I've heard some respected detailers use Mineral Spirits. They are much gentler than straight alcohol and will strip any old wax build-up.

I'm not sure if it will pull out imbedded contaminants that clay will grab, but it will remove residues and films that don't wash off.

Regards,
- Ryan

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'95 E420 - 'Shadowfax' 138kmi.
'92 Volvo 740Turbo Bard 193kmi
'74 240D - 'Ol' Green' 4spd Manual 104kmi. (sold )
'77 300D - 'Red' 223kmi (sold)
'75 240D - 'Bianca the Blue Bomber' (sold)