Paint sheen and paint systems

What is a paint system?

A paint system is every layer from the bare surface up. On new or bare timber that means a primer that soaks in and grips, an undercoat that builds and evens the surface, then usually two finish coats that carry the colour and take the weather. Skipping a layer saves time on the day and costs years of life.

On a sound repaint, the old paint already does the job of primer and undercoat. Bare patches are spot-primed, then the whole wall gets two finish coats. That is why the condition of the old paint changes the price so much.

Why two finish coats instead of one?

One coat rarely closes the film evenly. It goes on thin over edges, corners and board ends, which are the places where paint fails first, and the old colour can ghost through. A second coat fills the gaps and builds the film to the thickness the paint was designed for, which is what gives the paint its expected life.

Which paint sheen should you use where?

Sheen runs from flat, through low sheen and semi-gloss, to gloss. Flatter finishes hide flaws and suit ceilings and broad walls. Shinier finishes are tougher, shed dirt and wipe clean, so they suit doors, trims, sills and wet rooms. Most houses use two or three sheens: one for walls, one for ceilings, one for joinery.

A higher sheen makes surface flaws more obvious. On an old villa wall with uneven boards, a low sheen will look better than gloss, even though gloss is tougher.

Acrylic or oil-based paint?

Most house paint today is water-based acrylic. It stays flexible as timber moves, resists cracking, and cleans up in water. Resene gives a quality acrylic system on weatherboards 7 to 10 years and an oil-based system 4 to 6. Oil-based enamels still appear on some trims and doors because they level to a hard, smooth finish.

BRANZ notes that older oil-based paints kept hardening and turned brittle with age, while modern formulations do not. To find out what is on the wall, BRANZ Bulletin 633 describes a test: rub a hidden spot with a cotton ball dipped in methylated spirits. If paint comes off on the cotton, it is water-based.

Painting acrylic over old enamel

It can be done, but not straight over the top. Glossy old enamel is cleaned, sanded to a dull finish, and coated with a primer-undercoat that suits both the old and new paint. Without that step, the new acrylic can peel off the old gloss in sheets. A written quote should say which primer will be used and why.

• Wash off grease and dirt, then rinse.
• Sand the gloss back so the surface is dull and scratched.
• Wipe or rinse off the sanding dust.
• Apply a compatible primer-undercoat.
• Lay two finish coats in the chosen sheen.

On a house from the 1980s or earlier, that sanding is done wet because the old enamel may contain lead.

Does the paint brand matter?

Less than the prep and the number of coats. Premium paint does cost more per litre, but labour is most of the price of a repaint, so the better paint adds only a little to the total. What matters more is that the system suits the surface and that the paint maker’s data sheet is followed for drying times and conditions.

For lower-odour paint inside, look for the Eco Choice Aotearoa label, which has its own standard for paints.

Ask the painter to name the range, the sheen and the number of coats for each surface in the written quote. If two quotes name different ranges, ask why; there is often a good reason, such as a paint suited to a particular cladding or a high-wear area, and the answer tells you how carefully the job has been thought through.

Asked before a repaint

What sheen should I use on exterior weatherboards?

Low sheen is the usual choice for weatherboard walls. It has enough sheen to shed dirt and rain but hides small dents and uneven boards better than gloss. Trims, sills and window frames are often done in semi-gloss or gloss, which wears harder and is easier to wipe clean.

Can you put acrylic paint over oil-based paint?

Yes, with the right preparation. The old oil-based paint is cleaned, sanded to dull the gloss, and coated with a suitable primer-undercoat so the acrylic can bond. BRANZ describes a simple solvent test to tell which type is on the wall before you start.

Do I need an undercoat?

On bare timber, filler, repairs or a strong colour change, yes. The undercoat builds an even base, seals the surface and helps the finish coats cover. On sound paint that is being repainted in a similar colour, two finish coats over a cleaned and lightly sanded surface are often enough.