I have been painting the exterior of my mother-in-laws house with oil based primer and then an oil based “gloss” top coat. She says it has too much shine to it. It needs a second coat and I was wondering if there is a way to add something to the rest of the paint I have to make it a semi-gloss or flatter? The paint is from Sherwin Williams and I have about 10 one gallon cans left.
All you can do is add some flat oil base paint to tone down the sheen. I'm not aware of any additive that will do this. I would just use what you have and let the natural weathering of the paint tone it down over time. Besides, the gloss oil will last a little longer than 'flatter' alternatives.