Seeing as birds have been using my Mini as a toilet for the past few weeks, I thought it was about time to clean it, but before I do, I could use some advice -

The last time I washed it, beads of water sat on the surface (this was especially bad on the roof) then collected dirt. when they dried, they left hundreds of these horrible circles on the paint. I’ve tried using microfibre cloths on them before they dried, but it didn’t really do anything. Is there a good way to prevent these?