Full paint stripping is overkill if the existing coat isn’t peeling or cracked. A thorough sanding with 120-grit to knock down the gloss, a wipe-down with mineral spirits, and one coat of bonding primer got my door ready for new paint in an afternoon, no chemical stripper involved.
The part everyone skips is removing the door from its hinges. Painting it flat on sawhorses instead of vertical on the hinges avoided every single drip and gave a smoother finish than the two coats I’d previously done standing up.
Total time from bare door to two coats of finish paint: about five hours spread across a Saturday, with drying time built in between coats.