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What Undercoat Should You Use on Each Substrate?

A good finish starts long before the topcoat goes on.

Choose the right undercoat and the job covers better, sands easier and holds up longer. Choose the wrong one and you can end up fighting poor adhesion, flashing, stains bleeding through, patchy sheen or peeling down the track.

The tricky part is that “undercoat” is not one product for every surface. Different substrates need different prep and different primers.

Here’s a simple guide to what undercoat to use on what substrate.

First, what is the job of the undercoat?

Before you pick a product, work out what you need the undercoat to do.

Sometimes you just need to seal a porous surface.
Sometimes you need adhesion over a slick or glossy one.
Sometimes you need stain blocking.
Sometimes you need to lock down a tired, chalky or peeling surface.

Once you know the problem, it gets much easier to choose the right product.

New plasterboard

For new plasterboard, a sealer undercoat is usually your starting point.

Its job is to seal the paper face, even out suction between the board and stopping, and help reduce flashing through the finish coats. If the surface has texture differences between the paper and jointing compound, or you want extra fill and smoothness, a sanding sealer style product is often the safest pick.

Best fit: Acrylic sealer undercoat, especially one designed for plasterboard.
Watch for: Thin spots, heavy suction and patchiness around stopped areas.

The R&S Option – https://rstradecentre.co.nz/shop/paint-plus/undercoats-paint-plus/wallboard/paintr-easy-sand-sealer-10l/

Bare timber

Bare timber usually needs an undercoat that seals the surface and helps control tannin bleed, especially on resinous timbers.

Interior trim, doors and woodwork need a stable base so the topcoat lays off evenly. Exterior timber may need a more robust primer depending on exposure and timber type.

Best fit: Wood primer or stain-blocking primer where bleed is likely.
Watch for: Knots, tannins, sap bleed and inconsistent absorption.

The R&S Option – https://rstradecentre.co.nz/shop/paint-plus/undercoats-paint-plus/wet-area-undercoats-paint-plus/paint-plus-quick-prep-acrylic-undercoat/

Glossy enamel, laminate, melamine and similar slick surfaces

These are the surfaces that often catch painters out.

A standard sealer undercoat may not have enough bite for laminate, melamine, old enamel, tile surrounds or other hard, slick substrates. This is where a true bonding primer matters.

Best fit: High-adhesion bonding primer.
Watch for: Assuming a general-purpose wall primer will do the job.

The R&S Option – https://rstradecentre.co.nz/shop/paint-plus/undercoats-paint-plus/wallboard/zinsser-cover-stain/

Galvanised steel and other metal surfaces

Metal needs more than just coverage, it needs grip.

Galv, aluminium and other metals can reject the wrong coating system fast. Always make sure the surface is clean, sound and free from oils or oxidation before priming.

Best fit: Metal-suitable bonding or anti-corrosive primer, depending on the substrate and exposure.
Watch for: New galv, weathered galv, rust and contamination.

The R&S Option – https://rstradecentre.co.nz/shop/paint-plus/undercoats-paint-plus/galv/paint-plus-galv-coat/

Masonry, concrete, block and brick

Masonry can be porous, dusty and alkaline, so sealing is often the main priority.

Some jobs only need a good masonry sealer. Others need stain blocking, better adhesion over previously painted surfaces, or extra help on powdery areas.

Best fit: Masonry-suitable sealer or bonding primer, depending on the condition of the surface.
Watch for: Dusting off, alkalinity, moisture and patchy suction.

The R&S Option – https://rstradecentre.co.nz/shop/paint-plus/undercoats-paint-plus/masonry/paint-plus-masonry-sealer/

Wet areas

Bathrooms, laundries and similar areas usually need more than a basic wall undercoat.

You want good adhesion, moisture resistance and, where needed, stain blocking. If the surface is new plasterboard, a sealer undercoat may still be the right first step. If the area has been previously painted, slick, stained or hard to bond to, step up to a better-performing primer.

Best fit: Moisture-tolerant bonding primer or stain-blocking primer, matched to the substrate.
Watch for: Steam, mould history, water staining and soap residue.

The R&S Option – https://rstradecentre.co.nz/shop/paint-plus/undercoats-paint-plus/wallboard/zinsser-cover-stain/

Stains, smoke, water marks and bleed-through

This is where many standard water-based primers fall short.

If you are dealing with nicotine, smoke, water marks, marker, crayon, tannin bleed or other stubborn stains, use a stain-blocking primer rather than hoping the topcoat will hide it.

Best fit: Stain-blocking primer sealer.
Watch for: Water damage, smoke damage, tannin bleed and repeat staining.

The R&S Option – https://rstradecentre.co.nz/shop/paint-plus/undercoats-paint-plus/wallboard/zinsser-b-i-n-primer-sealer/

The simple rule

If the surface is porous, think sealing.
If the surface is slick, think adhesion.
If the surface is stained, think blocking.
If the surface is failing, think binding.

That quick check will usually point you in the right direction.

There is no one undercoat for every job.

The best result comes from matching the product to the substrate and the problem in front of you. A plasterboard sealer, a stain blocker, a bonding primer and a binding primer all do different jobs, and choosing the right one up front saves time, extra coats and call-backs later.

Not sure what’s under the old coating, or dealing with a tricky surface like glossy trim, MDF, stained ceilings or weathered exterior cladding? Talk to the team before you start. Getting the undercoat right is one of the easiest ways to make the topcoat look better and last longer.