Clear coat of flat stripe

Aren't decals normally made out of vinyl? I wouldn't clear over one. You can lay out your stripe with fine line and spray black basecoat and then clear it with a flattened clear. Most paint lines have a flattening agent that you can add to the clear and they give you percentages to add to get flat, 10% gloss, 50% gloss etc. it's not hard, though you'll always have a paint line.