Can you put water-based paint over oil-based on exterior timber?

Yes. Water-based paint goes over oil-based paint on exterior timber all the time, provided the old oil coat is sound, clean and de-glossed, and you use a primer or topcoat made to bond to it. What fails is water-based paint rolled straight onto glossy, chalky or brittle enamel with no preparation, and that failure usually shows up as peeling in sheets.

The reverse is the one to be wary of. Resene's own list of paint failures names solventborne paint applied over soft waterborne paint as the usual cause of crocodile-pattern cracking. Water over oil, done properly, is the normal direction of travel on older houses.

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

Use the methylated spirits test. Wet a clean white rag with meths and rub a small, out-of-the-way patch firmly. If the paint softens or starts to dissolve, it is acrylic (water-based). If nothing happens, it is probably an enamel or oil-based coating.

Do it on each part of the house you plan to paint, not just one wall. It is common on older houses to find acrylic on the weatherboards and old enamel still on the windows, doors, fascias and barge boards, because joinery was commonly done in gloss enamel.

A few other clues back up the test:

• Old enamel tends to be glossier than acrylic of the same age, and feels hard and slick rather than slightly rubbery.
• Where it has failed, old oil paint tends to crack and flake in brittle chips, while acrylic tends to peel away as a flexible skin.

Why does water-based paint sometimes peel off oil-based paint?

Adhesion is mechanical as much as chemical. A new coat needs something to grip. Oil enamel cures to a hard, smooth, non-porous film, and a water-based coat applied over it without preparation is sitting on a surface with very little tooth. It can look fine for months, then lift when the timber moves through a wet winter and a hot summer.

The other common causes are what is sitting on top of the old paint rather than the paint itself:

• Chalk. As exterior paint weathers, the top layer breaks down into a loose powder. New paint sticks to the chalk, and the chalk does not stick to anything.
• Mould and dirt. Resene's exterior timber guide is blunt that loosely bound contaminants or mould must be removed or the new paint will not bond to the old surface.
• Brittleness. The same guide notes that old enamel which has not been exposed to the sun, such as the inside faces of window frames, gets harder and more brittle over time and needs thorough sanding before painting.

If the old oil coat is already crazed into a network of cracks or curling away from the timber, no primer will save it. That paint has to come off, because a new coat will only ever be as good as the layer it is stuck to.

What preparation makes it work?

1. Kill mould first. A moss and mould treatment before washing, particularly on the south side.
2. Wash and de-chalk. Scrub with a proper house wash product, the way you would wash a car, and rinse thoroughly. Rub a dark cloth on the dry surface afterwards. If it still comes away white, wash again.
3. Scrape and sand anything loose. Resene suggests scrubbing flaking areas and sanding with 120 to 150 grit paper, feathering the edges so the repair does not show through.
4. De-gloss the rest. A scuff sand over sound enamel gives the new coat a key. On weatherboards in good condition this is a light job; on joinery it is the step that matters most.
5. Spot prime bare timber the same day. Resene warns that overnight dew creeps under freshly sanded edges and makes them curl and lift, so plan the work so sanded areas are primed before evening.
6. Topcoat with a system that states it suits old enamel. Check the data sheet before you buy.

Do you need an oil-based primer or undercoat first?

There are two accepted routes, and which one applies depends on the product you are putting on top.

Route	How it works	Trade-off
Oil-based undercoat as a bridge	Sand, apply a solventborne undercoat, then acrylic topcoats	Grips old enamel well; slower drying, solvent clean-up, and one more product on the job
Waterborne topcoat with built-in adhesion	Wash, de-chalk, scuff sand, spot prime bare areas, then topcoat directly	Faster and simpler; only suitable where the manufacturer states the product bonds to aged enamel
Strip back to timber	Remove the old system and start again with primer	The only fix for cracked or badly adhered paint; slow, and lead precautions apply

On the second route, Resene states that several of its waterborne exterior topcoats are formulated with adhesion promoters so they adhere to aged paints, including old enamels, without extensive sanding of otherwise sound paint. That is a specific product claim, so read the data sheet for whatever you are using rather than assuming every acrylic behaves the same way. Where the data sheet does not say it suits old enamel, use a primer or undercoat that does.

There is more on how the two paint types behave in our guide to the differences between oil-based and water-based paints.

Where is it most likely to go wrong?

Almost always on joinery rather than weatherboards. Window sills, sashes and door frames take the most sun and water, carry the thickest build of old enamel, and get handled. Those are the places to spend the sanding time.

The other trap is patchy preparation. A wall that was washed and sanded in one section and just washed in another will fail in one section. The result is one elevation peeling while the rest holds, and the difference is nothing more than the day it was done.

Can you do this yourself?

On a single-storey house with sound paint, yes. It is mostly washing and sanding, and the products are sold at any paint shop with advice on the counter. Take a sample chip or the meths test result in with you.

Two things change that. Homes painted before the 1980s may contain lead-based paint, and old enamel on joinery is exactly where it tends to be. Have it tested before sanding or stripping, and if it is positive, do not dry sand. And if the meths test shows oil paint that is cracked or lifting over large areas, you are looking at a strip-back rather than a repaint, which is a different scale of job. Our exterior painting quotes spell out which parts of the house need which level of preparation.

If you are not sure what is on your house or whether it will take a new coat, get in touch and we will test it and tell you straight.