I’d feel more comfortable with oil base, but the door has been painted once and I don’t know with what, so I went with latex, which I know can cover both. It didn’t cover well, but I would have been happy to apply four coats, if it wasn’t for the fact that the paint peeled off in one giant piece after it dried.
I sanded beforehand, and cleaned with mineral spirits. I did not use a primer, but as I said, the door had been painted at least one time previously. Any ideas?
Hmm. WAG territory here, as I’ve not painted steel doors before.
This NZ site to do with garage doors recommends using a primer first before painting with latex, no matter what was used before. That should stop the coats peeling off again.
Ideally you should strip the old paint off the door, prime, then repaint. If your old paint layers arnt bonded to the steel then it wont matter how many layers of new paint you put on.
Start with 60 or 80 grit sand paper on a palm sander (random orbital) to remove the paint and switch to 180 or 220 to smooth it out.
There are some good water borne DTM (direct to metal) paints if you prefer to not use an alkyd. A paint store would have them, but a Home Depot type place probably won’t.
If you don’t prep the door right, you will quickly have a peeling, cracking paint job on your hands.
Just an FYI: I did a cross hatch and tape test on the existing paint, and it adhered just fine. Should it still be necessary to sand down to metal? Several sites advise against exposing the metal.
It’s what I do on commercial sites. YMMV.
If the old paint is bonded well, then a light sanding and a good cleaning should work.
The most important thing is using the right kind of paint and/or primer.
Temperature can make a difference, too.
One other thing, if you clean with spirits, it can sometimes leave a residue that latex won’t stick too.
A degreaser or TSP is good to use.
As a rule of thumb, I use spirits to clean if I’m applying alkyds, TSP and water if using water borne or latex.
You don’t put latex on top of oil. You can put oil on top of latex (haven’t tried this but some doper said so) I bet they used enamel before as that’s the correct stuff to use. When you visit the store, they should let you know what to do.
handy, I’m only here asking because the paint store gave me the original advice. 
The LadyVor (who worked at a good paint store) says to wash it good with just soap and water, then use a “bonding primer” like “Bullseye 123”, because it will stick to either latex or oil based paint. Then let that dry and put on the latex. Also, don’t use a flat paint, or else it will look like crap in a week.
-lv
Actually, you can put latex on top of (completely dry) oil, but you will be double coating as the sheen flashes through the first coat.
I often prime with an alkyd over old stained wood (after prepping) and then use what the customer/home owner prefers for the top coat, whether it be an alkyd (oil), latex, or some other water borne product.
The best stain blockers are the oil or shellaq based Zinser or Kilz products. I only use latex primers for exteriors, as they don’t cover stains good enough for my taste, but almost never flash under exterior latex.
The paint store told you to clean it with spirits? Maybe they assumed you going to use an oil based product.
there are some very special primers formulated for all the different variables you might run in to.
I sometimes a Xylene based metal primer for raw metal, esp on new commercial construction.
There are many ways to do almost any painting job.