Product Filter works with global and class-specific attributes only. Unfortunately, it won't work with product-specific attributes.
On other skis that I have only one size of, I have a global attribute to put the size. But when I use this same approach for items with multiple sizes, I can either select only one size, or enter multiple values for size and end up with two drop down menus in the product, one with the product specific sizes and another with the global sizes. This is kind of a tricky issue to explain, but I can provide examples if need be.
You don't have to fill both the global attribute, and the product specific attribute. But I think I may misunderstand you. An example would help