Should I wipe behind my flat boxes?

We are often asked whether or not you should wipe behind or "trace" behind your flat boxes. This article aims to give you some things to consider when deciding if tracing behind your flat boxes is best for your finishing system.

How clean can you run the box? 

One of the biggest factors to consider when deciding whether or not you should trace behind your flat boxes is how clean you are able to run the box. The flat box is probably one of the easiest tools to learn when getting started with automatic taping tools, but running a flat box so clean that you don't have to touch it with a knife is an art form.

If you are still learning the flat box, "tracing" or wiping behind the flat box is a great way to tighten up your work, and is helps maximize the efficiency that flat boxes offer. We will go over a few pointers on tracing at the end of this article. 

How clean is your environment? 

Running the flat boxes without tracing behind them requires clean mud, clean tools, and clean walls. The drywall loading pump comes with a screen which filters out large chunks, but some junk will still get through even a fine screen if you aren't paying particular attention to keeping your mud clean. You will also want to walk through the joints and check for dried globs on the walls from your previous coat. Those small globs will catch in where the blade meets the brass bar, and cause scratch lines. 

The first coat with the box is more difficult. 

The first coat after tape is often tougher to run clean without wiping behind. This is because there is more fill on the first coat, making it more likely that you will see pock holes or air bubbles. There are products like No Pock that can help reduce this. The other thing that can help is running the box as far as you can in one direction, then running the entire joint back the other way to help remove the extra air. The finish coat is more forgiving with this as you can often get a clean finish with one pass if your pressure is right.

Smooth or texture? 

Of course, the level of finish is a factor as well when considering whether or not you are going to wipe behind your flat box. Smooth walls and ceilings will be far less forgiving than textured, so you might want to tighten things up by wiping down to minimize touch up. 

Inside corner intersections require extra attention 

There are a couple of things about where the flat joints meet the inside corners that you will need to pay attention to if you are trying not to wipe behind your flat box. 

One factor is that when you put the flat box all the way in the corner to start, it leaves about a 1/4" of no mud at the very point of the inside corner. Some guys who don't wipe at all, touch these up when they dry, but that can leave extra odds and ends that can get missed. When you wipe behind the boxes, you can wipe out about a foot from the corner and the tuck your knife back into the corner. The little bit of mud that you picked up on your knife will fill this in. 

Another factor is that there is often a little extra mud in the corner where you start. Some call these "Christmas trees" (they almost look like a sideways mountains). If you don't wipe these out at all, then you can end up with high points that can create touch up or even sanding. If you do wipe these out, my opinion is that you might as well wipe the flat out to the center of the wall when it's a wall with 2 inside corners. Then you'll only have one lap mark instead of two. If it's a wall with a door or window, wiping the flat out to the opening will of course leave you no lap mark. 

Ergonomics and tool wear are factors

If you are going to walk the length of each joint twice when running the box without wiping, you might as well consider that you could do that second pass around the room with a knife in your hand, rather than pushing the box around for a second pass. The knife is much lighter, and wiping with a knife instead of making two passes with the box will save you miles of wear and tear on your boxes. You can also catch all those odds and ends by hand while you are in the room and save yourself from having to backtrack later. 

Tips when tracing behind the box

Use a larger knife than the size box you are running. Wipe behind the 10" box with a 12" knife, wipe behind the 12" box with a 14" knife etc. This will help prevent the mud from rolling over, and creating an edge from too much center pressure. 

Keep your knife as flat against the wall as possible, without dragging your fingers through the joint. You can also use knives like the Sheetrock offset knives to help get the knife as flat as possible. 

In the corners, wipe out from the corner with a clean knife, and use the mud that collected on the edge to dip back into the corner and fill that 1/4" void we spoke about. 

Invest in some wipedown knives for ceilings. The wipedown knives with extendable handles are in my opinion one of the greatest inventions since sliced bread! The soft blades leave very little lap. Even if you just start with the 14" wipedown knife and handle, you will  to be able to wipe behind your 10" and 12" boxes on high ceilings from the floor.