When buying threaded rods, hex nuts, bolts, or washers, one of the most important decisions you'll make isn't the size or grade — it's the finish. The finish determines how well your fasteners resist corrosion, how long they'll last, and whether they're suitable for the environment they'll be used in.
At Northern Fasteners we stock all three of the most common metric fastener finishes: Bright Zinc Plated (BZP), Hot Dip Galvanised (HDG), and A4 Grade 316 Stainless Steel. Here's exactly when to use each one.
Bright Zinc Plated (BZP)
BZP is the most common finish for general-purpose metric fasteners. A thin layer of zinc is electroplated onto the steel surface, giving a bright, silvery appearance and a reasonable level of corrosion protection.
When to use BZP
- Indoor applications — workshops, warehouses, domestic construction
- Light outdoor use in sheltered or dry conditions
- General fabrication, engineering, and assembly work
- Where a clean, bright finish is preferred
When not to use BZP
- Exposed outdoor environments subject to rain and moisture
- Marine or coastal locations
- Contact with treated timber (preservatives accelerate corrosion)
- Underground or buried applications
BZP offers a good balance of cost and corrosion resistance for everyday indoor fastening. The zinc coating is typically 5–8 microns thick, which provides useful but not indefinite protection once damaged or worn.
Hot Dip Galvanised (HDG)
Hot dip galvanising involves immersing steel fasteners in molten zinc at around 450°C. This creates a much thicker, tougher coating than electroplating — typically 45–85 microns — that bonds metallurgically with the steel surface.
When to use HDG
- Outdoor structural applications — fencing, gates, agricultural buildings
- Construction and civil engineering
- Contact with treated timber (ACQ, CCA preservatives)
- Marine-adjacent environments
- Underground or buried applications
- Any application where long-term corrosion resistance is critical
When not to use HDG
- Where the thicker coating would affect thread fit — HDG nuts may need running down the thread before assembly
- Direct, continuous saltwater immersion (use A4 stainless instead)
Always match your finish throughout an assembly — use HDG nuts with HDG threaded rod for consistent protection.
A4 Grade 316 Stainless Steel
A4 stainless (Grade 316) contains molybdenum in addition to chromium and nickel, giving it significantly better resistance to chloride attack — the mechanism that causes standard stainless to corrode in marine environments.
When to use A4 stainless
- Marine and coastal environments — boats, harbours, sea walls
- Chemical processing and food industry applications
- Swimming pools and wet rooms
- Medical or pharmaceutical environments
- Any application requiring long-term corrosion resistance without maintenance
When not to use A4 stainless
- High-temperature applications above 300°C
- Where cost is a primary concern — A4 stainless costs significantly more than BZP or HDG
Quick reference guide
| Application | Recommended finish |
|---|---|
| Indoor general use | BZP |
| Outdoor sheltered | BZP or HDG |
| Outdoor exposed | HDG |
| Structural / construction | HDG |
| Contact with treated timber | HDG |
| Marine / coastal | A4 Stainless |
| Chemical / food environments | A4 Stainless |
| Swimming pools / wet rooms | A4 Stainless |
