Direct Answer
Structural steel fire protection is typically applied as spray-applied fire-resistive material (SFRM — 'spray fireproofing'), intumescent paint, board systems, or concrete encasement. The choice affects fabrication: SFRM-protected steel is typically shipped with primer only (no topcoat), and SFRM must adhere to primer. Intumescent coatings may be shop-applied. Connection surfaces that require inspection after erection must be accessible after fireproofing.
Full Explanation
SFRM (spray-applied fire-resistive material like Monokote or Cafco) is the most common system for structural steel in commercial buildings. It is applied in the field after erection. For SFRM adhesion: (1) Primer must be compatible with the specific SFRM product — confirm with the fireproofing subcontractor. (2) The primer must not be topcoated in the shop. (3) Faying surfaces in slip-critical connections cannot be coated with standard primer. Intumescent coatings (thin-film intumescents like Nullifire or Sherwin-Williams Firetex) provide fire resistance in a thin decorative coat — suitable for exposed architectural steel where SFRM aesthetics are unacceptable.
What This Means for Your Shop
The fireproofing system must be specified before you order primer. Using a primer that's incompatible with the specified SFRM creates a bond failure and requires stripping and re-priming — an expensive rework. Get the primer spec from the fire protection engineer or fireproofing subcontractor before the first blast job.
Common Mistakes
Applying red oxide or standard epoxy primer when the specification calls for a specific SFRM-compatible primer. Some SFRM systems require bare steel; others accept specific primers. Using the wrong primer causes SFRM to delaminate.
Recommended Process
Before ordering primer, obtain the fireproofing specification and confirm the approved primer products. Use only the approved primer. Document the primer type, lot number, and application in your coating records.