I get my best satin black results by using flattener in a lower solids urethane single stage black. The higher solids clears seem kinda finicky to me as far as repeatability and uniform gloss level. It's important to get about the same amount of paint everywhere on each pass which can be hard with tubes. If there are spots where it goes on too heavy, it will be shinier, especially when higher solids material is used. Don't use any slower reducer than called for, if anything go a step faster and over-reduce for lower solids and faster flash. You can spray the flattened material over sanded clear, but depending on expectations, it might be a lot more labor and cost effective to have the letters cut from vinyl.