Cracks telegraphing: repair options

Cracks telegraph through epoxy because the slab moves and the cured film cannot. The repair is not another coat over the line; it is opening the crack, cleaning it, filling it with a semi-rigid epoxy or polyurethane, grinding the patch flat, and coating over a stable surface. A topcoat applied over an active crack splits again in the same place.

What telegraphing means

Telegraphing is the visible transfer of a substrate defect through a coating. A crack in the concrete shows as a line or a ridge in the epoxy, even when the film itself is intact.

The film is usually not the problem. It is thin, rigid, and bonded, so when the slab opens or closes by a fraction of an inch with temperature or load, the film has to move with it and splits along the weakest line.

That is why the repair targets the crack, not the coating.

Crack types and what they mean

A structural crack with vertical displacement is not a coating problem. It needs an assessment of the slab before any cosmetic repair, because a filled and coated crack over a moving foundation will reopen.

The repair sequence

1. Open the crack with a grinder or a crack chaser to a clean, V-shaped channel about 1/4 inch wide.
2. Vacuum the channel and remove all loose material and dust.
3. Fill with a semi-rigid epoxy or polyurethane crack filler, working it into the channel.
4. Let the filler cure per the product data sheet.
5. Grind the filled area flat with the surrounding concrete.
6. Clean and coat the repaired area with the base and topcoat.

The opening step matters. Filling a tight hairline from the surface leaves a weak bond along the sides, and the filler pops out under traffic. The channel also gives the filler enough depth to flex with the slab instead of sitting as a thin cap on top of it.

Filler choice

A rigid epoxy patch is strong but brittle. On a crack that moves, it can crack again and telegraph through the new film.

A semi-rigid polyurethane filler flexes slightly with the slab and is the usual choice for active cracks. It bonds well and tolerates the small movements that cause telegraphing.

A cementitious patching compound is the wrong product for a coating job. It shrinks, it is porous, and it does not bond to the coating the way an epoxy filler does. On a garage floor, a cement patch in a crack often shows as a light line through the finished coating.

When a crack isolation membrane is worth it

On a slab with several active cracks, or a crack that has reopened after a previous repair, a crack isolation membrane across the line separates the coating from the movement.

The membrane adds a line item and a step, but it is cheaper than a second repair. It is most often specified on garage floors with post-tension slabs or slabs on expansive soils, where movement is expected rather than exceptional.

A membrane does not fix a structural problem. It manages movement at the coating level, and it should be specified before the base coat rather than added afterward.

Why a recoat alone fails

A recoat over a telegraphing crack bonds to the old film on either side of the line, and the crack continues to move underneath. The new coat splits along the same line, usually within the first heating and cooling cycle.

Worse, the old film may already be debonded along the crack, so the new coat has nothing solid to hold. That is why the repair has to go back to concrete at the crack, not to the old coating.

Cost of the repair

A localized crack repair is usually priced per linear foot, with a typical range of $10 to $30 per foot for routing, filling, and grinding, depending on access and the number of cracks. A 20 foot crack at $20 per foot is $400. Where the cracks run across a whole garage, the total can exceed the cost of the coating itself, which is why some owners choose a membrane system or a different finish entirely.

A full floor recoat after crack repair is the coating rate: on 400 square feet at $8 per square foot, 400 × $8 = $3,200. Grinding at $2 adds 400 × $2 = $800, for $4,000.

Compare that with leaving the cracks and recoating anyway: the recoat costs the same and the cracks return, which means paying twice.

What to ask a contractor

Ask how the crack will be opened, what filler will be used, and whether a membrane is included. Ask whether the crack is active, and how that was determined.

Ask, too, whether the slab has a moisture issue. A crack is a path for water, and a wet slab under a repaired crack can produce blisters along the repair line.

For the prevention side, see cracks telegraphing prevention. For what the defect looks like at each stage, read cracks telegraphing warning signs, and for the cost arithmetic across repair routes, check the cracks telegraphing cost guide.