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Normally they can be lightly sanded, zinsser 123+, under coat to match the top coat (i.e. same manufacturer, appropriate tint), 2 x top coat of choice. I usually hand brush them because they have panels so a roller is a bit less effective, you can use a mix of brush and roller if you struggling with a modern acrylic eggshell.
Lots of options in top coats though, gloss/eggshell/various sheen levels/oil based/acrylic etc..
If the the door has flaking/cracking paint or bubbling I'd probably strip it to the wood and start with Zinnser cover stain.
What would you use for a front door?