Can I Paint Over Bitumen or Tar-Based Coatings on Concrete?

Painting over bitumen or tar-based coatings on concrete is one of the trickier DIY painting jobs you'll encounter. The short answer is: yes, it can be done, but only with the right preparation and compatible products — skip either and you'll likely end up with a peeling, bleeding mess within months.

Why Bitumen Is Such a Problem for Paint

Bitumen (also called tar, pitch, or bituminous paint) is an oil-based, petroleum-derived coating commonly used on concrete floors, retaining walls, water tanks, and foundations for waterproofing. The trouble is that bitumen bleeds. The dark, oily compounds in bitumen migrate upward through almost any standard paint film, causing brown or black staining — even years after application. This is called bleed-through, and it's the number-one reason painting over bitumen fails.

Bitumen also remains slightly flexible and never fully cures in the traditional sense, which means rigid topcoats can crack and delaminate over time.

Step 1 — Assess the Existing Coating

Before you do anything, figure out what you're dealing with:

• Is the bitumen still well-bonded? Press a fingernail into it. If it's soft, tacky, or flaking, it needs to be removed or stabilised before painting.
• Is it thick or thin? Heavy, built-up bitumen (common on old concrete floors or tanked basements) is much harder to paint over than a thin brush-applied coat.
• Is it wet or damp underneath? Bitumen is often used as a damp-proof membrane. If moisture is still pushing through from behind, no paint system will last — fix the moisture source first.

Step 2 — Preparation Is Everything

Good prep is non-negotiable:

1. Remove loose or flaking bitumen using a scraper, wire brush, or angle grinder with a suitable disc. Wear a P2 respirator, safety glasses, and gloves — old bitumen products may contain polycyclic aromatic hydrocarbons (PAHs). Check WorkSafe NZ guidance on working with bituminous materials.
2. Clean the surface thoroughly. Remove grease, dirt, and any efflorescence with a concrete cleaner or diluted hydrochloric acid wash (follow manufacturer safety instructions carefully).
3. Allow to dry completely. Concrete must be dry before painting — typically at least 28 days for new concrete, or test with a plastic sheet taped down overnight.
4. Stabilise any remaining loose areas with a suitable penetrating primer if the bitumen is powdery or friable.

Step 3 — Choose a Bleed-Blocking Primer

This is the critical step. You must use a primer specifically formulated to block bitumen bleed-through. Standard acrylic or water-based primers will not stop it.

Look for:

• Shellac-based primers — highly effective bleed blockers, though they have strong solvent fumes (use in well-ventilated conditions)
• Solvent-based stain-blocking primers — products designed to isolate bitumen and tar stains
• Epoxy primers — two-pack epoxy systems can encapsulate bitumen effectively and are a popular choice for concrete floors

Always read the product data sheet and confirm the primer is rated for use over bitumen. Ask at your local Resene or Dulux trade store — their technical staff can recommend the right system for your specific situation. Wattyl also has technical advisors who can help.

Step 4 — Apply Compatible Topcoats

Once your bleed-blocking primer has fully cured (check the product data sheet for recoat times), you can apply your chosen topcoat:

• Concrete floors: A two-pack epoxy or polyurethane floor paint will give the most durable result.
• Walls and foundations: A quality exterior masonry paint or elastomeric coating works well.
• Water-retaining structures: Check that your chosen system is rated for potable water contact if applicable.

Apply at least two topcoats for adequate coverage and durability, following the manufacturer's recommended spread rates.

What About Removing the Bitumen Entirely?

In some cases — particularly where the bitumen is thick, heavily degraded, or you want a guaranteed result — full removal is the better option. Methods include:

• Mechanical grinding or scarifying (angle grinder, floor grinder)
• Chemical strippers formulated for bitumen removal
• Shot blasting for large floor areas (usually a hired-in service)

Full removal is more work and cost upfront, but gives you a clean concrete substrate and the widest choice of paint systems.

NZ Climate Considerations

New Zealand's variable climate — particularly in wetter regions like the West Coast, Northland, and Wellington — means moisture management is critical. BRANZ research consistently highlights that moisture behind coatings is a leading cause of paint failure on concrete. If your bitumen coating is on a below-ground or partially buried surface, consider whether the underlying waterproofing is still fit for purpose before painting over it.

When to Call a Professional

Consider hiring a qualified painter or concrete coating specialist if:

• The bitumen is on a large floor area or complex surface
• You're unsure whether moisture is coming through from behind
• The existing coating is heavily built up or in poor condition
• You need a result that will last — professional applicators have access to commercial-grade systems and the experience to apply them correctly
• The surface is part of a water tank or structure where coating failure could cause real problems

A professional painter will also know which products are currently available in NZ and can provide a guarantee on their work. You can find qualified local painters on tradedirectory.co.nz.

Sources & Further Reading

• Resene — Technical advice and paint systems
• Dulux — Concrete and floor coatings
• BRANZ — Building research and moisture guidance
• WorkSafe NZ — Hazardous substances and safe work practices
• MBIE Building Performance — NZ Building Code guidance
• Wattyl — Protective and decorative coatings