Bilt Hamber Dynax UB | Underbody Anti-Corrosion Wax
Why not just use underseal?
Because bitumen underseal cracks. It goes on thick, it dries hard, and the first stone that hits it opens a fissure. Water gets behind the coating, and now you have a sealed, dark, damp pocket against bare steel — which is a better corrosion cell than no coating at all. Every restorer has peeled back underseal and found a horror show underneath.
What Dynax UB does differently
UB contains elastic polymers that produce a tougher, more abrasion-resistant film than a conventional wax — but it stays a wax. It flexes. It doesn't crack, and Bilt Hamber state it won't peel. It's touch-dry rather than tacky, so it doesn't stay a dirt magnet forever, and it carries Bilt Hamber's metal-seeking corrosion inhibitors into every seam it reaches.
It's a solvent-borne aliphatic hydrocarbon system — Bilt Hamber's SDS declares C7 hydrocarbons and hydrotreated heavy naphtha as the carriers. The wax itself and the inhibitor package are not disclosed, so we won't invent them.
The numbers that matter
Target film thickness is 80–100µm — noticeably heavier than Dynax S50's 50µm, because an underbody takes physical abuse a cavity doesn't. Build it in at least two coats, 24 hours apart. One heavy coat is not the same thing and will not cure properly.
Coverage: Bilt Hamber state roughly 4 × 750ml aerosols to treat a medium-sized vehicle, or 1 litre per 8–10m² brushed or sprayed. Buy accordingly — running out halfway through an underbody is a miserable afternoon.
How to use it
Surface must be dry. Degrease first — Surfex-HD is the right tool — rinse and let it dry properly. Brush, roller or spray. Two coats minimum, a day apart. Around 24 hours to dry at 15°C.
Cold wax is thick wax. Bring it up to about 15°C for 24 hours before you start.
An honest note on the test data
You'll see Bilt Hamber's 2000-hour University of Hertfordshire salt spray result quoted across the internet for the whole Dynax range. That study tested Dynax S50 only. Bilt Hamber separately claim UB outperforms competitors under ASTM B117 conditions, but they publish no report for it. We're not going to lend UB a result it didn't earn.
Handle it properly
Aerosol: Danger — H220, H280, H304, H315, H336, H411. Ventilate, no ignition sources, wear respiratory protection when spraying overhead.
Safety Data Sheets (Australia)
Download the Dynax UB SDS – bulk (PDF)
Download the Dynax UB SDS – 750ml aerosol (PDF)