Mixing Paints of Different Brands

Will I have major problems if I mix paints of the same type, e.g. exterior acrylic latex, but different brands (and colors)?

Paints get their colour from the solid coloured particles in them (called "pigments"). These pigments don't interact with each other in any way (except that they do have a tendancy to clump together, but they do that anyway. So, mixing latex paints of different colours is no different than mixing different colour pigment particles, and that alone will never be the cause of a paint problem.

Similarily, paints get their gloss level from the amount and coarseness of something called "extender pigments" in them. These are huge rocks almost large enough to see with the naked eye. Were it not for extender pigments, or paints that come pre-tinted from the factory, all paints would dry to a high gloss. That's because when they tint paint in the paint store, the pigments in the colourant tubes of the tinting machine are all far too small to impart any roughness to the paint as it dries. That's because those same pigments in each colorant have to tint high gloss paints, and so they can't be so large as to reduce the gloss if the customer wants high gloss paint. So, mixing paints of different gloss is just a matter of mixing different size rocks, and so that alone will never be the source of a paint problem either.

Usually you can get away with mixing latex paints with latex paints, but it's not recommended that you do it because of the concern that the additives in one paint won't be compatable with the additives in another. If you get a problem, the problem you can expect is called "flashing", which means that the paint won't be of uniform gloss. It'll be fine in some areas and dull in other areas.

Where you CAN confidently mix latex paints is if you mix different paints from the same line of paints from the same manufacturer. For example, you would never get a problem mixing flat white Sherwin Williams interior Superpaint with high gloss red Sherwin Williams interior Superpaint to get a satin pink Sherwin Williams interior Superpaint. But, as soon as you cross lines even within the same manufacturer, like mixing either of those with some Sherwin Williams ProClassic interior paint, you run the risk of potential incompatability problems between the additives in each paint. And, of course, as soon as you start mixing paints from different manufacturers, then all bets are off.

However, that said, all you have to do is Google "paint recycling" to find all kinds of provincial, state and municipal programs where people drop off unused latex paints at Household Hazardous Waste collection sites. That paint is sorted for colour, and latex paints of similar colour are batched, filtered and treated to make recycled latex paints for sale to the public. Here's one company selling such recycled latex paints:

http://www.recyclepaint.com/

You might want to contact that company and ask how often they encounter problems with mixed latex paints due to incompatabilities.

Oil based paints have a much more robust film formation mechanism, so you're safer mixing oil based paints than you are latex paints. It's hard to prevent oil based paints from forming a solid film properly, but lots of things will prevent latex paints from forming a proper film. (including temperature, humidity, the amount of tinting done to it, and incompatability in the additives).

You can learn more than you need or even want to know about latex paints at:

http://www.paintquality.com

That web site is operated by the Paint Quality Institute, which was established by the Rohm & Haas Company who are the largest manufacturer of the acrylic resins used to make latex paints, acrylic floor finishes, acrylic grout and masonary sealers, acrylic nail polishes and acrylic sheeting (sold under the name Plexiglas if made by Rohm & Haas) in North America. The purpose of the Paint Quality Institute is to make professional painters, paint specifiers (like architects and designers) and consumers (presumably people who consume paint) aware of the benefits of using high quality latex paints.

Outstanding answer. Well-written, informative, complete. First class work.

Thanks for the fan mail, CAEMI.

Imagine how much better it woulda been if I was sober whan I wrote it!