Can you paint water-based enamel over old oil-based doors and trims?

Why do so many older homes have oil-based trims?

For decades oil-based (alkyd) enamel was the usual choice for doors, window frames, skirting boards and architraves. It dries hard and glossy and stands up to knocks. Plenty of houses built before the switch to water-based trim paints still have layers of it under later coats.

Most trim paint sold now is water-based. Dulux's own comparison lists water-based enamel as non-yellowing, touch dry in about 20 to 30 minutes and recoatable in about two hours, with water clean-up and lower odour. Oil-based enamel is listed as usually touch dry in 6 to 8 hours, recoatable in about 16 hours, cleaned up with mineral turpentine, and prone to yellowing.

Why has the white paint on my doors gone yellow?

If your white doors and skirtings have turned cream or yellow, especially inside cupboards, behind furniture or on door jambs that don't get much light, that's a sign of oil-based enamel. Dulux's technical advice says all alkyd enamels can be expected to develop a yellow tone over time, and that it's most noticeable where natural light is poor.

Cleaning won't fix it, because the colour change is in the paint film itself. Dulux's recommended fix is to let the enamel harden, lightly sand off the gloss, and repaint with a non-yellowing water-based system.

How do you tell if existing paint is oil-based?

A common trade test uses methylated spirits. Put a little on a clean white rag and rub a hidden spot, like the top edge of a door, for a few seconds. Acrylic (water-based) paint usually softens and some colour comes off on the rag. Oil-based enamel usually doesn't soften at all.

It isn't a lab test, and a surface can have both types in its layers. If the result is unclear, treat the surface as oil-based and prepare it that way. It costs a little more time and avoids a failed finish.

• Very glossy, hard and slightly yellowed: probably oil-based.
• Softens or rubs off with metho: probably acrylic.
• Unsure, or several layers: prepare it as if it's oil-based.

What goes wrong if you paint straight over oil?

Water-based paint needs something to grip. A hard, glossy oil-based surface gives it very little, so the new coat can look fine for a while and then scratch off with a fingernail, chip where doors knock, or lift in strips when masking tape is pulled off. Once that happens the loose paint has to come off before the job can be done again.

The same thing happens on window sills and exterior trims, where sun and rain add stress. If you're already dealing with flaking paint, our guide on why paint peels and bubbles goes through the other common causes.

What's the right way to prepare old enamel?

The steps vary a little between products, so always follow the instructions for the exact paint going on. Dulux's Aquanamel product page, for example, says to lightly sand glossy surfaces to a dull finish, remove all the sanding dust, and apply its water-based prep primer where required, with two coats of enamel and a light sand between coats.

• Wash off grease and grime first. Kitchen doors and handle areas carry more than you'd think.
• Fill dents and cracks, and let the filler dry fully.
• Sand the whole surface until the shine is gone, not just the rough spots.
• Remove every bit of dust with a damp cloth or tack rag.
• Apply an adhesion primer or undercoat suited to glossy surfaces, as the paint maker specifies.
• Apply two thin coats of water-based enamel rather than one heavy coat, sanding lightly between them.

Is sanding old trims safe?

Not always. The Australian Government's Lead Alert guide says homes built before 1970 are most at risk of having lead paint, and it names architraves, skirting boards and sash windows among the surfaces most likely to be knocked, chipped or rubbed. Dry sanding or scraping lead paint creates fine dust that's harmful to breathe or swallow, and the guide says young children and pregnant women are especially at risk.

If your house is older and the trims have never been stripped, test before anyone sands, and never use a dry power sander on them. The Lead Alert guide sets out the safer ways to do it. We cover this in more detail in our guide to repainting old weatherboards.

Should I do trims myself or get a painter?

A single door is a reasonable weekend job if you're patient with the sanding and primer. A whole house of doors, frames and skirtings is a different scale: it's the most time-consuming part of most interior repaints, because every surface needs the same careful prep and two coats.

When we quote an interior repaint we look at what's on the trims now, because it changes the prep and the time involved. Your written quote spells out what's being done, so you can compare it properly with others.

Quick questions

Sources

What this guide relies on. Product advice changes, so check the maker's current instructions for the exact paint you're using.

1. Water-based vs oil-based enamel paints — Dulux Trade
2. Yellowing of enamels — Dulux Trade
3. Aquanamel Semi Gloss: preparation and application — Dulux
4. Lead Alert: the six step guide to painting your home (5th edition) — Commonwealth of Australia