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But Bonsal American's Crenson says the compatibility issue is no longer a concern. Crenson says many of the industry's initial efforts to combine asphalt and coal tar in a sealer involved mixing an SS-1-type anionic asphalt emulsion, (the same type of emulsion commonly used on paving tack coats today) with coal tar emulsion. Historically, some coal tar sealer formulations have used cationic emulsifiers, and the two types of emulsions can't be combined because the mixture becomes unstable.

"That was the state of the art at that time," Crenson says. "More recently, sealers made from asphalt/coal tar blends do not use SS-1 emulsions. The industry has learned that it is possible to blend these materials together, and has developed formulations to solve that kind of problem. The compatibility of the emulsifier and other ingredients has been worked out."

Maclean agrees, saying that today's technology facilitates more compatible blends of asphalt and coal tar and while not ideal, successful performance on the pavement has been achieved. The equipment used to put it down should not be a concern. The primary concern he thinks contractors need to be aware of is that curing of the film takes longer for blends than for sealers made of either 100% asphalt or 100% coal tar.

"Film formation takes longer with blends," Maclean says. "Individual particles of tar and asphalt do not coalesce together so the film will remain softer relative to coal tar sealer throughout curing, as well" Crenson agrees.

"Typically blends tend to cure a little more slowly and remain somewhat softer longer," Crenson says. "The cured coating of dried film remains somewhat tender longer than the contractors are used to with coal tar sealer, and that's because of attributes of asphalt emulsion sealer that are brought into the coal tar sealer."