Trailer Undercoating:
Stop Frame Rust
on Any Trailer

Why trailers rust faster than trucks

• They sit in the spray zone. The trailer tongue, front crossmembers and fenders take the full blast of whatever the tow vehicle's tires pick up, including gravel and brine. The front wall of campers and enclosed trailers gets chipped the same way a hood does; our rock chip protection guide covers that side.
• Open channel and tube frames trap debris. C-channel and tube frames collect mud and salt on their inside faces, where nobody washes and paint is thinnest.
• Thin factory finishes. Many trailers are painted or powder coated once, with little or no primer inside the tubes. A chip from gravel goes straight to steel.
• Long idle periods. A trailer parked on grass or gravel for months stays damp underneath, which keeps any salt residue active.
• Salt water immersion. Boat trailers get submerged, and salt water inside tubes and around axles, springs and brakes keeps corroding long after the boat is on the road. Rinsing with fresh water as soon as possible after a saltwater launch is the most important habit.

How long galvanizing lasts on a trailer

Hot-dip galvanized trailers are protected by a sacrificial zinc layer that slowly corrodes in place of the steel. ASTM A123 sets minimum zinc thickness by steel thickness: 75 microns for structural shapes from just over 1/8 inch up to 1/4 inch, and 100 microns above 1/4 inch. How long that zinc lasts depends on the environment. The American Galvanizers Association lists typical zinc loss of 0.7 to 2.1 microns per year in urban (C3) settings, 2.1 to 4.2 in high-corrosivity settings with substantial chloride (C4), 4.2 to 8.4 in very high (C5) settings, and 8.4 to 25 in extreme (CX) settings with extended wetness and chloride deposits. A trailer that sees road salt or regular saltwater launching sits at the high end of that scale, which is why galvanized boat trailers still rust out at the axles and crossmembers.

Trailer undercoating material, weight and cost

Enter the deck size and how much of the underside you want covered. The estimator works out the area, how much oil or polyurea it takes, how much weight a polyurea coat adds to your payload, and a cost range. Coverage factors are our rule of thumb for typical trailer frames; product figures are from published data sheets and cost guides.

How we undercoat a trailer

1. Strip it to what you need to coat

   Pull the deck boards or mats if the floor underside is getting coated, and remove fenders if possible. Unplug and protect wiring, lights and brake magnets or drums.

2. Clean and assess the steel

   Pressure wash with a degreaser, then let it dry. Probe crossmembers and the tongue for thin or perforated steel and weld in repairs before coating. Coatings do not add strength.

3. Blast or grind, then prime

   For polyurea on bare or painted steel, abrasive blast or grind to clean metal with a profile, then prime the same day with a compatible ArmorPrime primer. On galvanized trailers, follow a galvanized-specific prep instead of blasting through the zinc.

4. Mask the moving and hot parts

   Axles, hubs, bearings, brakes, spring shackle pivots, the coupler, jack, safety chains and lights stay uncoated.

5. Spray the polyurea

   A plural-component rig sprays ArmorLiner polyurea to the specified thickness on the frame, crossmembers, floor underside, fenders and tongue. It sets in seconds, so trailers usually go home the same day.

6. Oil what the coating cannot reach

   Fog the inside of tubes, spring packs and hardware with an oil or lanolin product once the coating has cured. See our oil undercoating guide for products and amounts, and our rust proofing guide for a yearly treatment schedule.