Have some old cast aluminium garden furniture, paint peeling off, white powdery 'salts' underneath. Some of the paint scrapes easily and other places seem well stuck. Do I need to remove it all? How shall I treat the 'salts'. With acid? What is the best type of paint to use for repainting.
Thanks.

I think you need to use an Etch Primer for ferrous and non ferrous surfaces.

Ensure surfaces are clean, dry, free from dirt, grease and other contamination.