+1. Using old paint to prime is penny wise and pound foolish. A good primer (I use a long of Zinsser 123) greatly improves the paintability of many surfaces.Paint shouldn't be used as a primer. Primers are specifically made for adhesion and sealing. Self priming and paint & primer in one is a load of crap. Years ago we called adding primer to paint for colors that didn't cover a "split coat" which served as our first coat but it does nothing for priming.
I had some leftover paint from a recent job. I was thinking of using it to prime white wood on my house, not a customers house.Does that also mean you can use primer for paint? Each has specific qualities and do specific things. And really how are you saving aren't your customers buying your paint and primers.
Using the paint for a primer coat on white wood that I'm using to trim a screen porch.
Totally agree some substrates such as new sheetrock requires a primer specifically formulated for sheetrock.
Can't always believe what you read.