Painting interior walls in your home is one of those tasks that seems pretty quick and simple, but usually ends up being far more work than you anticipated. That’s because it’s not only a matter of brushing or rolling on a coat of fresh paint—there’s also all the prep work and cleanup that goes along with it (and tends to be edited out of TV shows featuring home makeovers).

And along with prep work like moving furniture out of the way and covering everything in drop cloths, there’s the matter of washing the walls. This is where it can get confusing. Most DIY guidance and paint can instructions indicate that washing walls before painting them is a crucial step.

But if you’ve seen professional painters at work, you may have noticed that they often skip this step. So, which is it—an important part of the process, or one that isn’t really necessary? Here’s what to know.

Washing walls before painting them is the rule, not the exception. However, as The Spruce points out, instructions on how exactly to wash your walls varies between manufacturers, ranging from wiping them down with a sponge and warm water, to cleaning them using TSP (trisodium phosphate)—a powder that is mixed with water to form an alkaline solution used to dissolve grease and oil.

Generally speaking, if you’re going to paint interior walls, it’s best to wash them first.

When you might be able to avoid washing walls before painting

The reasons professional painters tend not to wash walls before painting depend on the situation, but if they’re getting paid by the job (rather than the hour), it’s not hard to see why they skip that step.

Beyond monetary reasons, some professional painters, like Eric Regan of Mission Painting & Home Improvements in Overland Park, Kansas, say that most of the time, interior walls in some rooms of your home don’t have to be washed before they’re painted—including if you do the job yourself.

So how can you tell whether you might be able to skip that step? First of all, if you’re painting your kitchen, bathroom, or any other room where you cook or bathe, there’s no getting out of at least wiping down your walls. The same apples to vertical surfaces in any room, like trimming and ledges.

Aside from that, according to The Spruce, if you’re dead-set against washing your walls before painting them, they should meet all, if not most of these conditions:

• Not being excessively dusty

• Absence of crayon and/or grease stains on the walls

• The home does not have a wood-burning fireplace

• The walls have little or no contact with hands

• There are no pets in the house

Even if all of the criteria above apply to the room you’re going to paint, you should still give the walls a quick (dry) wipe-down to brush off any lingering dust or debris that you might not be able to see.