Why would I make the crack bigger?
Because as found it is too small to fix (§2a): filler cannot enter a hairline, so surface smears fail as skins over a moving joint. A cut channel gives filler walls to bond and a shape to lock into. Counterintuitive, and the whole trade agrees on it.
Can't I just put compound over it?
You can, and it will crack again along the same line — usually within a season or two of movement. That skin failure is the exact outcome this tool exists to prevent.
The same crack comes back every winter — will this fix it?
This tool is stage one of three for a returner: open (45400), fill and iron (45450), then bridge — tape and compound, or a Wet & Set strip off the 09911 roll — because a moving crack needs its movement distributed, not just filled (§2b).
Does it work on brick or concrete?
Scoped honestly (§2d): soft, crumbling mortar at a hairline, yes — raking and cleaning. Sound mortar, concrete, stone — no ; carbon steel loses that argument, and masonry crack-chasing tools exist for it.
What's the blade shape?
Unclaimed — no packet describes the grind, so this store describes the work instead: a drawn edge that deepens a line and can angle to undercut it.
Can I use it to dig out old caulk or grout?
That is the cross-line call, and the answer is the routing sentence both stores' pages share (§5): dead caulk bead, failed grout, crumbling mortar joint → 43670; hairline crack you're about to patch → this tool, then the 45450.
Old plaster house — anything to know before I start?
Yes, and it is the page's most important paragraph (§2c): pre-1978 patching compounds can contain asbestos (16 CFR 1304 is the dividing line) and pre-1978 paint is presumed lead — test before cutting; abatement questions precede technique.
