The only thing worse than painting is unplanned repainting. So it’s good to ask questions first. One good question is this: Can you paint semi gloss over flat paint? Let’s explore.
Can you paint semi gloss over flat paint?
As long as the flat paint is in good condition, there’s usually no problem painting semi gloss over flat. I’ve always found it’s easier to paint semi gloss paint over flat paint than the other way around.
One concern I have is whether the existing paint is oil or water based. Water based paint doesn’t go well over oil based paint. To test, dip a cotton ball in some alcohol, and try to rub off the paint with it. If some paint comes off, it’s water based. You can safely use water based paint, of any finish. If no color comes off, it’s oil based. Either use an oil based paint of your desired finish, or apply a bonding primer first.
Flat paint tends to take subsequent coats pretty easily. I typically find the color to be a bigger problem than the finish. Covering a neutral color tends not to be bad. Covering non-neutral colors is more difficult. Red and yellow are always hard to cover. Loud colors tend to be difficult to cover, and so do very dark colors.
I get painting a room black. I went through that phase where I wanted a room black too. Whoever ended up painting that room after me didn’t like me very much.
The trick when you have to cover a tough color, whether it’s flat or another finish, is to get some Kilz primer, and get it tinted close to the color of your topcoat. The hardware store may say they can’t tint it quite as dark, or get a perfect match. That’s OK. The primer’s job is to seal out the old coat and make it easier to cover. The tint helps it do a better job of covering. I once had to paint a pink bathroom, and it took five coats. Had I known the tinted primer trick, I would have gotten it done in two or three.
Of course, primer is flat. So any time you paint semi gloss paint over primer, you’re painting semi gloss over flat.
Dealing with flat paint in poor condition
When the paint is in poor condition, that makes things tougher. If the paint is peeling, your best bet, if you don’t want to replace drywall, is to paint the walls with a peel stop primer. This primer can actually get under the flaking paint and stick it back down. It sounds too good to be true, but I’ve found it works surprisingly well. I’ve used it, and several years later, the paint jobs I used it on are still holding up. It costs $10/can more than regular primer, but if the paint is in poor condition, it’s worth it. Well, unless you like messing with drywall.
I don’t especially enjoy painting but I like drywall even less, so I keep a can of peel stop primer on hand to deal with peeling paint. It serves me well.