I would like to know if I can mix a gloss black and flat black and create a semi-gloss paint? Is there a kind of ratio I need to get the semi-gloss mix?
If the gloss and flat paints are of the same composition, you can mix them to obtain a lower gloss finish. Only experimentation will allow you to get to the semi-gloss level you desire. I suggest you start by mixing small quantities of the flat paint to the gloss paint and then applying it to a smooth surface. Perhaps you can start by mixing equal volumes of each paint. If the results is too glossy, then start again by adding more of the flat paint. Keep a note of the mixing ratio that results in the desired gloss level.