I'm thinking that you may want to protect the matte finish of the weathering while still keeping the gloss finish of the helmet, right?
In that case, there is not much you can do, unless you take a paint brush and paint a matte clear coat over the areas of weathering.
In my opinion, you may just want to leave it as is, and see how the acrylic weathering wears with use. After all, a real helmet will wear, lose its gloss finish by getting scratches and get dirty, especially in battle. Also, some of the dirt will rub off to get replaced with different types of wear, scratches, gouges, dents.
TazMan2000