Decorating flaky steel garage door - can I use filler?

After wire brushing two old galvanised steel garage doors I am left with a very uneven surface from where the (thick) paint has come away. Most of the paint is still intact and I think stripping it all would be a huge effort. The edges of bare patches don't seem to want to 'feather' - the paint is brittle. Any way to get a reasonably flat surface for a finish coat? I started filling in with undercoat but the bare patches are way below the rest and it looks a bit like the surface of the moon. Is there any kind of filler I could use?
Thanks for any advice.
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Car body filler, apply with a flat bit of plastic
NT
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