Bobsticle said:

I wouldnt go powered at all unless going down to bare aluminium. Even if you want to remove all the paint it will be a nightmare getting it all off, especially round the rivets etc.

As said, if you get down to brimerbright your going to need an etch primer. If this is applied over any original paint left over it will bubble.

A rock and a hard place I know but its a choice of all or nothing.

I would only use a block, wet n dry and soapy water.

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Bobsticle said:

Unless the original paint is releasing itself from the aluminium it’s fine to use this as a solid base to work from. Starting afresh could bring more problems than it cures.
Key the surface with a medium wet and dry removing any nibs. Use soapy water but not one with silicone in it like some washing up liquids.
Mask off the car then apply a thin coat of contrasting primer. You can then use large flat blocks with the wet and dry to show up dints.
Fill as required, flat the filer and begin high build primers with progressively finer wet n dry until ready for top coats.
Try to imagine your going to varnish the primer rather than paint it. That way you will get the filling and priming spot on. Once you start the top coats it’s too late to hide anything.

I don’t do any of the above as the bodywork is last on my priority list. One day when everything else is done I may go for a decent finish but I’m happy that it looks like a shed.

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