Strip away a small amount of aluminium with household chemicals
Citric acid will not etch pure aluminum on any reasonable timescale, at room temperature, though commonly-used aluminum alloys, high temperatures, or the presence of salt can change that. However, it may be better to try some other chemicals: here is a decent resource for finding metal etchant solutions. The two that are probably accessible with household chemicals are hydrochloric acid (can be found at hardware stores with pool supplies) and sodium hydroxide (often found at grocery stores as lye). Neither is super dangerous, but you'll want to take precautions to avoid getting them in your eyes, on your skin, or on your clothes and HCl produces hydrogen gas as it dissolves aluminum, so don't put big pieces of aluminum in concentrated HCl unless you have adequate ventilation.

It will probably take some experimentation to get the concentration right: you want an etch rate fast enough that you're not there for hours etching, but slow enough that you can control it. The big problem with wet etching aluminum is that most etchants take a long time to get through the surface layer of aluminum oxide (and the time it takes is not really reproduceable due to differences in thickness and such) so when you put a piece in the etchant, nothing may happen for several minutes, then you look away for a few seconds and turn back to find all the aluminum gone. This can be avoided by adding HF, which happily removes the oxide, but is very nasty stuff that is rather dangerous to work with.

It might be possible to do this by picking a low concentration etchant and checking the piece frequently, but you may run into issues if the surface is dirty (I recommend washing the surface with acetone, isopropanol, and deionized water in sequence) or depending on how the aluminum was deposited. Some deposition techniques produce large variations in film thickness across the surface and you might run into the situation where the aluminum will be completely etched away in one place but is still totally reflective in another.