Galvanised vs Painted RSJ Beams – Which Is Better for Outdoor Use?

When RSJ beams are exposed to weather, moisture, or aggressive environments, corrosion protection becomes essential. The two main protective options—hot-dip galvanizing and protective painting—offer different benefits, costs, and maintenance requirements. This guide compares both methods to help you choose the best solution for your outdoor or exposed beam applications in 2026.

Understanding Corrosion

Why steel corrodes:

• Iron + oxygen + water = rust (iron oxide)
• Rust is porous, allowing further corrosion
• Progressive degradation reduces section thickness
• Eventually compromises structural integrity

Corrosion rates vary by environment:

• Indoor/dry: <0.01mm/year
• Outdoor/sheltered: 0.05-0.10mm/year
• Outdoor/exposed: 0.10-0.20mm/year
• Coastal/aggressive: 0.20-0.40mm/year

Over 30 years unprotected outdoor beam could lose 3-12mm thickness = significant strength loss!

Hot-Dip Galvanizing

Process:

1. Steel cleaned (degreasing, pickling in acid)
2. Fluxing prepares surface
3. Immersion in molten zinc (450°C)
4. Zinc bonds metallurgically to steel
5. Thick zinc coating forms (typically 85-200 microns)

Coating composition:

• Iron-zinc alloy layers bonded to steel
• Pure zinc outer layer
• Total thickness: 85-200 microns typical

Advantages of Galvanizing

1. Exceptional durability:

• 50-100+ year lifespan in most environments
• Zero maintenance required
• Coating thickness provides long-term protection

2. Complete coverage:

• Every surface coated (inside, outside, hard-to-reach areas)
• No missed spots or thin areas
• Edges and corners receive extra thick coating

3. Self-healing:

• Zinc corrodes preferentially (sacrificial protection)
• Minor damage self-seals via zinc corrosion products
• Steel beneath protected even if coating scratched

4. Immediate full protection: -Ready to install straight from galvanizer

• No drying/curing time needed

5. Tough coating:

• Resistant to mechanical damage
• Can be transported and handled without special care

Disadvantages of Galvanizing

1. Higher initial cost:

• £30-60 per meter additional (vs. unpainted)
• Can double beam material cost for small beams

2. Lead time:

• Requires specialist galvanizing facility
• Add 2-3 weeks to procurement
• Not all sizes/lengths accommodate every galvanizer’s bath

3. Size limitations:

• Bath size limits: typically 12-13m length maximum
• Very large beams may not fit standard baths

4. Appearance:

• Dull grey metallic finish
• Cannot be aesthetically painted initially (poor adhesion)
• Zinc patina develops over time (can be uneven)

5. Weld considerations:

• Welding after galvanizing produces toxic fumes
• Damaged coating at welds needs repair (zinc-rich paint)
• Pre-galvanizing detailed fabrication preferable

6. Hydrogen embrittlement risk:

• High-strength steels (>1100 N/mm²) at risk
• Not issue for standard S275/S355 RSJ beams

Protective Painting Systems

Multi-coat paint systems:

Typical 3-coat system:

1. Surface preparation: Wire brush or grit blast
2. Primer coat: Zinc-rich or epoxy primer (50-75 microns DFT)
3. Intermediate coat: Epoxy or polyurethane (75-100 microns)
4. Topcoat: Polyurethane or acrylic (50-75 microns)

Total system thickness: 175-250 microns

Common Paint Systems

1. Alkyd/Oil-Based:

• Traditional economical option
• Lifespan: 5-10 years outdoors
• Requires frequent maintenance
• Not recommended for structural steel

2. Epoxy Systems:

• Excellent adhesion and chemical resistance
• Lifespan: 10-15 years
• Good for industrial environments
• Can chalk/fade in UV

3. Polyurethane Topcoats:

• Excellent UV resistance
• Good color retention
• Often used over epoxy primer/intermediate
• Lifespan: 15-20 years with maintenance

4. Intumescent Fire Protection Paint:

• Provides both corrosion and fire protection
• Swells when heated to insulate steel
• Lifespan: 10-15 years
• Higher cost but dual-function

5. Micaceous Iron Oxide (MIO):

• Contains plate-like iron oxide particles
• Creates barrier protection
• Lifespan: 20-30 years
• More expensive, very durable

Advantages of Painting

1. Lower initial cost:

• DIY application possible: £8-15/m materials only
• Professional application: £15-35/m
• Much cheaper than galvanizing

2. Color Choice:

• Any color available
• Can match architectural requirements
• Easy to specify RAL/BS colors

3. No size limitations:

• Can paint any size beam
• Suitable for very long or large sections

4. Easier touch-up:

• Damaged areas easily repainted
• Site repairs straightforward

5. Fire protection option:

• Intumescent paints provide fire protection
• Galvanizing doesn’t offer fire resistance

6. Aesthetic finish:

• Smooth professional appearance
• Better for visible applications

Disadvantages of Painting

1. Requires maintenance:

• Inspect annuallyfor damage
• Repaint every 10-20 years depending on environment
• Maintenance cost over lifetime can exceed galvanizing

2. Surface preparation critical:

• Poor prep = poor performance
• Grit blasting ideal (expensive)
• Hand prep (wire brush) less effective

3. Vulnerable to damage:

• Coating can chip, scratch, peel
• Damage exposes steel to rapid corrosion
• Transport/installation must be careful

4. Application conditions:

• Temperature limits (5-35°C typically)
• Humidity affects curing
• Rain prevents work
• Delays possible in bad weather

5. Drying/curing time:

• Each coat needs drying time
• Total system: 3-7 days
• Delays installation

6. Coverage issues:

• Hard-to-reach areas may have thin coating
• Back of flanges, web-flange junctions vulnerable
• Requires careful application

Cost Comparison

Material Costs (2026 UK)

203×133×30 RSJ, 4.5m length:

Unpainted (mill finish):

• £68/m × 4.5m = £306

Galvanized:

• Beam: £306
• Galvanizing: £40/m × 4.5m = £180
• Total: £486 (+59%)

Paint System (DIY):

• Beam: £306
• Prepare + primer + 2 coats: ~£15/m × 4.5m = £68
• Total: £374 (+22%)

Paint System (Professional):

• Beam: £306
• Professional multi-coat: £25/m × 4.5m = £113
• Total: £419 (+37%)

Initial cost winner: Painted (especially DIY)

Lifetime Cost Analysis (30 Years)

Galvanized:

• Initial cost: £486
• Maintenance: £0
• Recoating: £0
• Total 30-year cost: £486

Painted (professional system):

• Initial cost: £419
• Maintenance inspections (annual): £30/year × 30 = £900
• Repainting (years 15, 30): £113 × 2 = £226
• Total 30-year cost: £1,545

Painted (DIY maintenance):

• Initial cost: £374
• DIY inspections: £0 (your time)
• DIY repainting (years 15, 30): £68 × 2 = £136
• Total 30-year cost: £510

Lifetime cost winner: Galvanized (if excluding labor value)

ROI: Galvanizing pays for itself typically within 20-25 years even compared to DIY painted

Performance by Environment

Dry Internal (not exposed to weather)

Recommendation: Either or none!

• Indoor environments: minimal corrosion risk
• Mill finish acceptable with occasional inspection
• Prime coat adequate if any protection desired
• No need for galvanizing

Winner: Mill finish or basic primer (cost saving)

Sheltered Outdoor (under eaves, canopies)

Recommendation: Good paint system OR galvanizing

Paint system:

• 3-coat polyurethane system
• Lifespan: 15-20 years before recoating
• Inspect annually

Galvanizing:

• 50+ year lifespan
• No maintenance

Winner: Galvanizing if budget allows, otherwise quality paint system

Exposed Outdoor (fully weathered)

Paint system:

• High-quality MIO or polyurethane
• Recoat every 10-15 years
• Regular inspection essential

Galvanizing:

• 30-50 year+ lifespan
• Zero maintenance

Winner: Galvanizing strongly recommended

Coastal/Marine (salt exposure)

Paint system:

• Specialized marine coatings required
• Frequent inspection
• Recoating every 5-10 years
• High maintenance burden

Galvanizing:

• 25-40 year lifespan (shorter than inland but still excellent)
• No maintenance
• Proven track record in marine environments

Winner: Galvanizing essential

Aggressive Industrial (chemicals, pollutants)

Depends on specific chemicals:

• Some chemicals attack zinc (acids)
• Others attack paint

Recommendation: Consult corrosion engineer for specific environment

Often: Galvanizing + compatible paint topcoat = best solution

Application-Specific Recommendations

Garden Structures (pergolas, gazebos)

Exposure: Moderate outdoor Recommendation: Quality paint system or galvanizing if budget allows Reasoning: Aesthetics important, paint offers color choice

Garage Headers/Lintels

Exposure: Sheltered in soffit typically Recommendation: Galvanizing preferred Reasoning: Maintenance access difficult once building complete

Balcony Supports

Exposure: Fully exposed Recommendation: Galvanizing mandatory Reasoning: Safety-critical, difficult/expensive to maintain, permanent exposure

Carport Beams

Exposure: Partially sheltered Recommendation: Galvanizing strongly recommended Reasoning: Constant weathering, wet vehicle drips, long-term exposure

Extension/Conservatory Beams

Exposure: Variableinterior, some exterior exposure at connections Recommendation: Painted if visible (aesthetics), galvanized if budget allows and hidden Reasoning: Balance aesthetics and protection

Temporary Works/Scaffolding

Exposure: Short-term outdoor Recommendation: Basic paint or even mill finish Reasoning: Temporary use doesn’t justify galvanizing cost

Hybrid Solutions

Galvanizing + Paint

Best of both worlds:

1. Galvanize for excellent corrosion protection
2. Paint over galvanizing for aesthetics

Waiting period required:

• Allow 6-12 months weathering before painting
• OR etch surface with acid wash (“T-wash”)
• Use compatible primer designed for galvanizing

Cost: Higher initial but maximum longevity (60-80 years possible)

Applications:

• High-end architectural projects
• Heritage/conservation work
• Where both aesthetics and ultimate durability required

Selective Protection

For partially exposed beams:

Galvanize critical exposed sections (ends, outdoor portions) Paint indoor/less critical sections for cost saving

Requires: Masking during galvanizing (specialized) OR welding galvanized sections to painted

Environmental Considerations 2026

Galvanizing

Pros:

• Long lifespan = less frequent replacement
• Zinc from spent coating recyclable
• Energy-intensive initially but amortized over 50+ years

Cons:

• Fossil fuel energy for 450°C bath
• Toxic acid pickling waste
• Zinc mining environmental impact

Sustainability: Improving with renewable energy use, closed-loop acid recycling

Painting

Pros:

• Lower energy manufacturing
• Water-based systems reducing VOCs

Cons:

• Frequent recoating = repeated environmental impact
• Paint waste disposal
• VOC emissions (reducing with new formulations)

Sustainability: Improving with bio-based resins, low-VOC formulations

Overall: Galvanizing arguably more sustainable due to longevity despite higher initial impact

Conclusion

For outdoor or exposed RSJ applications, hot-dip galvanizing provides superior long-term protection with zero maintenance, making it the best choice when budget allows. Quality paint systems offer lower initial cost with good protection but require ongoing maintenance.

Quick decision guide:

Choose galvanizing if:

• Beam will be fully outdoor/exposed
• Coastal or aggressive environment
• Maintenance access difficult
• Budget allows initial premium
• 50+ year lifespan desired
• Safety-critical application

Choose painting if:

• Budget constrained
• Aesthetics/color important
• Indoor or sheltered location
• Regular maintenance feasible
• Fire protection needed (intumescent)

Hybrid (galvanize + paint) if:

• Maximum durability AND aesthetics required
• Budget not constrained
• Prestigious project

No protection if:

• Fully indoor dry environment
• Non-critical temporary structure

For most outdoor residential RSJ applications (garage headers, balconies, carports), galvanizing provides best overall value considering lifetime cost and zero maintenance.

Disclaimer: Corrosion rates vary by specific environment. Consult a corrosion engineer for aggressive or unusual environments. Cost figures are 2026 UK market estimates.