If for some reason you need to grind out this achievement, get all 3 weapons dirty, find a workbench, quicksave, clean, quickload, repeat.
Higher difficulties require more chemicals per clean so if you're on reader bump it up to Hardcore at the benches to spend more chems.
Sometimes it decides to stop counting them. If this is the case just move forward until your next autosave, and then reload the quicksave. Should fix the counter.