Telegraphing

Telegraphing, also called show-through or ghosting, is the visible transfer of features in the substrate or underlying layers to the surface of a cured coating. Cracks, joints, seams, fastener heads, trowel marks, patches, sanding scratches and texture differences can all telegraph. The coating follows the shape of what lies beneath it, so even a perfectly applied film can reveal defects that were not corrected before coating. Telegraphing is a frequent cause of appearance complaints on floors, walls, roofs and vehicle panels.

Common forms

Crack and joint telegraphing on concrete

Cracks and control joints in a concrete slab often reappear as lines in floor coatings, especially thin films and self-leveling systems. If the crack is active, it can also crack the coating (see concrete crack repair before coating).

Patch and repair telegraphing

Patched areas have a different texture, porosity or color than the surrounding substrate. They can show as dull or glossy patches, or as outlines. This is common in walls repaired with filler and in floors patched with mortar.

Fastener and seam telegraphing

On roofs and panels, fastener heads, laps and board joints show through coatings, particularly on metal roofs and insulation boards (see roof coatings). On plywood and OSB, panel joints and edges commonly telegraph (see coating plywood and OSB).

Sanding scratch and body filler telegraphing

In automotive refinishing, sanding scratches and filler edges can appear in the topcoat, sometimes after solvents from later coats swell earlier layers (see automotive refinish coatings).

Profile and texture telegraphing

A rough blast profile, grinding marks, or a coarse texture can show through thin coatings, appearing as unevenness or gloss variation.

Fiber pattern telegraphing

On fiberglass and composites, the fiber weave can appear on the surface over time as the resin shrinks, often called print-through (see coating fiberglass (FRP)).

Causes

• Uncorrected substrate defects. Cracks, holes and joints left open or poorly filled.
• Differential porosity. Patches absorb primer and topcoat differently than surrounding areas, giving gloss and color differences.
• Shrinkage. Fillers, patching compounds or resins that shrink after the coating is applied.
• Solvent swelling. Topcoat solvents swell softer underlying layers, enlarging scratches or edges that later shrink back.
• Movement. Active cracks, panel joints and expansion joints move with temperature and load.
• Thin films. Thin coatings cannot hide substrate features; thicker or filled systems mask more.
• Lighting. Low-angle light exaggerates minor irregularities.

Prevention

Surface preparation

• Repair cracks with suitable fillers and allow full cure.
• Fill holes and pores so that the surface is uniform.
• Feather patch edges and sand smooth.
• Grind or blast concrete to a uniform profile (see concrete surface preparation).

Priming and sealing

• Prime patches and the whole surface to equalize porosity.
• Use a primer or sealer recommended for patched or mixed substrates.

Joint treatment

• Honor moving joints in concrete; do not bridge them with rigid coatings. Use flexible sealants or joint fillers designed for coatings.
• Tape and treat panel joints on wood and board substrates.

Coating selection and thickness

• Use thicker or filled coatings where a smooth appearance is critical.
• Consider elastomeric coatings that can bridge minor movement (see elastomeric coatings).
• For flooring, broadcast or textured finishes can disguise minor substrate features (see broadcast flooring systems).

Timing

• Allow fillers and patches to cure fully before coating.
• Follow recoat windows and avoid applying solvent-heavy coats over soft layers.

Repair

Repairing telegraphing usually means removing or sanding the affected area, correcting the substrate feature, priming, and recoating. Simply adding more coating may reduce visibility but often does not eliminate it, and active cracks will continue to show.

Setting expectations

Some telegraphing is normal. Control joints in floors, for example, are intentional and will typically remain visible. Specifications should state which features must be filled and how smooth the finished surface must be, and owners should understand that coatings follow the substrate (see writing coating specifications).

In practice

In practice, telegraphing is easiest to prevent at the preparation stage. Grinding high spots, filling joints and cracks with suitable products, and applying a leveling or fill coat where needed all reduce the chance of underlying features showing. On roofs and floors with moving joints, the joints should be detailed so they can move without cracking the finish. Owners should also understand that some reflection of substrate texture is normal with thin coatings.

Key terms

• Telegraphing. The appearance of underlying features, such as joints or texture, through a coating.
• Print-through. A related term, especially for fiber or fabric patterns in composites.
• Ghosting. Faint marks or shapes visible through a finish.
• Joint reflection. Lines over substrate joints that move or shrink.
• Fill coat. A leveling coat used to fill surface irregularities before the finish coat.

Frequently asked questions

Is telegraphing a coating failure?

Usually it is a substrate or preparation issue. The coating may be performing as designed.

Why did lines appear months after coating?

Fillers can shrink over time, and active cracks or joints move with seasons. Both can make features appear later.

Will a thicker coating stop telegraphing?

A thicker or self-leveling system can hide static features, but moving cracks will still show unless treated.

Related entries

• Floor coating failures
• Concrete floor coatings
• Coating fiberglass (FRP)
• Coating existing coatings
• Concrete crack repair before coating (new)
• Coating plywood and OSB (new)