What's nice about working at a hardware store, if you don't know something someone you work with does. Everyone, and I mean every single person I work with, recommends that you prime before you paint. Everything. Every time.
So when someone asks, "Is priming really necessary?" the real question is: "Is making it look good really necessary?" Because if it's not, you should skip the primer. And if you're going to skip the primer, you might as well skip the patching and the sanding, and the washing, and the taping. Just buy a cheap brush and a cheap roller, slap up some paint, and hope for the best. Odds are it'll look perfectly awful.
It's November. I'm behind schedule, and I need levels (as in something to put products on at different elevations) for a window display. I decide to create cake stands with plywood rounds and sofa legs (I'll call them sofa buns, since I'm making cake stands) and paint them white. It's next to impossible to find a place to spray paint in San Francisco, but a parking spot next to the store opens up, and I lay claim to it with slabs of cardboard and my cake stand assemblies.
And, no, I did not put money in the meter.
Pressed for time, I spray the cake stands with plain white spray paint. Did you catch that? I skipped the primer. I figured it would be fine for the stands to be imperfect, perhaps rustic, as the theme of the display was sort of a white-country-cottage Christmas.
After the second coat of plain ol' spray paint, it was apparent I was nowhere near obtaining a shabby chic finish. Shabby for sure. Chic, not so much.
So I used another can.
Three cans and three coats later, the wind picks up. Leaves, dirt, and I don't even want to know what other nasty trash that flies up off the city streets are now adhered to the wet cake stand surfaces. Defeated, I haul the cake stands inside. The next day, lo and behold, miracle of miracles, another parking spot opens up. The cake stands have been lightly sanded, wiped down, and are now ready for (yep, you guessed it) PRIMER! That's right, I primed them. In a parking spot. And I still did not put money in the meter.
Take the time to prime