The phrase "flatten out" is used to describe the process of removing the bumps and irregularities from a surface. However, sometimes it can be helpful to use synonyms for this term to add variety to your writing and avoid repetition. Some synonyms for "flatten out" include "smooth over," "level off," "straighten," "even out," and "iron out." Depending on the situation, you may find that one of these synonyms better conveys your intended meaning. For example, "smooth over" may be more appropriate when discussing conflicts or issues that need to be resolved, while "iron out" may be more suitable for discussing wrinkles or creases in fabric.