I have found a hairline crack between 2 rows of cinderblock on my house. Can anyone suggest what I can use to fix and fill the crack before I repaint?
Hello wmcritter, and welcome to the Home Depot Community!
Small cracks in the mortar joints are fairly common, and also easy to repair. Horizontal cracks may indicate structural problems, but “hairline crack” repair is likely just a tuck point type process.
Are there any leaks into the house? Is this a below grade repair or above grade? Finally, whether this in inside the house or on the exterior might change how I would tackle this.
For most repairs of this type, I would use mortar mix; the same material used to build the wall in the first place. You will want to grind/carve out the crack so that the new mortar will fill in as far as possible. I would use a grinder to do this outside. Inside you will want a helper with a wet/dry vac to catch as much dust as possible. Instead of a grinder you can also use an old can opener or a grout saw to do this. Just keep in mind that this will take awhile doing it by hand. The goal is to create a “V” shaped groove.
To fill in the crack, it’s handy to have a grout bag, pointing trowel and a brick jointer. The bag is not absolutely necessary, but makes it easy to shove the mortar mix far into the crack. You will also want to have a spray bottle filled with water to wet down the crack before filling it.
For below grade repairs and especially when the cracks leak I prefer to use hydraulic cement as the filler. This is made up in very small batches and filled using a trowel. For above grade repairs mortar mix will work just fine.
Finally, if this crack is really a small job with no structural issues, there is a "mortar in a tube" type product that you may find useful:
Quikrete Mortar Repair
The time you spend doing the repair will go down, but you will want this to set up completely (minimum 24 hours) before you prime and paint.
I hope this helps, and please let us know if you have any other questions.