Most wood houses built before the late 1980s spent the first part of their lives under oil paint. Linseed oil house paints at first, then alkyd enamels on the trim and alkyd body paints on the siding, applied every eight or ten years by whoever owned the place at the time. At some point a later owner switched to latex, and the house now carries a layered record of both chemistries, often with no paperwork saying which coat is which. That buried history matters more to the next paint job than the colour, the sheen or the brand on the can.
The reason is simple and widely ignored. Old oil paint and new acrylic paint do not behave the same way, and the boundary between them is where exterior coatings most often let go. A crew can do everything else correctly, wash, scrape, caulk and apply two generous coats, and still watch the new film peel off in sheets a year later because it was rolled onto a glossy, embrittled alkyd surface that it never gripped. On an older wood-sided house, repainting starts with one question that is rarely asked out loud: what exactly is already on the wall?
In Pasadena the question comes up constantly, because the housing stock is old and wooden in a way most of Southern California is not. Craftsman bungalows with shingle and clapboard siding, the streets around Bungalow Heaven, the older houses along the Arroyo, all of them have been painted many times over a century, and almost all of them have oil somewhere in the stack. Add hot, dry summers and hard ultraviolet, which age an alkyd film faster than a mild climate would, and the transition from oil to acrylic becomes the single most important technical decision on the job.
Why oil and acrylic fight each other
An alkyd or linseed oil paint cures by reacting with oxygen. That reaction does not stop when the paint feels dry. It continues slowly for years, and the film keeps getting harder and less flexible as it goes. A fresh alkyd coat is tough and slightly elastic. A thirty year old alkyd coat is closer to a thin shell of glass, and under strong sun it also chalks at the surface and develops a fine network of cracks, the pattern painters call alligatoring or checking.
Acrylic latex works differently. It dries as water leaves and the polymer particles fuse into a film that stays flexible for its whole life. That flexibility is why acrylic is the better finish on wood that swells and shrinks with the seasons.
Put the two together and the problem is obvious. The wood moves, the flexible acrylic moves with it, and the brittle oil layer underneath cannot. It cracks, and when it cracks it takes the acrylic with it, because the new coat is only as well anchored as the old coat it is sitting on. The failure looks like the new paint was bad. It was not. It was applied to a layer that had already finished its working life.
There is a second, more immediate problem. An aged alkyd enamel, especially on trim and doors, is often still hard and somewhat glossy. Acrylic needs some surface texture or porosity to grip, and a slick oil surface gives it almost none. Paint applied straight over it can look perfect for months and then peel with a fingernail.
How to find out what is on the wall
Guessing is how this goes wrong. There are three simple tests, and any contractor worth hiring should run at least the first two on every elevation and on the trim separately, since body and trim were often painted with different products.
The alcohol rub test
Soak a cotton ball or a white rag in rubbing alcohol, or better, in denatured alcohol, and rub a small patch of the paint firmly for twenty or thirty seconds. If colour transfers to the rag and the surface softens, the top coat is latex. If nothing comes off and the surface stays hard, the top coat is oil based.
The test reads only the top layer. A house with latex on top can still have a brittle oil layer two coats down, which is why the next test matters.
Looking at a cross section
Find a place where paint has already chipped, or use a sharp blade to cut a small V shaped notch through the film to bare wood somewhere inconspicuous. Look at the exposed edge in good light, ideally with a magnifying glass. The layers show as thin bands of different colours. Oil layers tend to be hard and fracture cleanly. Latex layers tend to be softer and slightly rubbery. A thick stack with many bands, cracking through its full depth, is telling you the whole system has reached the end of its life, not just the top coat.
The tape test for adhesion
Press a strip of strong tape firmly onto a cleaned patch of wall, rub it down with a thumbnail, then pull it off sharply at a steep angle. Paint on the tape means the existing coating is not well attached to whatever is under it. On houses with oil under latex, this test often lifts the latex cleanly off a glossy oil layer beneath, which is the exact failure you are trying to prevent on the new job.
Lead comes first
Any house painted before 1978 should be treated as having lead in the older layers until a test says otherwise, and old oil paint is where it usually sits. That changes how scraping, sanding and cleanup are allowed to happen, and it is a separate subject from the chemistry discussed here. The only point worth making in this context is that the surface preparation choices below assume the crew is already working under lead safe practices where they apply.
Three ways to deal with old oil paint
Once the tests are done, the condition of the oil layer decides the approach. There are really three cases.
Sound, well bonded oil paint
If the alkyd is still firmly attached, the tape test is clean and there is little cracking, it can stay. The job is to give the new paint something to hold. That means washing off chalk and dirt, then dulling the gloss by sanding or with a liquid deglosser, and then applying a bonding primer formulated to stick to hard, glossy surfaces. Some painters use an oil based primer over old oil for the same reason, then top coat with acrylic once it has cured. Either route works. What does not work is skipping the primer and rolling acrylic directly onto glossy alkyd because the paint can says it is self priming.
Chalking oil paint
A chalky alkyd surface is common on sun facing walls. The film is still attached, but its surface has broken down into loose pigment. Acrylic applied over chalk bonds to the powder, not the wall. The fix is a thorough wash with a stiff brush and a cleaner that cuts chalk, a rinse, and then a check with a dark cloth once it is dry. If a light chalk residue remains, a penetrating primer designed to bind chalky surfaces is the right base coat.
Cracked, alligatored or failing oil paint
When the oil layer is checked through its full depth, peeling, or failing the tape test across large areas, no primer will save it. The new system can never be more stable than the old one under it. The answer is removal down to sound material or bare wood on the affected surfaces, usually by scraping and controlled sanding, sometimes with infrared heat tools or chemical strippers on detailed trim. This is slow, and it is where the real cost difference between bids tends to hide. A low quote on a house with alligatored oil siding has usually assumed the old paint can be painted over.
Trim, doors and windows need their own decision
Oil enamel survived longest on trim, doors and window sash because it levels well and wears hard, so these are the surfaces most likely to still carry glossy alkyd even when the body of the house has been latex for twenty years. They also take the most handling and the most water, which makes adhesion failures there obvious within a season.
Window sills and the lower rails of sashes deserve particular attention. They hold water, they face the sky, and on older houses they often have thick oil buildup that has cracked into a fine grid. Taking those surfaces down to sound material and priming them correctly is the difference between a sill that looks good for one summer and one that looks good for eight.
Doors are their own small project. A front door with years of oil enamel on it may need a full strip, a bonding primer, and then a durable acrylic or waterborne alkyd finish that gives the hard, smooth feel people expect from a door without the brittleness of the old chemistry.
Waterborne alkyds and hybrid products
Many modern trim paints are waterborne alkyds or alkyd and acrylic hybrids. They clean up with water but cure partly through an oil like reaction, giving a harder, smoother finish than a standard acrylic. They are useful on trim and doors, particularly where an owner wants something close to the look of old oil enamel. They do not remove the need for proper preparation over old oil. A hybrid applied over glossy, unprepared alkyd will fail in the same way a standard acrylic would.
What to ask before signing anything
The questions that reveal whether a crew understands this are short. Ask whether they tested for oil based paint, and how. Ask where they found it, body, trim or both. Ask what primer they plan to use over it and why. Ask which surfaces they expect to strip rather than prime, and how that shows up in the price. Ask what happens if they find more failing oil than expected once scraping starts.
Good answers are specific. A contractor who says they ran an alcohol test on every elevation, found oil enamel on all the trim and a chalky alkyd under latex on the west wall, and plans a bonding primer on the trim and a chalk binding primer on that wall, has actually looked at the house. A contractor who says their paint sticks to anything has not.
The practical sequence
On an older wood house the order of work follows the chemistry. Test first, elevation by elevation and body and trim separately. Wash and let the house dry fully. Remove anything that is failing, and accept that this is the expensive part. Dull or degloss the sound oil surfaces. Prime with a product chosen for what is actually on the wall. Then apply the acrylic finish coats, which will finally be sitting on something that can hold them.
Done in that order, the switch from oil to acrylic is the last difficult transition the house will need. Every paint job after it is latex over latex, which is far more forgiving. Done out of order, the same switch produces the peeling, flaking exterior that gives older houses a reputation for not holding paint, when the real problem was a layer of thirty year old enamel nobody bothered to test.
