Bitumen Varnish - Technical Guide for Coal Tar

Overview

Coal tar epoxy is the toughest, most chemically resistant heavy-duty coating family there is. It was originally developed to line the inside of potable water tanks and burial tanks, where nothing else would survive, and it earned its reputation in oil and gas pipelines, sewage treatment works and marine environments.

The chemistry is a polyamine-cured epoxy heavily loaded with the heavy aromatic fractions of coal tar pitch. The tar component is what gives the coating its extraordinary resistance to crude oil, fuels, bitumen, acids and alkalis — and its characteristic black colour.

Modern practice has moved towards the higher-performance epoxy coal tar grades, including solvent-free and high-build formulations, and towards the non-coal-tar alternatives such as epoxy novolac and vinyl ester where specification or hygiene rules forbid the tar. Delshine supplies both the traditional and the modern families.

Product identification

The identifiers below are the ones printed on the drum and quoted on the purchase order. Always order and dispatch by code, because the grade and the zinc or temperature variant cannot be inferred from the trade name alone.

Where this product is used

• Internal lining of potable and raw water storage tanks
• Buried pipelines and pipeline external coating
• Oil and fuel tank interiors, including crude oil tanks
• Sewage treatment tanks, digesters and clarifier walls
• Marine hulls, ballast tanks and shipboard bilge areas
• Tunnel, culvert and underground structure protection
• Chemical bunds and secondary containment
• Refinery and petrochemical tank farm interiors

How it works in the coating system

The topcoat is the visible, exposed layer of the system. It provides the colour, the gloss and the final resistance to weathering, UV, chemicals and mechanical wear — the properties the specifier and the owner actually see and rely on.

Because it is the only coat fully exposed to the service environment, the choice of topcoat chemistry is the single decision that most affects how a coating system ages. A system that is under-specified at the topcoat will look good on day one and disappoint in five years.

Purpose in the system: The final coat of the system, applied over the intermediate coat.

Technical properties

The values below are the typical published range for this product family. The exact figures for your batch are confirmed on the batch certificate and the current technical data sheet.

Surface preparation

1. Abrasive blast steel to Sa 2½ (ISO 8501-1) as the default standard. Coal tar epoxy has no tolerance for rust or scale.
2. Test the surface for soluble salts and wash with fresh water where contamination is suspected — coal tar systems are particularly prone to blistering over residual salt.
3. Remove oil, grease and wax completely. Weld spatter must be removed and the area abraded.
4. Round edges to a minimum 2 mm radius and grind all welds smooth.
5. Control ambient conditions strictly — substrate at least 3 °C above dew point, air above 10 °C, humidity below 75%.
6. Plan the coating sequence carefully: coal tar epoxy often requires a tie coat between incompatible coats, and the recoat window must be respected.

Application procedure

Apply by spray for the best appearance and film uniformity, by roller for large flat areas, or by brush where access is restricted.

Aim for a consistent wet film across the whole area. Overlapping passes should be kept wet to avoid a lap line in the finish.

Plan the work so that the coating is applied in manufacturable quantities — a full kit should be mixed and used within the pot life, and any surplus should be discarded rather than saved.

Mixing, ratio and induction time

Two-pack finishes are mixed at the ratio stated on the data sheet, commonly 4:1 or 5:1 by volume for epoxy and 3:1 for polyaspartic and acrylic polyurethane. Use a proper mixing gauge — a mix that is even a few percent off ratio will cure differently and may not meet the published properties.

Cure, recoat window and service readiness

Surface dry in 2 to 6 hours, recoatable in 12 to 24 hours, and fully cured in 7 days. Do not place the structure into service, immersion or heavy loading before full cure, even though the surface will feel dry much earlier.

Coverage and thickness calculation

Typical dry film thickness is 60-100 µm for a conventional finish and 40-60 µm for a high-build polyurethane finish. At 80 µm, one litre covers roughly 8-10 m². Two coats are usually specified where colour, gloss uniformity or coverage hiding matter.

Technical note. Coverage (m²/litre) = (volume solids × 10) / DFT (µm)

Worked example

Advantages

• Exceptional resistance to crude oil, petroleum products, bitumen and fuel — no other coating family handles these materials as well.
• Very high chemical resistance to acids, alkalis and inorganic salts.
• Excellent abrasion and impact resistance, making it ideal for buried and submerged service.
• Long track record in potable water tank lining, with certified grades available.
• Solvent-free formulations available for low-VOC and confined space work.
• Strong adhesion and film build at relatively low film thickness compared with many barrier coatings.

Limitations and precautions

• The black colour is permanent — this is not a decorative coating and cannot be overcoated in a light colour.
• Coal tar pitch is a suspected carcinogen. Modern safety legislation in many jurisdictions restricts or prohibits its use, and the material must be handled with full respiratory protection.
• It cannot be overcoated with many other coating types because of solvent incompatibility — a tie coat is usually required.
• The epoxy component can blush in humid conditions, which will destroy intercoat adhesion if not addressed.
• UV resistance is poor — external service needs an aliphatic topcoat or a suitable alternative coating system.

Storage and shelf life

• Store component A and B in their original sealed containers in a cool, dry, ventilated store between 5 °C and 30 °C.
• Keep the coal tar component away from heat and naked flames — the pitch is a combustible product.
• Do not allow the containers to freeze and keep them tightly closed.
• Rotate stock first-in-first-out and observe the labelled shelf life.

Safety, PPE and ventilation

• Coal tar pitch may contain polycyclic aromatic hydrocarbons. Use supplied-air respiratory protection and follow the full respiratory protection programme for the product.
• Wear chemical-resistant gloves, goggles and coveralls and wash thoroughly after handling.
• Keep all ignition sources away from the storage and mixing area.
• Ensure maximum ventilation when mixing and applying, and use closed mixing where possible.
• Dispose of residues through a licensed hazardous waste contractor.

Quality control and inspection

• Check colour and gloss against the approved sample or RAL reference before starting the full area.
• Measure DFT and check for thin spots, particularly over welds, edges and connections.
• Inspect the finished surface for lap marks, sag, orange peel, dust inclusion and solvent entrapment before accepting the coat.

Pricing and how to order

Delshine quotes this product per square metre at a specified dry film thickness, or per drum/litre depending on the project. The material cost is usually the smallest part of the installed cost — surface preparation, access and application labour dominate. Send us the substrate, the service conditions and the required DFT and our technical team will return a full system recommendation with a firm price.

Coal tar epoxy is priced per square metre at a specified dry film thickness, normally with a premium for the solvent-free grades. Because it is often used in confined spaces and on buried infrastructure, the ventilation and confined space controls are a significant part of the installed cost and should be included in any comparison.

Frequently asked questions

The questions below are the ones our technical team is asked most often about this product family.

Related products

These products are normally used in the same system as the product described above.

• High-Build Epoxy Coal Tar Pitch Primer
• High-Build Epoxy Coal Tar Pitch Finish
• Solvent-Free Epoxy Coal Tar Pitch Paint
• Chlorinated Rubber Asphalt Paint

See all products