Both exterior and interior paints are available in various finishes including eggshell, flat, gloss and semigloss. Each finish has its own advantages and disadvantages, which you should take into consideration before purchasing the paint. A flat finish, for instance, has little or no shine to it and hides slight imperfections on a wall; semigloss has a slight shine that reflects light and shows surface imperfections.
The two common bases for paint are oil-based and water-based. Oil-based paints have a non-water-soluble base to it, while water-based paints have a water-soluble base, which means you can clean up water-based paints with water; oil-based paints require paint thinner to remove unwanted paint. In addition, water-based paints do not produce the intensity of fumes that oil-based paints do. However, oil-based paints last longer and are more durable than water-based paints. Typically, interior paints are water-based, while exterior paints are oil-based.
When mixing paint together, it is the base not the finish that is most important. Trying to mix oil-based with water-based paint will not work as intended. In essence, you are trying to mix oil and water, which separates. As long as the flat and semigloss paints both have the same base, you can successfully mix the paints together. Before combining the paints, pour them in a clean container. For best results, attach a long mixer attachment to a power drill and mix the two paints together for several minutes.
When you mix different types of paint together, you are reducing the effectiveness of each of the specific paints. For example, if you mix interior paint with exterior paint and use it outdoors, you will degrade the durability of the exterior paint, and it probably will not last as long as you would like. In addition, if you happen to run out of the mixed concoction, the paint center will not be able to recreate it to match.