Why white paint turns yellow indoors

White paint that yellows indoors is almost always a solventborne, oil-based alkyd enamel: Resene's paint experts say this yellowing occurs in darkened areas such as the inside of cupboards, which is why cupboard interiors and skirting boards behind furniture go first. Waterborne enamels are sold as the non-yellowing alternative, while grease and smoke discolour any paint but sit on the surface and mostly wash off.

So the first job is working out which of those you have, because one is fixed with a bucket and the other with a repaint.

Why does oil-based enamel go yellow?

Alkyd enamels are made from resins based on vegetable oils. They flow out beautifully and dry hard, but they keep changing chemically as they age. Resene's own comparison of alkyd and acrylic binders rates alkyds poor for yellowing and acrylics good, and lists yellowing in shady situations among the known weaknesses of alkyd resins. It also notes that alkyds lose flexibility with age, which is the same slow chemistry showing up another way.

The counter-intuitive part is that light slows it down. Resene puts the yellowing down to a photochemical reaction of the vegetable oils in the alkyd resin, and says it occurs in darkened areas. That is why you get the classic signs:

• the inside of a wardrobe noticeably creamier than the doors;
• a pale rectangle where a picture used to hang is actually the more yellow patch, not the cleaner one;
• skirting behind a couch darker than skirting in the open;
• a hallway with no window yellowing faster than the sunny lounge next to it.

Resene adds two more causes that catch people out. Yellowing also occurs when solventborne paint films are cleaned with ammonia-based cleaners, and pale solventborne enamels yellow faster with heat, so trims and walls near heaters, fireplaces and radiators tend to go first.

Is it the paint, or something on the paint?

Before repainting anything, rule out a dirty surface. Wipe a hidden patch with warm water and a mild detergent and compare.

Cause	Typical clue	Washes off?	Fix
Oil enamel yellowing	worst in dim or covered spots, even colour through the film	no	repaint in a waterborne enamel after proper preparation
Cooking grease	tacky, concentrated near the cooktop and rangehood	mostly	degrease, rinse; repaint only if still discoloured
Tobacco or fire smoke	brown-yellow, heaviest on ceilings and upper walls	partly	clean, then a stain-blocking primer before new paint
Clear varnish over paint or timber	amber tone on trims, benchtops, stair treads	no	strip or paint over; the varnish, not the white, is yellowing
Water stain	brown ring or tide mark on a ceiling	no	fix the leak, dry out, seal with a stain blocker

Kitchens often have two problems at once: an oil enamel yellowing on the cabinets, with a layer of cooking grease on top. Clean first, then judge.

Clear coatings deserve a separate mention because they are often mistaken for yellowing paint. Resene notes that solventborne clears tend to yellow as they age, and that its solventborne Qristal polyurethanes impart a yellow tone to timber, while waterborne clears such as Resene Aquaclear do not yellow in the same way. A pale floor or trim under a solventborne polyurethane can drift warmer over time for that reason, whatever colour is underneath.

Which finishes stay white?

Waterborne finishes. Resene describes its waterborne enamels, Enamacryl and Lustacryl, as non-yellowing alternatives to solventborne paints, and the same Resene comparison rates acrylic binders good for yellowing where alkyds rate poor. Other manufacturers make comparable waterborne enamels; the product data sheet will say whether it is waterborne.

The trade-off is honest rather than one-sided. Resene lists the strengths of alkyds as excellent initial gloss, excellent flow and levelling, drying hard, and resistance to oil and grease in kitchens. Some painters still prefer oil for exactly those reasons on doors and joinery. If the finish is going on a white or pale colour indoors, and especially anywhere dim, a waterborne enamel is the choice that stays the colour you picked.

Can you paint waterborne enamel over yellowed oil paint?

Yes, with preparation. The old enamel is hard and glossy, and waterborne paint will not grip it reliably without help. The usual sequence is to clean off grease and grime, sand to dull the gloss, dust off, and apply a primer that the paint maker lists as suitable over old enamel before the waterborne topcoats. The detail varies by product, so follow the data sheet for what you are using rather than a general rule.

Testing matters here too. Health New Zealand advises presuming lead-based paint on buildings built in the 1980s or earlier, and old trims and doors are exactly where it turns up, so have it tested before sanding.

Our article on waterborne enamel on joinery covers the switch from oil in more detail.

For the broader choice beyond trims, see the differences between oil-based and water-based paints.

When is it not worth fixing?

When nobody sees it. The inside of a linen cupboard that has gone cream is a cosmetic issue, and the paint film is usually still doing its job. Repainting it is a nice-to-have, not a repair.

It is worth fixing when the yellowing is on joinery in a room you have just repainted in a clean white, because the contrast only gets worse, or when you are about to sell and the trims look tired against new walls. In those cases, doing the trims in a waterborne enamel at the same time as the walls is far cheaper than coming back for them later.

If you are weighing up whether to repaint yellowed trims and doors, get in touch and we can tell you what is under the yellow and what it will take.