What are the best conditions for sealing an asphalt driveway? What time of year, and under what kind of weather would be best?
A sunny day in the 70's-80's range is ideal for sealcoating. It is warm enough to ensure proper drying and cool enough to prevent the sealcoat from drying too fast to correct a mistake.
One of the key things to remember when sealcoating is CLEANLINESS MATTERS! By far the most simple way to do this is to have one person with a leaf blower and another with a stiff bristle push broom. Do a quick once-over with the leaf blower then have the broomer start at one end of the driveway while the leaf blower operator directs the stream of air at the broom. Spend extra time at cracks and dark spots.
Another option is to thoroughly power wash the driveway. This is less than ideal as you will have to wait a day or more for the driveway to fully dry. Failure to do so will trap moisture in the asphalt after sealing and damage the asphalt when it freezes. Powerwashing also leaves the possibility for wind to blow more dirt/debris onto the driveway while waiting for it to dry.
Properly cleaned asphalt will guarantee a good seal. :)
My preferred application method for sealcoat is a squeegee versus a broom or sprayer.
Pick a sunny day, with ABSOLUTELY no chance of rain that day. If it rains within a few hours of the sealing job, you won't be happy. 24 hours will be plenty of time for it to dry though.
The perfect day would be sunny, but not incredibly hot, as sealing a driveway would be miserable work on a blistering hot day. Remember, you will be standing on black pavement in the sun.
Start work in the morning, to give it time to dry in the sun. Keep anything off it for a day.