We often have people new to automatic taping tools ask us how much water to add to their joint compound for certain applications with automatic taping tools. This article is intended to give some basic guidelines for the different phases of the taping and finishing process when just starting out using automatic taping tools. The suggestions on this page should not replace manufacturer specifications, but are field use suggestions.
Most every joint compound is different
The most important thing to remember when adding water to your joint compound is that even the same compound, from the same manufacturer can vary significantly from one run to another. This means it's difficult to give hard and fast measurements. Pro Form mud will likely respond different to adding water than USG mud, etc. You will ultimately want to train yourself on how the mud feels, looks, and responds hanging from the mixing paddle rather than relying on "how many ounces" to put in every time. You should also mix the compound as is, without water before adding any water to it.
Mixing mud for taping flat & butt joints with the automatic taper
We discuss which type of compound to use with the automatic taper in "What joint compound should i use in the automatic taper?". Be sure to give that a read as well.
When mixing mud for taping flat & butt joints with the automatic taper, you should consider whether you are pre-filling the joint first with a set-type compound, or if you are going to attempt to tape over small gaps without pre-filling. We highly recommend pre-filling gaps 1/8" and above to eliminate cracks. It will also allow your tape to lay a little flatter. If you are not pre-filling, you are going to want your mud for taping flats and butts to be a little on the thicker side. You will also want to leave a little more compound behind the tape as this will help prevent cracking. Taping over gaps with "runny" mud, or wiping you tape too tight can cause shrink cracks.
Depending on how thick the compound is right out of the box, add approximately 16-32 ounces of water for a full bucket or box. Add it a little bit at a time to see how your mud reacts to adding water. If you are pre-filling your joints with set type compound, then you can run your compound a little thinner. Try a 32 ounce cup of water to start, and you might even be able to add a little more, depending on how thick your mud was to start. This will make it easier to pump through your loading pump, and will also make it run easier in the automatic taper.
Mixing mud for taping corners with the automatic taper
When taping corners with the automatic taper, you will definitely want to address large gaps in some way, as you will want your corner taping mud to be quite a bit looser than your flat joint mud. Pre-filling the gaps with set-type compound is a pretty fool proof method for address large gaps, and you end up having dry and workable corners relatively quickly. We have also seen guys install mesh in the corner to reinforce the corner. Another popular method is to tape and roll the gapped sections prior to Taping the entire corner. This provides a second layer of tape underneath your corner tape where the gap is.
Mixing your mud loose for taping corners will allow the tape to roll tighter, allowing more compound to end up on top of the tape when you glaze, or skim it. This will make for better coverage with your corner box on the next phase. Looser corner taping mud will also allow the automatic taper to run smoother in the corner, with less drag. For mixing corner taping mud, again start with 32 ounces added in increments, and test it to see where you are at. You may want to go as much as a second 32 ounce cup of water, or more but make sure that you are using a high glue content mud, as thinning down lightweight joint compound can cause peeling and blistering issues.
Mixing mud for the flat boxes
Like mixing taping mud, how much water to add depends on what brand mud you are using, and what consistency it is out of the box. Also, with the flat boxes, the weight of the mud you are using is a factor in how much water you add. Heavier compounds will have a tendency to "fall" out of the box faster because of gravity. We recommend using a lightweight joint compound for running boxes. If you are used to finishing by hand, you will want your flat box mud creamy and workable similar to what you would want it to be in bed coating flat joints. The tension is getting it loose enough to push out of the box with ease, but stiff enough that mud isn't pouring down the walls. You could start with 16 ounces of water, and test the consistency.
Mixing mud for the angle box
Mud consistency for the angle box again depends on what type of mud you are using. The outlet of the angle box is much smaller than the outlet for the flat box, so you don't need to worry so much about mud falling out of the corner box, like you do with the flat box. When you run the angle box, you are often operating it with your arms over your head, so thinning the compound down will make the box easier to run. You can go pretty thin with the corner box mud, but you don't want to have it so thin that the mud sags if you have a section like a deep beveled edge that fills heavy. Start with 32 ounces of water added in increments and test it from there.
Mixing mud for the Mudrunner®
Mud consistency for the Mudrunner again depends on what type of mud you are using. The nice thing about the Mudrunner is that because it is piston driven, it take some of the wear and tear off of your joints. This means you can have your compound slightly thicker than what you would when mixing mud for the corner box. A consistency somewhere in between corner box mud and flat box mud is a good place to start. Mud consistency with the Mudrunner will determine how "fast" the piston pushes out the joint compound. Thinner mud will of course make the tool run faster, thicker mud will make the tool run slower.