or the surface tension in general is to high on the aluminium. a good way to clean it is to look for this rubbing powder like comet (there are european solutions that are exactly the same except the name) check if it holds bleach !!!, since thats what makes it work. you can clean it with that, after that you can do a water test and it should create an even film. i believe aaplied Science made a video about it once check his channel https://www.youtube.com/@AppliedScience it has more cool stuff when it comes to etching really small things !
here is the video
here is the video