It won't really matter as using an interior door outside is a doomed enterprise.
Casey

And though Casey is most likely dead-on, it never hurts to prime the so-called pre-primed doors. Since you're using it as an exterior door, you might as well use an exterior primer under your exterior paint.

Some of the first apartments I looked at had interior doors for the front door. My parents said no and drove to a different part of town to look...

If it's a hollow door...obviously forget it!!

If it's a cheap pine door....forget it!!

IF it's a decent solid door, yes...scuff-sand thoroughly and re-prime ALL sides.
THEN 2 coats of new color, at least Satin sheen.

Faron

"Some of us parents actually are smart."

Indeed! Though back then I was just upset that living in a nice apartment meant I couldn't have a pet. ;)

Don't bother painting it at all. It will not last more than a couple of years no matter what you do to it.

And why do you "have to"?

Can I use old indoor door for trellis for my plants outside.

the doors have no use for me otherwise and I would have to throw it out. This way I can use it for atleast 2-3 yrs.. please Advise