Will brass door numbers lose their shine? How do I care for each material?

Brass will naturally dull over time, depending on exposure to the environment - some love this mellowed patina, but if you prefer the original lustre it can be restored with a metal polish such as Autosol Chrome and Metal Polish.

Stainless steel won't tarnish or develop a patina, so upkeep is an occasional wipe rather than metal polish. ACP numbers just need an occasional wipe with a soft damp cloth.