Substrate flatness before installing a ceramic lining

Flatness is not only a global shell dimension. The ceramic assembly responds to the local surface beneath each unit, the transition between neighbouring units and the stiffness of the plate behind them. A broad panel can meet its overall dimensions yet contain a weld crown, abrupt mismatch or shallow dish that prevents uniform support. Mark the exact lining limits, because conditions outside the field may not matter while a defect at the starting edge does. Include seams, attachments, patch plates and previous repairs in the survey. If the substrate is intentionally curved, define the design radius and local smoothness rather than calling the entire surface flat.

Choose a survey method suitable for access, required resolution and geometry: a straightedge and feeler method may suit a small accessible panel, while a larger or curved assembly may need a gridded or instrumented survey. The method matters less than traceability. Identify datum points, grid spacing, tool status, surface condition and who recorded the values. Show both high and low deviations on a marked drawing rather than reporting one worst number. Repeat questionable readings after cleaning loose debris. Measurements taken over scale, weld spatter or temporary fixtures describe contamination, not the steel that will carry the lining.

Grinding a weld crown can improve local geometry but may not be permissible without engineering review, and it does not remove wider distortion created by welding. Likewise, filling a depression may provide a smooth face while leaving a flexible plate that moves under process or installation loads. Identify whether the observed condition is a surface feature, a fabrication deviation or a structural behaviour. Review support members and attachments on the opposite side where accessible. Any correction must preserve required weld integrity, corrosion allowance, pressure boundary and structural function. The lining contractor should not be left to decide those changes during adhesive application.

An adhesive layer has an approved application range and a defined role in the complete system. Using it as uncontrolled fairing material can create variable cure, trapped air, excessive squeeze-out, unsupported corners or a finished bore outside requirement. Where local filling is allowed, state the approved material, preparation, maximum lift or sequence from current instructions and how the repair will be checked before tiling. Where it is not allowed, correct the substrate by an engineered method. Record the disposition of each mapped deviation. A verbal instruction to ‘make it fit’ is not an acceptance criterion and cannot be audited after the surface disappears.

Before adhesive mixing begins, the responsible parties should confirm that geometry, cleanliness and environmental conditions meet the approved procedure. The release record should include the survey drawing, accepted corrections, unresolved restrictions, representative photographs and the time of final surface preparation. Protect the accepted surface from new contamination and fabrication work. If another trade welds, grinds or walks material through the area, re-inspect the affected zone. This hold point separates substrate responsibility from lining workmanship and preserves evidence. It also gives future failure analysis a reliable answer to a basic question: what condition was actually covered by the installed system?