Main reason kung baket nababawasan ang bula and water spots ay hard ang water sa area nyo. There's really nothing you can do to change that unless may water softener ka, which, of course, doesn't make sense kung pang hugas lang ng oto.

What you can do to minimize it is to wash by part. Meaning, when you wet the roof/side windows, wash mo na agad yun and dry (kahit 80% dry lang), then you wash the hood, and fenders... Ganun ulit, then trunk and rear quarter panels. Then doors.

Or better yet, just wash at night when it's cool.