As mentioned by others, I do appreciate the ability to walk over areas and clear them as it also helps to clear up any mistakes! One small suggestion below, but nice work! (I also really like how the spray-paint effect turned out btw)
One recommendation for a possible improvement is to give a tiny bit of leniency in amount covered vs uncovered. So instead of something like 100% coverage, maybe 95% required? and if you still want to encourage the 100% you could start a small countdown at that point for someone to try and sneak in the last 5% -- Players who planned ahead would have an easy time finishing up, others might undo their progress unintentionally while trying to search for those remaining patches!