It certainly is tempting. You have tile on the wall with broken grout and need to patch it up. Removing and replacing the grout will take all weekend. A tube of caulk costs $5, and you could probably cover those gaps in 15 minutes. So can you caulk over grout? It sure would be easier.
While caulking over grout is a fairly common quick fix, there are several reasons not to recommend it. It seems to work sometimes, but when it doesn’t work, you end up with two problems.
The problems with caulking over grout
Caulk and WD-40 may have been my dad’s favorite quick fixes. Caulk has its uses. But this isn’t the best one. The major problem with caulking over grout is that caulk doesn’t adhere all that well to grout. It stays on the wall, but it doesn’t seal tightly with the grout. The gaps allow water to water get in behind the caulk and mold grows. Now you have two problems.
Sometimes it takes weeks. Sometimes it takes years. And there’s no way to know which it will be. I’d be more inclined to do it if I knew it would last years, but there’s no way for me to know that.
The problem compounds when you go to remove the old caulk. Some of the old caulk doesn’t come out, making it even harder to get a good seal the second time. So the second repair takes longer than the first and is likely to develop mold sooner.
While some people report doing these fixes and having no problems, it’s risky. If it works, it saves you time, but if it doesn’t, you’ll have to come back, remove all the old grout, and regrout.
But there’s an aesthetic problem too. Unless the grout is white, you probably won’t get a good match with your caulk. You can get caulk in black and several shades of brown and tan, but those caulks are meant for windows and siding. That means the colors are a much better match for windows and siding too. When you use them with grout, they’re probably going to stand out.
Getting the versatility of caulk with the look of grout
You can buy caulk grout, which is caulk mixed to have the color and texture of grout so it matches your grout. This isn’t intended for patch jobs, however. It’s meant for the gap between the tub surround and the first row of tiles. This is where cracks tend to develop first, from the stress of the tub taking on water. To use it, fill the tub with water, then apply a line of caulk grout into the gap. This way you have enough material to stretch far enough without cracking. It’s a good preventative measure, but it doesn’t help much once you already have the problem.
Or you can cheat and use white, and then you can use white silicone caulk, which is cheaper and stronger. White goes with everything. It’s a little less forgiving than a sand color when it comes to hiding dirt, but cleaning caulk and grout is easier than replacing it. I don’t especially enjoy either of them, but I’d rather clean it than replace it.
The easiest way to solve this problem is to remove the old grout using a carbide blade on an oscillating tool. Then you can float new grout in between the tiles. In areas where cracks are prone, such as against the tub or where the wall meets the ceiling or another wall, you can use caulk or caulk grout for extra flexibility. Ideally, use white grout so you can use white silicone caulk for maximum durability and mold resistance.