Then you have to use it again. A 12-volt auto windshield defogger gun is somewhat less effective. A better way is to apply a little heat continuously. Or with just a little electrical know-how you can make an antidew heater to any size, shape, and specification you want. Here are the details.
Warmed optics can have unexpected benefits.
Dew works its first subtle evils before you notice anything. The late Walter Scott Houston used electric warmers on both the objective and eyepiece holder of his 4-inch refractor. When he turned off the power, the telescope could lose a whole magnitude of light grasp before the objective actually looked dewy. This may be because, contrary to what you might think, gentle heating keeps a telescope close to the temperature of the surrounding air. After all, the whole idea is to stop it from growing colder than the air.