Planning · 7 min read
Prep is the paint job: what good preparation looks like
Paint is only as good as what it’s stuck to. Here’s what careful preparation involves, inside and out, and how to tell when it’s being skipped.
Key takeaways
- Most paint failures, from peeling to stains bleeding through, trace back to preparation rather than the paint itself.
- Outside, good prep means washing and drying, scraping and feathering, priming bare wood quickly, and caulking only where caulk belongs.
- Inside, it means clean surfaces, properly repaired plaster, dulled glossy trim, and stain-blocking primer wherever stains could bleed through.
- Test older trim for oil-based paint before latex goes over it, and ask every painter to walk you through their prep in detail.
Most paint failures are prep failures. Paint is a film a few thousandths of an inch thick; put it over chalk, grease, loose paint or damp wood, and it fails along with them, no matter what was in the can.
On an older house, prep can take longer than the painting, and it’s the easiest part to cut short. Here’s what it looks like done thoroughly, outside and in. Our process page explains how we approach a project as a whole.
What does good exterior prep include?
Outside, prep has two jobs: give paint a clean, sound surface, and keep water out of the wood. Michigan’s freeze–thaw cycles find every open joint, and a short exterior painting season tempts everyone to hurry.
Washing, then waiting
Siding collects dirt and pollen, and aging paint turns chalky as sunlight breaks it down. Wipe a sunny wall with a dark cloth: if it comes away coated in powder, new paint would stick to that chalk instead of the wall. Mildew, the gray or black specks on shaded walls and under eaves, has to be killed and scrubbed off, or it can grow back through the new coat.
A pressure washer helps, but too much pressure gouges soft wood and forces water behind siding. Then comes the step most often rushed: drying. Paint over damp wood tends to blister and peel, so plan on at least a day or two of dry weather, longer when it’s cool or humid. It’s one reason timing an exterior job in Michigan takes planning.
Scraping, sanding and feathering
Anything loose comes off: if a scraper can lift it, new paint won’t hold it down. That doesn’t mean stripping to bare wood, since old paint that’s still tight is a sound base.
Scraping leaves a ridge where old paint stops and bare wood begins, and new paint outlines that ridge instead of hiding it. Sanding feathers the edges so they don’t telegraph through as lines. Wood that has weathered gray gets sanded back to clean fibers, because a weathered surface holds paint poorly.
Bare wood and rot
Bare wood should be spot-primed within days, not weeks, before sun and moisture degrade its surface. Cedar and redwood generally need a stain-blocking primer, or their tannins can bleed through as brownish streaks.
Small soft spots can be dug out to sound wood and rebuilt with exterior filler; two-part epoxy fillers are especially durable. But when rot runs deep, spreads along a board, or reaches a sill or anything structural, the honest answer is carpentry: cut out the bad wood and replace it. Rot also means water is getting in, so the source (a leaky gutter, missing flashing, an open joint) needs fixing too.
Caulk: where it belongs, and where it doesn’t
Caulk seals joints where water would otherwise get in: where trim meets siding or other trim, and around windows and doors. Cracked old caulk should be cut out, not buried, and new caulk goes on over primer, which helps it stick.
Just as important is where caulk doesn’t belong. Never seal the bottom edges of lap siding, where each board overlaps the one below; those gaps let moisture escape from behind the siding. Seal them and the water stays in the wall, where it can surface later as peeling paint and rot. Weep holes in windows and storm windows should stay open for the same reason.
Window glazing on old wood sashes
On original wood windows, glazing putty seals the glass into the sash and sheds water. Old putty dries, cracks and falls out, letting water into the wood. Failed glazing should be replaced before painting and given time to firm up; drying times vary widely, so follow the label.
The paint should then lap about a sixteenth of an inch onto the glass to seal the joint. Reglazing is slow, fiddly work, so if a bid for a house full of old windows never mentions it, ask why.
Protecting plants, paths and patios
Shrubs should be tied back, covered with breathable drop cloths (plastic can cook plants on a hot day), and rinsed with water before and after any cleaning solution is used nearby. Walks, patios, decks and air-conditioning units get covered, and paint chips get collected, not left in the mulch.
What does good interior prep include?
Inside, walls and trim are seen up close, often with sunlight slanting across them, so good interior painting prep aims for smooth as well as sound.
Protection and cleaning
Furniture moves to the middle of the room and gets covered. Floors get canvas drop cloths or taped-down paper rather than slippery plastic, and switch and outlet covers come off instead of being painted around. Kitchen walls carry cooking grease and bathrooms carry soap and hairspray residue; paint won’t stick well to either, so surfaces get cleaned, then rinsed of any cleaner.
Filling holes and dings
Nail holes, dents and dings get filled, sanded flush and spot-primed. Skip the primer and patches can show through the finish as dull or shiny spots, which painters call flashing.
Plaster cracks in older homes
Many older homes in Grosse Pointe and across Metro Detroit still have plaster walls, which move as the house settles and the seasons change. Hairline cracks are common and usually cosmetic. A lasting repair means opening the crack slightly, bridging it with mesh tape, and filling it with setting compound, a powder mixed with water that hardens chemically and shrinks less than premixed compound. A quick smear of spackle often lets the crack come right back.
Loose plaster is different. If a section bulges, sounds hollow when tapped or gives when pressed, it has likely pulled away from the lath behind it, and no crack filler will hold. It has to be reattached, or cut out and patched. A wall covered in cracks and old patches may need a skim coat: a thin layer of compound troweled over the whole surface.
Stain-blocking primer
Water stains, smoke residue, marker and pine knots can bleed through ordinary paint, sometimes reappearing as it dries. A stain-blocking primer seals them in; oil-based and shellac-based primers are common choices for heavy stains and knots, but check the label for what each one blocks. With water stains, fix the leak and let the area dry first, or the stain will return.
Dust and trim caulk
Dust left on a surface ends up in the finish, so everything gets vacuumed and wiped down before painting. For heavy sanding, closing off the room and shutting off forced-air heating or cooling keeps dust from traveling. Last, gaps where trim meets walls get paintable caulk. The rule of thumb: caulk is for joints between parts that move; filler is for holes in a surface.
Is your old trim oil-based or latex?
Many older homes still have oil-based paint on their trim. Latex brushed directly over glossy oil paint often fails to grip and can peel off in strips, sometimes with just a fingernail.
To check, clean a hidden spot, such as trim inside a closet, and rub it firmly with a cotton ball dampened with rubbing alcohol. If the paint softens and color comes off on the cotton, it’s latex. If nothing happens, it’s most likely oil-based.
Either way, glossy trim should be scuff-sanded or wiped with a liquid deglosser to dull the shine; the goal isn’t to remove paint, just to give the new coat some tooth. Oil-based trim should also get a bonding primer made for use over oil-based paint before latex goes on. Skip that step and peeling is a common result.
What about lead paint in older homes?
Homes built before 1978 may contain lead-based paint, and prep is exactly the kind of work that disturbs it: scraping and sanding create chips and dust. The EPA’s Renovation, Repair and Painting (RRP) rule requires certified firms and lead-safe work practices for paid work in pre-1978 homes that disturbs more than 6 square feet of painted surface per room inside, or more than 20 square feet outside. The EPA’s RRP program page has the details.
What should you ask a painter about prep?
A thorough painter will walk you through their prep step by step. A breezy “we prep everything” tells you something too.
- How will you handle loose paint and bare wood? Listen for scraping, feathering and prompt priming.
- How long will the house dry after washing? Painting the same day as washing is a red flag.
- Where will you caulk, and where won’t you? The bottom edges of lap siding belong in the second answer.
- What if you find rot or failed glazing? Ask whether repairs are included, and who decides when filler isn’t enough.
- Which primers, and where? Spot priming, stain blocking and bonding over oil are different jobs.
- How will you protect plants, floors and furniture, and control dust?
Prep belongs in the written estimate, too, and our guide to comparing painting estimates covers what else to look for. Put these questions to every painter you talk to, us included.
Go deeper
- Why exterior paint peels — and how to diagnose the cause.
- Painting plaster walls in an older home — cracks, loose plaster and calcimine.
- Painting old wood windows — sashes, glazing and storms.
- Lead paint in older homes — what to know before any project.
Questions
How long should a house dry after pressure washing before painting?
Usually at least a day or two of dry weather, and longer when it’s cool, humid or shaded. Wood absorbs water during washing, and paint applied over damp wood can blister or peel as that moisture tries to escape. A moisture meter takes the guesswork out of it.
Can you paint latex over oil-based paint?
Yes, but not directly over glossy oil-based paint. The surface should be cleaned, scuff-sanded to dull the shine, and primed with a bonding primer made for use over oil-based paint. Skipping those steps is a common reason latex peels off older trim.
Should you caulk the bottom edges of siding?
No. On lap siding, the small gap where each board overlaps the one below lets moisture escape from behind the siding. Sealing it can trap water in the wall and lead to peeling paint and rot. Caulk belongs at trim joints and around windows and doors.