6banger,
I dont know what product you are using , but make sure you use the same brand of primer surfacer that is compatiable with your Epoxy!

You can try taking a Grey scotch bright pad & lightly dry scuff your epoxy & it will take some of that sheen off .

Now days most brands have tintable sealers that allow you to have about any color of your choice, but they also have a slight sheen to them also , but you lightly scuff it & it will dull down some.
Hope this helps out .
Spray