Re: Priming/painting tips on an interior metal door
Oil primer isn't going to lay down very smooth unless it has a lot of oil in it. Some (low-VOC) primers have no oil. You can add a small amount of boiled linseed oil to the primer to enhance the brushing characteristics, but also add a quantity of Japan Dryer to it to make sure it will cure in a reasonable time.
For ultra-smooth enamel work you need to block sand the primer, then fully dust off w/compressed air and wipe down with naphtha or paint thinner.
Rustoleum oil paint is not great, but usable, especially on an exterior door. I would avoid any of the big box brands of glossy acrylics. I have never seen one of them do a decent job on a front door.
Moore's acrylic/alkyd emulsion paints are really good. They apply very smooth, low-odor, dry as hard as oil paint, water clean up.
For a super-shiny front door (which requires immaculate surface and prep work, and dust-free conditions) use "Hollandlac Brilliant" from Fine Paints of Europe. It's very expensive but in its own class of gloss, durability and finish.
Remove not the ancient landmark, which your fathers have set.