Sealcoating asphalt pavement is usually a good idea. Application of a coal-tar based coating helps to prolong pavement life in a number of ways. Properly applied, sealcoating retards the loss of volatile constituents of the asphalt. Loss of these components results in a brittle, weakened pavement. The sealcoating process helps protect the petroleum-based asphalt against attack from petroleum-based automotive fluids such as gasoline and motor oil. It also helps to keep out water, which attacks virtually everything. Coal-tar is considerably more resistant to water degradation than asphalt.
Having sung the praises of sealcoating, it's only fair to present the downside. Sealcoating is not a cure-all and can be a waste of money. Among other misconceptions is the notion that sealcoating will rehabilitate distressed pavement. About the only thing it does for faded, failed asphalt is turn it into black, failed asphalt. It's analogous to painting rotted wood. The cost, compared to that of overlayment or other conventional remedial measures, is modest (which is surely why we see it so often in this context) but the benefits are, at best, negligible.
Timing of sealcoat application is a factor in determining benefits. It should be applied within a year or so after asphalt placement and repeated every three to five years to net the best results. If you wait too long initially or between applications, critical volatiles can be lost. Once gone, they cannot be effectively reintroduced, the claims of some manufacturers of restorative products notwithstanding. Other protective aspects of the sealcoating would still pertain, but there is a point past which the benefits would not justify the cost. Coating asphalt that may have only a year or two left may improve the appearance of the pavement a while, but it's probably not the best way to spend the pavement budget.
If there were a simple formula for determining whether sealcoating is appropriate for your pavement, we would gladly reveal it here. The fact is that it's usually a judgment call as to the merits of applying sealcoat to older pavement and engineering criteria should apply.
Kirk Parsons
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