Walk down the paint prep aisle at your local hardware store and you'll find shelves full of primers, sealers and undercoats — often with overlapping claims on the tin. Understanding what each product actually does will save you time, money and a frustrating repaint job down the track.
What Is a Primer?
A primer is the first coat applied to a bare or previously unpainted surface. Its main job is to bond to the substrate — whether that's bare timber, plasterboard, concrete, or fibre cement — and create a stable foundation for the coats that follow.
Primers are formulated to penetrate the surface and grip it firmly. Without a primer on a porous or bare surface, your topcoat can peel, flake or absorb unevenly. Different substrates need different primers:
- Bare timber – use a timber primer to seal the grain and prevent resin bleed
- New plasterboard (GIB®) – a PVA-based primer sealer is typically recommended; GIB publishes specific finishing systems for their boards
- Fibre cement (e.g. James Hardie) – James Hardie specifies approved primers in their technical documentation; always follow their system
- Concrete or masonry – a masonry primer or sealer is needed to deal with alkalinity and porosity
What Is a Sealer?
A sealer is designed to lock down a surface — stopping it from absorbing too much of the next coat, or preventing stains, tannins, and other contaminants from bleeding through. Think of it as a barrier coat.
Common situations where a sealer is the right choice:
- Stain blocking – nicotine, water stains, or timber tannins can bleed through standard paint. A shellac-based or oil-based stain-blocking sealer locks these in.
- Highly porous surfaces – raw concrete, old plaster, or weathered timber can drink up paint. A sealer evens out the porosity so your topcoat goes on smoothly and consistently.
- New plasterboard – a PVA sealer prevents the paper face from raising and ensures even sheen in the finished coat.
Some products are marketed as primer sealers, combining both functions. These are handy for general prep work but may not be the best choice when you have a specific problem (like heavy tannin bleed) that needs a dedicated product.
What Is an Undercoat?
An undercoat (sometimes called an intermediate coat) goes on after the primer or sealer and before the topcoat. Its job is to build film thickness, improve opacity, and give the topcoat the best possible surface to bond to.
Undercoats are typically:
- Pigmented – they contain more pigment than a primer, helping to hide the substrate colour and reduce the number of topcoats needed
- Sandable – once dry, a good undercoat can be lightly sanded to a smooth finish before the topcoat goes on
- Topcoat-compatible – always check that your undercoat is recommended for use with your chosen topcoat (water-based with water-based, oil-based with oil-based, unless the manufacturer states otherwise)
For interior trim and doors, a dedicated timber undercoat is still widely used in NZ, particularly under oil-based or alkyd topcoats. For walls, a tinted primer or primer sealer often does the job of an undercoat in one step.
Comparison: Primer vs Sealer vs Undercoat
| Product | Primary Purpose | Typical Use |
|---|---|---|
| Primer | Adhesion to bare substrate | New timber, plasterboard, concrete |
| Sealer | Block stains / control porosity | Tannin bleed, porous surfaces, new GIB® |
| Undercoat | Build film, improve opacity | Before topcoat on trim, walls, joinery |
| Primer Sealer | Adhesion + porosity control | General prep on new or sound surfaces |
NZ-Specific Considerations
New Zealand's climate varies enormously — from the humid, salty air of coastal areas to the dry heat of Central Otago and the damp winters of the South Island. Choosing the right prep system matters:
- In coastal or high-humidity areas, moisture-resistant primers and topcoats are important. BRANZ provides research and guidance on paint systems suited to NZ conditions.
- Wet areas (bathrooms, laundries) must comply with NZ Building Code clause E3. Use products specifically rated for wet areas, and ensure waterproof membranes are in place before tiling or painting.
- Many NZ homes have bare or repainted weatherboards — check whether you're dealing with oil-based existing paint before applying a water-based primer over the top, as adhesion can be poor without proper preparation.
Resene and Dulux both publish detailed paint system guides on their websites, including which primer or sealer to use for specific substrates. Wattyl also offers technical data sheets for their prep products. Always read the technical data sheet (TDS) for the specific product you're using.
Tips for Getting the Best Result
- Never skip prep – the topcoat is only as good as what's underneath it
- Tint your primer – ask your paint store to tint the primer close to your topcoat colour; this can reduce the number of topcoats needed
- Allow proper dry times – rushing between coats is one of the most common causes of paint failure
- Sand between coats on trim and joinery for a smooth, professional finish
- Check compatibility – mixing water-based and oil-based products in the same system can cause adhesion failure
When to Call a Professional
For most interior painting prep, a confident DIYer can handle primer, sealer and undercoat application. However, consider hiring a qualified painter when:
- You're dealing with lead paint in older homes (pre-1980s). Sanding or disturbing lead paint is a health hazard — check WorkSafe NZ guidance before proceeding.
- The surface has significant damage, rot, or moisture issues that need assessment
- You're painting exterior weatherboards or cladding on a multi-storey home (working at height safely requires the right equipment and experience)
- You want a high-quality finish on feature walls, joinery or cabinetry
A qualified painter will know exactly which prep system suits your substrate and conditions. You can find licensed, reviewed painters in your area at tradedirectory.co.nz.