Coal Tar or Asphalt Emulsion Sealer: Which Should an Oshkosh Driveway Get?
September 9, 2026
The two sealers go down the same way and cure by the same rules, but they differ sharply in what they leave behind in the water, and the map of where coal tar is banned keeps growing.
What the two sealers are
Both products are water-based emulsions applied by squeegee or spray in two coats over clean asphalt. The difference is the binder. A coal tar sealer is built on refined coal tar pitch, a byproduct of coking coal. An asphalt emulsion sealer is built on asphalt, the same petroleum binder that holds the driveway together, stabilized in water with clay or mineral colloids. Wisconsin's 2019 Assembly Bill 797 defined a coal tar sealant product as one containing coal tar, coal tar pitch or coal tar pitch volatiles, identified by Chemical Abstracts Service numbers 65996-93-2, 65996-89-6 or 28007-45-2, which is the cleanest way to read a label.
On the driveway, the application rules are identical: air at 50 degrees F and rising for 24 hours, dry pavement, sun and airflow, cracks filled first. This site does not promote one brand or formulation. The question is what the two chemistries do after they wear off, because a sealcoat is a wear layer and every bit of it eventually leaves as dust.
The PAH problem
Coal tar carries high concentrations of polycyclic aromatic hydrocarbons, or PAHs. A U.S. Geological Survey study released in December 2016 sampled stream sediment around Milwaukee and found that dust from coal-tar sealant contributed about 42 to 94 percent of the PAHs in the samples, and that 78 percent of the samples had PAH levels that could harm aquatic life such as aquatic insects. The pathways USGS named are stormwater runoff, wind and tires. Potential effects on fish and other aquatic life include fin erosion, liver abnormalities, cataracts and immune system impairment.
The Minnesota Pollution Control Agency reached a similar result: a sediment study found coal-tar sealants contributed 67 percent of the total PAHs in 15 metro-area stormwater ponds, and the agency states that asphalt-based sealants have far lower PAH concentrations. Oshkosh sits between Lake Butte des Morts, the Fox River and Lake Winnebago, and a driveway's runoff has a short trip to open water.
Where coal tar is banned, and where it is not
The list below includes only rules that could be verified as of this writing.
- Minnesota: the Legislature banned the sale and use of coal tar-based sealants in 2013, effective January 1, 2014.
- Washington: a statewide ban, and the District of Columbia: a ban or ordinance, as listed in the Minnesota Pollution Control Agency's July 2016 inventory of restrictions.
- Dane County, Wisconsin: the first community in the state to halt coal tar driveway sealants, roughly a decade before 2017.
- Milwaukee: the Common Council banned the sale and use of high-PAH coal-tar sealants in February 2017.
- Statewide Wisconsin: AB 797, introduced January 24, 2020, would have banned sales from January 1, 2021 and use from July 1, 2021, and set a high-PAH threshold of 0.1 percent by weight. It was referred to committee and shows no further action; no statewide ban turned up in this research.
- Oshkosh and Winnebago County: no local ordinance restricting coal tar sealants turned up in this research. That is not a permission slip, only a note that the choice is currently yours.
Performance: what actually differs on the driveway
The honest performance answer is that both chemistries fail for the same reasons and succeed with the same prep. A manufacturer application specification for an asphalt-based sealer requires cracks of roughly 1/4 to 3/4 inch to be cleaned and filled with crack sealant first, oil and grease spots scrubbed until water no longer beads on them, two coats on a low-traffic surface, and the second coat only after the first is dry, preferably 24 hours later. That is the same list a coal tar crew works from.
What kills a sealcoat in Oshkosh is not the binder choice. It is water in the base through the roughly 86 freeze-thaw days a year, the lakebed clay that holds it there, and the county's salt, 8.9 tons per lane mile in 2024-25. Neither product is a repair. Both are a two to three year wear layer over a driveway whose cracks have been filled and whose failed sections have been patched.
The decision for an Oshkosh driveway
For a residential driveway, asphalt emulsion is the default answer. It matches the direction Wisconsin's largest communities have already taken, it keeps PAH dust off the lawn and out of the gutter, and it gives up nothing in the maintenance schedule. Driveway sealcoating on this site is scoped the same way regardless of product: patch, crack fill, clean, two coats, a full day of cure.
For a commercial lot the same logic applies with a larger footprint. A parking lot sealcoating job sheds its wear layer onto sidewalks, storm inlets and tenants' floor mats, so the property manager owns the runoff question. Whatever the product, the price is driven mostly by prep, edge work, access and scheduling; the cost guide walks through those drivers.
Questions to ask before you sign
A few direct questions settle the matter faster than a sales pitch. Put them to an Oshkosh sealcoating contractor before the crew is scheduled, and expect plain answers.
- Is the sealer asphalt-based or coal tar-based, and can you send the product data sheet?
- How many coats, and how long between them?
- Which cracks get filled first, and what happens to the alligatored section by the garage?
- What is the overnight low forecast for the day after application?
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Quick Answers
Is coal tar sealer illegal in Wisconsin?
Not statewide. Dane County and the City of Milwaukee restrict coal tar sealants, Minnesota and Washington ban them statewide, and a 2020 Wisconsin bill to ban them was referred to committee, where its history ends. No Oshkosh or Winnebago County ordinance turned up in this research. Check current local rules before assuming either way, and ask the contractor to name the product in the written scope.
Does asphalt emulsion sealer last as long as coal tar?
For a driveway, the maintenance schedule is the same: a two to three year rotation, with cracks filled between coats. Both products wear away as dust, and both fail early when water gets under them or new asphalt has not cured. The prep, the cure conditions and Oshkosh's freeze-thaw count matter far more to the lifespan than the binder chemistry does.
How can I tell which sealer was used on my driveway?
Ask for the product data sheet and look for the words coal tar or coal tar pitch, or the CAS numbers 65996-93-2, 65996-89-6 or 28007-45-2 that Wisconsin's AB 797 used to define a coal tar sealant. A cured coat does not show its chemistry by eye. If the previous application is unknown, the next job still starts the same way: the surface is cleaned, cracks are filled, and the contractor checks that the old coat is sound and bonded before choosing the product to go over it.