One of the best ways to protect asphalt and concrete is to coat them with sealant, and when that sealant dries, you'll have surfaces that resist stains and mold. But the sealant can wear off and suffer breaches that allow contaminating substances – and mold-friendly moisture – to invade the asphalt and concrete. That means you need to have the sealant re-applied occasionally. That sounds easy enough, but because that sealant is clear when it dries, it's hard to tell when to have the layer redone. Look for a few tell-tale signs that something has happened to the sealant, and then have the surfaces cleaned and the sealant added once more.
Damage to the Surfaces
The most obvious sign that you need to have the sealing redone is if the asphalt or concrete suffer damage. Chips in the concrete from large hail, cracks forming in the asphalt after sub-freezing weather (which signals that moisture has already breached the surface), scrapes from machinery being dragged across – if you can see the damage, you need to get the surfaces restored or replaced, cleaned up, and sealed up once again.
Stains You Can't Wash Away
Sealant does a good job of repelling stains. Obviously, things can spill onto the sealed surfaces and leave what looks like a stain, but you should be able to wash those away. If you find that there are stains you can't remove, such as oil under cars or stains from spilled food, chances are that there was damage to the sealant layer or that it wasn't applied properly the first time. Have the sections repaired (this could involve anything from simple power washing to replacing the concrete or asphalt) and then properly sealed.
You See Efflorescence
If the surface in question is concrete, and you see a whitish, powdery layer appear on top, you've definitely got to call the sealcoating company again. That whitish powder is efflorescence, and it's basically a salty layer that appears over time when moisture in the concrete rises to the surface and evaporates. The salts are what is left behind. When you see this, it means the sealant is either damaged or gone. Cleaning the concrete and then sealing it should solve that problem.
A final sealing layer needs to be added to any surface that you use for driveways, walkways, patios, and private or service roads. Sealcoating is an essential part of keeping these surfaces in good shape for a long time.