While mortar clumps can be chipped away, a "film" is known to be just a very thin dried layer. Naturally, it is alkaline and can therefore primarily only be removed with acidic cleaners.
If you are concerned about chemicals, you should, for example, avoid COLA. A litmus paper dipped into the beverage shows a pH value of 1 (strongly acidic).
So (this is meant seriously) take a small bottle of COLA, pour it over the affected area, wait about 10 minutes, and then blot up the dirty liquid with a cloth. Rinse afterwards with clean water, and you will see: the cement film is gone!
Best regards, KlaRa
If you are concerned about chemicals, you should, for example, avoid COLA. A litmus paper dipped into the beverage shows a pH value of 1 (strongly acidic).
So (this is meant seriously) take a small bottle of COLA, pour it over the affected area, wait about 10 minutes, and then blot up the dirty liquid with a cloth. Rinse afterwards with clean water, and you will see: the cement film is gone!
Best regards, KlaRa