sun and the rain

[BMC]

The paintwork on my car reacts badly on days like these. Showers, then rain water sits on the flat surfaces and becomes baked when the sun comes back out. It ends up with semi transparent milky white blotches which appear to have sunk into the lacquer and are very difficult and time consuming to get out.

The car gets waxed and polished fairly regularly.

T cut is not very effective. Anyone got any ideas on (a) removing the marks and (b) stopping it happening