what type of primer is used for metal doors?
Alkyd (oil based) 1) Remove the door from the frame, and remove the hardware. 2) Install two 3 long screws at top and bottom towards the corners, (only screw them in about 1). 3) Set up a pair of saw-horses and lay the door flat on the screws to paint one side and edges, then flip the door over (with a helper) using the protruding screws as handles. 4) Paint the other side. After it's dried for a day, lightly sand brush marks with 220 sandpaper. Wipe with a tack rag, and then color coat with a good quality alkyd paint.
are they steel or aluminum doors and have they been previously painted? if they have been previously painted then all you have to do is give them a light sand and wash them , allow to dry. then apply 2 - 3 coats of an alkyd semi gloss a alkyd porch and floor enamel works very well also. if they are unpainted galvanized steel then give them a light sand and wash with some white vinegar, rinse off and allow to dry. you can use a red oxide primer but it is not needed just apply 1 - 2 coats of an alkyd primer under coater. then apply 1 - 2 coats of an alkyd semi gloss. again you can use a porch and floor enamel. i like to use a 4 inch fabric roller when i do them. make sure to wash the roller well and dry it before using to get rid of any small loose hairs. always give a light sand between coats and make sure you dust them off before painting. use a top quality paint and please don't let them talk you into using a water base paint at the paint store it will not stand up as well as an alkyd paint. if you are able lie your doors flat on some saw horses they will be alot easier to work on that way.
you can get Kilz for wood, metal, etc. Just look at the can to make sure what it will adhere to.
If they are outdoor metal doors, then i'd worry about rust forming so I'd use a rust preventive primer like RUSTSEAL first.
