Structural Repair: Sheet Metal, Welding and Finishing

Sheet-metal repair is investigation first and drilling second. Classify the damage — dents, scratches, cracks, corrosion, delamination — and apply the manual limits that decide between blending out, patching or replacing the part. A discovered crack is stop-drilled at both extreme ends before any repair begins, so that further growth is arrested and the true extent can be measured.

Rivet removal is centre-punching, drilling to shank size without enlarging the hole, then punching out the shank. An oversize hole requires the next rivet size up or an engineered repair. Layout respects edge distance of at least two diameters and pitch of about three diameters. A patch restores the load path using matching alloy and temper and is sealed against moisture. Every repair is mapped, measured and referenced to approved data.

Approved welding covers gas and TIG for steel tube structures and TIG or MIG for aluminium, with strict cleaning and heat control. Judge a weld on penetration, porosity, cracks and undercut — a neat bead can still be weak. Structural adhesive bonding has its own process controls and cure times.

Fabric-covered aircraft use approved envelopes or doped fabrics tensioned and sealed to the applicable data. Check strength with a punch tester, and watch for ultraviolet and chemical decay, tears and rib stitching failure.

Finishing is protection rather than appearance: strip, treat, prime and topcoat within the specified temperature and humidity limits, and observe isocyanate respirator rules. Each paint defect has a process cause — runs and sags indicate excessive film build, orange peel a poor atomisation, fish-eyes surface contamination such as oil or silicone, and blistering moisture or a chemical reaction under the film.