I tried to paint over a metal door going to my garage. I did not sand it and the paint can be easily scraped off with my fingernail. How to I remove the paint and what are the right steps?

I glazed my first wall. I painted it first with 2 coats of a matte latex paint which came out fine. Then I used a glaze, a darker color added to it, ratio 1 part darker, 8 parts glaze (recommended by someone who had done it) I also used an