How to choose paint for exterior steel on carport

This article provides a pretty good run-down.

1. Yes you should clean: soap and water or any other mild cleaner is good.

2. You may not need additional primer, but you should fix any dents/scratches -- they will stick out after you put the new paint on. Auto-body filler (eg Bondo) works for dents and deep scratches. Before you prime, the whole door should be lightly sanded using 120-150 grit sandpaper to roughen the surface, and then wipe the whole door down to get rid of any particles from sanding.

3. Use an oil-base primer, and again, be sure to lightly sand first.

If there is six or seven layers of paint over the concrete then it will be sufficiently thick to be noticeable if you attempt to just scrape a single area and paint. It sounds as if a number of different people over the years have been putting off the dreaded inevitable conclusion that you should completely scrape and remove paint from the entire surface and repaint it with a proper exterior enamel approved for concrete.

When scraping paint always make sure to wear a dust mask as one of those layers may be lead based paint and it would be extraordinarily bad to breathe this in. When scraping keep children, animals and pregnant women away as they are particularly susceptible to ill effects from lead exposure.

Most hardware stores sell lead testing kits that will let you know if any of this scraped paint dust contains lead. A word of caution though that if you happen to verify and know that lead is in the paint then if you ever go to sell the place that you would be legally obliged to add this information to the sellers disclosure. Sometimes it is better to just treat the paint dust as if it contains lead and not know for your own sake.