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Hi @HeadingEast,
Your new paint coating needs to be applied to a sound surface. So, anything that is currently on the sheds that could come off needs to come off. The unpainted one likely just needs a good scrub down to remove any surface contaminants. Run your fingers over the painted one and see if any coloured dust is left on your fingers. If so, you'll need to sand the surface with a Scotch Brite pad until you remove the powdery layer. You can then clean it down with Sugar Soap. The shed with the peeling paint will probably give you the most difficulty. Any peeling paint needs to be removed, and the remaining checked that it would not peel off in the future.
The unpainted and sound painted ones could benefit from a coat of Dulux 4L Precision All Metal Primer. It would be a good idea to paint the peeling areas of the third shed with Zinsser 0.95L Peel Stop Clear Binding Primer before the Dulux Precision primer.
You'll then be free to choose an exterior paint to top coat with. You could either use White Knight Rust Guard (for a harder finish) or Dulux Weathershield.
Please let me know if you have any questions.
Mitchell
