don't everyone screen at once, I have tried all sorts of cleaners, and yes if you leave some of the harsh ones to dry, your f@£$ed. I clean my bike once every 6 months if I'm lucky
the best thing I have tried is 0000 grade wire wool NOTHING HIGHER OR MORE COURSE and spray WD40 onto that and use a little bit of elbow grease, keep spraying it onto the wire wool. I am not saying scrub through to the bare metal, but a bit of light rubbing, like when you scratch your balls or haemorrhoids, but I can promise you it works a treat. Also I have found it to work better when wheels are dry, I then wipe off the residue with a dry cloth, or wash the bike after, the green scrubby kitchen pads don't work as well, although they might if you did it every week. but I definitely can't be arsed with that.
the wire wool and WD40 method will remove tar spots, any rust on engine bars/luggage racks (but that's probably just mine, as I've got better things to do than clean my bike every 3rd day and then every day in between) will clean the exhaust too.
if you don't believe the wire wool doesn't scratch, try it on an inconspicuous area, I was very worried at first. I would put a photo on but it's too dark out, might try later if anyone is interested.