If you have gaps in your mortar , you want to fix those to prevent animals and insects from getting inside, and also from water from running into the wall, because behind this is all wood and you don’t want it to rot. So what you can do is get some mortar mix from your home center, which is pretty easy to mix up - you just follow the directions. It’s just powder, you add some water to it. and if you have a different colored mortar other than your standard gray, you’ll want to go and find the cement color which you can add to it and tint it so it will match, and no one will be able to tell that you did this. <P> The next thing you want to do after you’ve gotten this mixed up properly is to wet the area because you always want to apply the patch onto moistened existing mortar. And once you’ve done that, I would recommend using rubber gloves but I don’t have any on me today but I’ve never minded getting dirty. And nothing beats just applying this with your fingertips. You’re going to just work really hard to work it in there and it looks like a terrible mess but it does clean up well. And once you’ve gotten it in there, smooth it out with your fingers and you’re done.