The Exterior Primer Map, Substrate by Substrate

Why this matters

An exterior repaint is never one substrate. The same house carries sound old paint, bare wood where it was scraped, new trim from the carpenter, cedar that bleeds, galvanized gutters and a stucco or masonry base, and each of those wants a different primer or none at all. Crews that prime everything with whatever is on the truck lose twice: they spend a coat where none was needed, and they put the wrong primer on the two or three substrates where the wrong one fails. Alkyd on galvanized peels in sheets; a general-purpose latex primer over cedar knots lets the tannin bleed brown through a white finish. This card is the map: which primer family each exterior substrate takes, and the condition that changes it. Whether a failing wall gets spot priming, full priming or stripping is a separate call, made in 'Spot Prime vs Full Prime vs Strip on Failing Exterior Decision Matrix'.

The governing rule: the data sheet is the last word

Primer families behave predictably, which is why a map is possible. But within a family, products differ in what they are labeled for, and the label decides. The order of authority is: the topcoat maker's data sheet for what it may go over, then the primer's data sheet for what substrate it is labeled for, then this map. Where a topcoat maker names a primer for a substrate, use it, because a failure on an off-label system is the shop's to fix. Spread rate, recoat time and thinning are per the product data sheet; see 'Reading a Paint Data Sheet for Coverage, Thinning and Recoat'.

Lead. On any pre-1978 house, presume lead until an EPA-recognized test kit or a certified renovator's sample says otherwise. Priming is not disturbance; the scraping and sanding that creates the bare spots is, and it follows 'How to Scrape and Feather Sand Peeling Trim on a Pre-1978 House'.

The map on one elevation

          /\
         /  \     <- gable siding, sound paint:
        /    \       no primer but bare spots
       /      \
 ====================  <- galvanized gutter: acrylic
 |                  |     primer for galvanized
 |  +-----+ +-----+ |
 |  | win | | win | |  <- new pine casing: exterior
 |  +-----+ +-----+ |     wood primer, all six sides
 |  cedar shutters  |  <- cedar, knots: tannin-
 |                  |     blocking primer
 |------------------|
 |  stucco skirt    |  <- stucco: alkali-resistant
 |                  |     masonry primer
 +==================+======= grade

Substrate by substrate

Bare wood and new trim. An exterior wood primer, either alkyd (oil) or acrylic latex labeled for bare exterior wood. Both work; alkyd penetrates and seals the grain of dry, weathered wood well, latex stays flexible and is easier around a crew working all day. The condition that matters more than the family: bare wood left in the sun grows a gray, weathered surface layer in a short time, and primer laid on that layer is only as strong as the layer. Sand back to fresh wood before priming any wood that has gone gray. New trim gets primed on all six sides, ends and back included, before it goes up where the carpenter allows, because an unprimed back or end grain wicks water and pushes the finish off the face. A cut end made on site gets primed before it is installed.

Knots, cedar and redwood. These woods carry water-soluble tannins (cedar, redwood) and resin in knots (pine) that migrate through a primer that does not block them and stain the finish yellow-brown. They take a stain-blocking primer: traditionally alkyd on cedar and redwood, a shellac-based primer spot-applied to pine knots where its label allows exterior spot use, or a waterborne primer that is specifically labeled to block tannin. A general-purpose latex primer is the common failure. Two coats over knots are normal. The proof is a white topcoat over the primer: if brown shows through after it dries, the block failed and the area gets more blocking primer, not more topcoat.

Galvanized gutters, downspouts and flashing. Never an alkyd primer directly on zinc. The zinc reacts with an oil film over time (saponification), the bond turns to soap at the interface, and the coating sheets off. The galvanized surface takes an acrylic primer labeled for galvanized metal, or a topcoat its maker labels as self-priming over galvanized. New galvanized also carries a mill oil and passivation film that blocks adhesion; it is degreased per the primer maker's instructions before priming, and some makers want it weathered or etched first. Factory-finished aluminum gutters are a different substrate: clean, scuff the factory finish and use a bonding primer or the topcoat maker's recommended system.

Masonry and stucco. New masonry, stucco and concrete are strongly alkaline (high pH) until they cure and carbonate, and an ordinary primer or alkyd will fail on them. They take an alkali-resistant masonry primer or sealer, applied only once the surface is within the pH limit on that primer's data sheet. New stucco is commonly given about four weeks to cure, but the data sheet's pH limit is the test, not the calendar: some primers are made to go on at a higher pH than others. Check with pH test strips on a dampened spot. Old, previously painted masonry that is sound and chalk-free often needs no primer; bare patches, new patches and efflorescence spots (cleaned off first) get the masonry primer.

Sound previously painted siding. No primer, when the old paint is sound, clean, chalk-free and the same chemistry family the new coat is labeled to go over. The checks: wash it, then rub a dark cloth across it; a cloth that comes away white means chalk is still there, and it either gets washed again or primed with a primer made to bind chalk. Old glossy oil on trim (the alcohol test reads it) takes a bonding primer before latex; see 'How to Test Glossy Trim and Prime Latex Over Oil'. Bare spots from scraping get spot primed with the bare-wood primer above.

What goes wrong, and how it shows

The last two have the same signature worth teaching: look at the back of the failed flake. Wood fibers mean the wood surface failed; white powder means chalk; clean primer with nothing on it means the primer did not bond to the substrate at all.

Worked example: a 1986 two-story repaint

The house. Built 1986, so no lead presumption. Wood lap siding, sound except for scraped bare spots on the south wall. The carpenter replaced the casings on four windows with new primed-on-face pine. Eight cedar shutters, previously painted, with bleed showing on two. Galvanized gutters, original, previously painted, peeling on the north side. A stucco skirt along the front with a new patch about 3 feet by 4 feet where a dryer vent was moved twelve days ago.

The map the lead wrote on the work order:

1. Siding. Washed, then the dark-cloth chalk test on each elevation: clean on three, light chalk on the west. West rewashed and retested clean. No full prime. South-wall bare spots spot primed with the exterior wood primer, after sanding off the gray on two spots that had been scraped the week before.
2. New casings. Factory primed on the face only. The backs could not be reached once installed, so the four casings came off with the carpenter's agreement, were primed on backs and ends with exterior wood primer, and went back up. The site-cut ends were primed before reinstallation.
3. Shutters. Two bleeding shutters were primed in full with an alkyd stain-blocking primer. The other six were sound and were spot primed only where scraped to bare cedar, with the same primer.
4. Gutters. The north-side peeling was scraped to sound paint. The flakes came off clean and left bare, shiny zinc: an earlier painter had probably put alkyd on the metal. The bare metal was degreased and primed with an acrylic primer labeled for galvanized.
5. Stucco patch. pH strips on the dampened patch read above the masonry primer's stated limit. The patch was left, and the crew came back to it at the end of the job, eleven days later; at 23 days the strips read within the limit, and the patch was primed with the alkali-resistant masonry primer. The rest of the skirt was sound and got no primer.

The tally. Five substrates, four different primer families (wood, stain-blocking, galvanized acrylic, masonry), and one substrate (the sound siding) with no full prime, only its scraped bare spots primed. Priming the whole house with one primer would have spent a full coat on the sound siding and put the wrong primer on the shutters, the gutters and the green patch.

What would change the answer.

• A pre-1978 house. The same map, but the scraping on the siding, shutters and gutters is RRP work once it passes the 20 square foot exterior minor-repair limit, as a whole-house repaint's prep will, and its cleanup has to pass before priming.
• PVC or composite trim instead of pine. Often needs no primer at all, but dark colors may be restricted because heat buildup can distort the trim; check the trim maker's instructions.
• The topcoat maker names a system. Their named primer replaces the family choice above.

How to verify the map was followed

• Each substrate's primer is written on the work order with the product, and matches its data sheet's labeled substrates.
• Back-of-flake check done on any failing area before choosing its primer.
• pH, chalk and alcohol test results recorded for the stucco, siding and trim that were tested.
• A white or light test patch over any tannin-blocking area checked dry before the full topcoat goes on.

References

• Primer and topcoat manufacturers' product data sheets for the system used
• See related: Paint Preparation + Priming Guide
• See related: Spot Prime vs Full Prime vs Strip on Failing Exterior Decision Matrix
• See related: How to Test Glossy Trim and Prime Latex Over Oil
• See related: The Gaps a Painter Must Never Caulk