A nonprofit whose budget is tight wonders if it would be possible to take donations of partial cans of white paint, mix them all together to come up with four or five gallons of paint and then use it.
Issues of course are:
1. Mixing of latex/oil – which I don’t think you would want to do.
2. Gloss/semi-gloss/satin/flat – which I have no idea if you can mix.
3. Different brands which have different formulations.
4. Paint Failure – will such a mix stand up over time or fail miserably shortly after application?
You are on the right track with you bullet points. Therefore I will respond as you listed.
1) Please do not mix the oil and latex paints together. Will not work.
2) Generally speaking mixing coatings of dissimilar sheens will result in the sheen being the least of what was mixed. I would, however, recommend against mixing flat paint with any sheen. Would likely result in flat spots in the resulting finish.
3) As a general rule, differing brands are not as different as you might think. For instance, Sherwin-Williams owns "Dutch-Boy" "Pratt and Lambert" "Martin-Senour" among 4-5 other regional brands. Pittsburgh owns 3-4 that I know of. Therefore you may not be mixing different formulations as much as you think.
4) As long as you are mixing for interior usage, I would have no real concern with intermixing of various coatings. However, I would recommend against such intermixing for exterior painting.
Finally, be absolutely certain to mix enough for the project. No paint store around will be able to mix a color to touch-up your concoction. Throwing out an un-needed couple of gallons will be ultimately cheaper that having to repaint with some bought paint. You might even ask your local paint store to donate some of their Mis-tints for a tax write-off. Offer them some good publicity, you might be surprised.