a sensible minimum for a homeowner too.

Bag paint chips and plastic as you go. The EPA says that for routine home renovation, most waste generated during these activities is classified as solid, non-hazardous waste,^[3] but check your locality's rules; see how to dispose of old paint in Virginia for leftovers.

Stop and call a pro if

• The house was built before 1978 and large areas of paint are peeling to bare wood. That is a lot of lead-paint disturbance; a painter who is lead-safe certified has the training and containment for it.
• Reaching the work means a second-storey ladder, a roof edge, or anywhere near the overhead lines to the house.
• Probing turns up rot in fascia, sills, corner boards or framing, or the peeling traces back to a leak you cannot find.

If the paint is failing in one area, why exterior paint peels will help you find the cause, and cracking window trim has its own answer. For budgeting, see what exterior painting costs in Virginia. Browse exterior painters, look up painting businesses around Blacksburg, or read more exterior answers. When you would rather hand it off, describe your exterior painting job once and licensed local businesses can quote it.