Painting over tar paint (also called bituminous paint or Stockholm tar) on old New Zealand weatherboard houses is possible, but it's not as simple as slapping on a fresh coat. The key challenges are bleed-through, adhesion failure, and the risk of trapping moisture — all of which can cause your new paint to fail within months if the prep work isn't done properly.
What Is Tar Paint and Why Is It a Problem?
Tar-based paints were commonly used on NZ weatherboard homes from the early 1900s through to the mid-20th century, particularly on the back faces of boards, around joinery, and sometimes as a full exterior coating. They were valued for their waterproofing and preservative qualities.
The problem is that bituminous compounds bleed through most standard topcoats, causing brown or yellow staining that worsens over time — especially in warm weather. Standard acrylic or oil-based paints don't bond reliably to a tar surface without the right preparation and primer system.
Step 1 — Assess the Existing Tar Coating
Before you do anything, assess the condition of the tar paint:
- Firmly adhered and stable? If it's not flaking, peeling, or powdery, you may be able to paint over it with the right primer.
- Flaking, cracking, or delaminating? It must be removed or stabilised before any new coating is applied.
- Thick, uneven build-up? Multiple layers of old tar can cause adhesion problems and may need to be stripped back.
Do a simple cross-hatch adhesion test: score a small grid with a knife, press masking tape firmly, then pull it off. If the tar lifts, it's not stable enough to paint over.
Step 2 — Check for Lead Paint
Old NZ homes built before 1980 may also have lead-based paint in the existing coating system. If you're sanding, scraping, or water-blasting old paintwork, you could be exposing yourself and your family to lead dust. Before disturbing any old paint, use a lead test kit (available at hardware stores) or have a professional test it.
WorkSafe NZ has guidance on working safely with lead paint. If lead is present, additional precautions — or a licensed professional — are strongly recommended.
Step 3 — Surface Preparation
Good prep is everything. Here's what's typically required:
- Clean thoroughly — wash down with a sugar soap solution to remove dirt, mould, and chalking. A low-pressure wash works well; avoid high-pressure blasting directly into board joints.
- Sand or scrape any loose, flaking tar paint back to a firm edge.
- Fill any gaps or cracks in the weatherboards with a flexible exterior filler or paintable sealant.
- Prime bare timber with an appropriate timber primer before applying your bleed-blocking primer.
- Allow to dry fully — NZ's humid climate (especially in Auckland, Northland, and the West Coast) means moisture in the substrate is a real risk.
Step 4 — Choose the Right Bleed-Blocking Primer
This is the most critical step. A standard acrylic primer will not stop tar bleed-through. You need a shellac-based or oil-based bleed-blocking primer specifically designed for bituminous surfaces.
- Shellac-based primers (such as Zinsser BIN) are highly effective at sealing tar and preventing bleed-through. They dry quickly but require methylated spirits for clean-up.
- Oil-based primers can also provide a reasonable barrier, though they may be less effective on heavy tar build-up.
Always check the primer manufacturer's data sheet to confirm suitability for bituminous surfaces. Resene and Dulux both have technical advisors who can recommend the right primer for your specific situation — it's worth calling them before you buy.
Step 5 — Apply Your Topcoat System
Once your bleed-blocking primer is fully cured:
- Apply two topcoats of a quality exterior acrylic paint, allowing full drying time between coats.
- Choose a paint rated for NZ exterior conditions — UV resistance and flexibility are important for weatherboards that expand and contract with temperature changes.
- Wattyl also offers exterior systems suitable for weatherboard homes.
Follow the paint manufacturer's recommended recoat times — rushing this step is a common cause of failure.
What About Removing the Tar Paint Entirely?
In some cases — particularly where the tar coating is very thick, heavily degraded, or you want a guaranteed long-term result — full removal is the better option. This can be done by:
- Heat gun and scraper (slow but effective; avoid open flame near timber)
- Chemical strippers suitable for bituminous coatings
- Mechanical sanding (generates dust — use appropriate respiratory protection)
Full removal is labour-intensive and may be best left to a professional painter, especially on a two-storey home or if lead paint is also present.
When to Call a Professional
Consider hiring a qualified painter if:
- Lead paint is confirmed or suspected
- The tar coating is heavily degraded or very thick
- The weatherboards themselves are damaged, rotten, or need replacing
- You're working at height (scaffolding and fall risks are serious — WorkSafe NZ has clear guidelines)
- You want a guaranteed, long-lasting result
A professional painter will have access to commercial-grade bleed-blocking systems and the experience to assess your specific situation. You can find qualified local painters on tradedirectory.co.nz.
Note on building consent: Repainting your weatherboards is generally maintenance work and does not require a building consent. However, if you're replacing weatherboards or making structural changes, check with your local council. MBIE Building Performance has information on Schedule 1 exemptions.
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