When tiling the bathroom, I cut the door frame higher up than I needed to and now have a large gap. Please advise the best method of filling this gaps so that it looks professional. Some choices - 1. Caulk - easiest, but wont be flush and I believe will look tacky 2. Cutting the frame higher up and adding a new piece of frame. This will look better, but will take time/effort. Is there a filler that can be moulded/sanded and then painted and that is waterproof?
The usual approach is to cut a thin strip of the same jamb and moldings and glue into place.
You can also fill in small spots with an epoxy based product like Bondo or other paste-like filler (would not use it for thicker vertical surfaces since it may slump). It can be carved and sanded like a hard wood.
A combination of these techniques should give you a pretty unnoticeable surface once painted.
The difficulty here is extending the door casing's pattern so that it nearly touches the tile. A stylistic device which helps with this problem is the plinth block which you simply install in place of a removed section of the moulding.
In your case, plinth blocks address what to do about the outward-facing door casing. The remaining issue to deal with is the door jamb itself, and the inner moulding running down the inside of the jamb.
The inner moulding, I would just remove and replace with a longer one.
Before installing the new inner moulding, I would extend the jamb with a plank of wood grafted underneath, bookended by the two plinth blocks (perhaps secured to them with adhesive, or finishing nails). The plank could be obtained by cutting from a piece of moulding (conveniently, already primed white).
