Surface Preparation, Placement, and Finishing
Good shotcrete starts with good preparation. The substrate must be clean, sound, and rough enough to bond. Remove loose material, dust, oil, and debris; chip out unsound concrete; and roughen smooth surfaces. Pre-wet absorptive substrates to a saturated-surface-dry condition so they don't pull water from the fresh material, but avoid standing water. For repairs, square off edges and provide adequate depth.
Placement should build uniform thickness without voids or cold joints. Apply in layers appropriate to the orientation and mix; overhead and vertical work may require thinner lifts. Keep reinforcement properly positioned and encapsulated. Where a cold joint is unavoidable, prepare the surface before continuing. Finishing follows placement: screed or trim to line and grade while the material is workable, then float or trowel as appropriate. Avoid overworking, which draws water to the surface and weakens it. Cure properly to support strength and durability.
Traps: skipping pre-wetting; leaving dust or laitance; building too thick in one pass; finishing too late or overworking; neglecting curing; failing to prepare cold joints.
Worked example / rule of thumb: prepare the surface to a rough, clean, SSD condition, then place in controlled layers; finish while plastic and cure promptly. If a cold joint forms, roughen and clean it before resuming so the new material bonds.