Woowee, this is a long one.
This is actually something I did for my job. Here’s the deal: I have this electrochemical bath that has a sample in it; one electrode is a piece of graphite, the other electrode is the sample itself. Don’t worry about what the bath does for now, but it’s important that the longer the sample is in the bath, the more the effect of the bath is on the sample. It’s pretty much linear with time.
Now, I often want to see the effect of different lengths of time in the bath, with all other variables staying the same. The way I’ve done this in the past was to paint nail polish over parts of the sample (nail polish protects the covered part of the sample from the effect of the bath), put it in the bath for some amount of time, then take it out, either remove that nail polish, or add more, and put the sample back in for more time, etc. So basically you can have different length times on the same piece, which is useful for consistency and visually looks good.