At 1420 MHz (λ ≈ 21 cm, the Hydrogen line), painting a steel dish with Hammerite (or similar metal paint) makes essentially no meaningful difference to RF performance, as long as it’s applied normally.
Why it doesn’t matter much
At microwave/radio wavelengths like this:
- The RF current flows only in the outer skin depth of the metal (microns deep)
- A paint layer is typically tens to hundreds of microns at most
- Most paints, including Hammerite, are:
- electrically insulating
- non-resonant at 1.4 GHz
- effectively “invisible” to the wave
So the signal “sees” the metal underneath, not the paint.
When paint can start to matter
There are only a few edge cases:
1. Very thick or uneven coating
- If the paint becomes thick enough to introduce significant surface roughness on the scale of millimetres to centimetres, it can slightly increase scattering loss.
- This is usually negligible unless the dish is already poorly formed.
2. Conductive or metallic paints (rare case)
- Some “metallic” paints contain flakes of aluminium or zinc.
- These can slightly alter surface current distribution, but at 1420 MHz it is still usually fine.
3. Water absorption / weathering
- The only real long-term issue is environmental:
- Water ingress or cracking paint can lead to rust
- Rust does increase surface roughness and RF loss over time
What does matter more than paint
For a 21 cm system, performance is dominated by:
- Surface accuracy of the parabola (cm-scale tolerances)
- Smoothness (no large ripples or dents)
- Feed illumination and phase centre alignment
- Blockage (feed support struts etc.)
Paint is far down the list.
Practical recommendation
- Hammerite is actually a good choice: durable, rust-resistant
- Apply it evenly, not excessively thick
- Avoid leaving bare rust spots underneath (those matter more than the paint itself)
- Don’t worry about RF loss from the coating
Bottom line
A Hammerite-coated steel dish at 1420 MHz behaves almost identically to a bare metal one electrically. The paint is essentially invisible to the signal; its value is purely mechanical and corrosion protection, not RF performance.