How do I prep previously painted steel exterior door to paint again?

If the paint is in good condition, you can paint over it. If you think it seems uneven, you could gently sand it for an even base coat.

You only need to lightly hand sand the old semi gloss finish to give the new paint something to adhere to, clean & dry thoroughly, then apply the paint of your choice :)

Hi Karrie! Steel exterior doors are great in so many ways. Here are some helpful links! Best wishes.

https://www.wikihow.com/Paint-a-Steel-Door

https://www.house-painting-info.com/articles/painting-a-steel-door/

http://www.hammerzone.com/archives/doors/maintain/paint1/steel.htm

https://homeguides.sfgate.com/prepare-paint-metal-outside-doors-26872.html

https://homeguides.sfgate.com/prepare-paint-metal-outside-doors-26872.html

Clean surface, make sure no flaking. Then just apply the paint. Problems occur when going from oil based to latex or covering stains. If you do have any stains that won't go away apply some Kilz to stain and let dry before painting.