If you bought a satin paint to match to an existing paint but it is a little too shiny can you add something to the paint to flatten it some?

Answer

if your paint is latex add some water to slightly dull the sheen, if you need to drastically dull the finish buy some flat paint in the same color and mix accordingly

Do not thin the paint too much as it will change the durability of the topcoat. Buying a 2nd qt, gallon of flatter paint will also change the sheen but cost you more in the end and you will have a bunch of paint sitting around. The easier fix is to ask your local Sherwin Williams, Quals or other paint supplier for some flattening powder which is the only real difference in the sheen of your paint and the flatter paint suggested above and costs pennies. About 4 oz of this powder to a gallon will change a satin, eggshell to a flat. You could do the math and mix a small pint to test it. Good luck.