There's no need Stuart. You can do it one of two ways, either spray very close to the roof so you don't get overspray or spray into the sponge.
On the topic of the cleaner, I couldn't disagree more with Jonny on the cleaner. I've tried snowfoam, tried baby shampoo, tried car shampoo, all sorts. The autoglym cleaner with the weird hard sponge is superb. The colour of he sponge when I'd finished was insane, really felt like it gave it a good clean. I'll seal it next weekend and post pictures as long as it doesn't rain.