Coal tar coatings have been protecting buried and submerged steel for over a century, and for good reason. They go on thick, water doesn’t get through them, they tolerate less-than-perfect conditions better than most high-performance coatings, and they’re cheap per mil.
They’ve also been pushed out of potable water service entirely (we cover that in Can You Use Bitumastic on Potable Water Pipe?). But for pipe and steel that’s buried, submerged, and never touches drinking water, Bitumastic is still one of the most practical coatings on the market.
The Two Bitumastic Products You’ll Actually See
Carboline’s Bitumastic line has several products, but two show up on most specs:
Bitumastic 50 — A single-component coal tar mastic. No mixing, self-priming, and it builds up to about 30 mils dry in one coat. It’s used for below-grade damp proofing and corrosion protection on steel and concrete: foundations, retaining walls, buried steel, and piling. Because it’s one-part, it’s the easy choice for smaller jobs and field touch-up.
Bitumastic 300M — A two-component coal tar epoxy polyamide. It builds 16 to 24 mils dry per coat, is self-priming, and holds up to chemicals, abrasion, and immersion far better than a single-component mastic. It’s compatible with cathodic protection, and its data sheet references SSPC-Paint 16, Corps of Engineers C-200/C-200a, and AWWA C210 for exterior service.
Rule of thumb: Bitumastic 50 for damp proofing and lighter-duty buried steel; Bitumastic 300M for pipe, immersion service, and anything with cathodic protection or aggressive soil.
Where Bitumastic Belongs on a Pipe Job
Exterior of buried steel pipe. Coal tar epoxy has been a standard buried-pipe exterior coating for decades. It’s thick, resists soil moisture and groundwater chemicals, and works with the cathodic protection systems commonly installed on steel pipelines.
Sanitary sewer and storm pipe. Steel force mains, storm culverts, and outfall pipe see wet, dirty, often corrosive service. Coal tar epoxy handles it where the pipe won’t carry drinking water.
Manholes, wet wells, and lift stations. Concrete and steel in sewer structures get hit with hydrogen sulfide and moisture. Coal tar epoxy has a long track record in this service, though many municipalities now spec newer non-coal-tar epoxies for H2S-heavy environments. Check the owner’s standard.
Non-potable process and wastewater lines. Cooling water, fire suppression lines that aren’t tied to potable supply, industrial wastewater, and treatment plant piping downstream of any drinking water use.
Piling, sheet piling, and submerged steel. Seawalls, dock piling, bridge piers, and dam gates — anywhere steel is buried or underwater full time.
Below-grade structural steel and foundations. Column bases, grade beams, and steel encased in backfill.
Where It Doesn’t Belong
- Inside any potable water pipe, tank, or fitting. Not NSF 61. Full stop.
- Anywhere in sunlight. Coal tar breaks down under UV. It chalks, cracks, and “alligators” when exposed. A pipe that runs above grade even for a short span needs a different coating on that section.
- Where you’ll need to topcoat later. Coal tar bleeds through most topcoats. If a section might need a finish color or be painted over down the road, talk to the coating manufacturer before using coal tar there.
- Where the owner’s spec prohibits it. Some owners and agencies have phased coal tar out entirely because of worker exposure concerns. Their spec rules.
Prepping New Pipe for Bitumastic
“It’s new pipe, it’s clean” is the most expensive assumption on a pipe coating job. New steel pipe arrives with mill scale, oil from manufacturing, and often surface rust from sitting in a yard. Coal tar epoxy is forgiving, but it’s not magic. Put it over mill scale and the mill scale lets go, taking the coating with it.
For Bitumastic 300M, Carboline’s data sheet calls for:
- Solvent cleaning per SSPC-SP 1 to remove oil and grease first. Blasting over oil just spreads it around.
- SSPC-SP 10 near-white metal blast for steel in immersion or buried service.
- A 2.0 to 3.0 mil angular anchor profile.
Bitumastic 50 is more tolerant for non-immersion damp proofing, but on buried pipe you still want mill scale and rust removed by blasting. We go deeper into the 300M prep requirements in Surface Prep for Coal Tar Epoxy.
Field Joints, Fittings, and Repairs
Most new pipe gets coated in a shop, but welds, field joints, fittings, and damage from handling all get coated in the field. That’s where most buried-pipe coating failures start:
- Field joints need the same prep as the rest of the pipe — blast the weld and the cutback on each side, feather the shop coating edges, and coat within the manufacturer’s overlap requirements.
- Fittings and specials often arrive uncoated or with just a shop primer. They need a full blast.
- Handling damage from lifting slings and bedding rock needs to be found and repaired before backfill. A holiday test (spark test) of the full coating before the pipe goes in the ground is the only way to find pinholes you can’t see.
Once pipe is backfilled, every missed holiday becomes a point where corrosion concentrates, and cathodic protection current drains out at that one spot.
Cost Makes Sense When It’s Done Once
Coal tar coatings are cheap to buy. The blasting, the field labor, the excavation, and the backfill are where the money goes. If the coating fails early on buried pipe, you’re paying for all of it again — plus the excavation to get back to it.
Blast it right, coat it to spec, spark test it, and a coal tar system on buried non-potable pipe can go decades without anyone needing to think about it.
Blasting Jack blasts new and existing pipe, fittings, piling, and below-grade steel throughout Michigan — in the yard, at the shop, or in the trench. Contact us for surface prep ahead of Bitumastic or any buried-service coating.