How to Seal the Underside Plywood of Hallmark Passport Trailer

Updated 04/02/2026 | Published 03/29/2026

Question:

Hey guys. Love your stuff and web site! Doing some trailer maintenance and am thinking about the underside of the trailer. Its an enclosed Hallmark Passport, 8.5x22 i made up as a racing car hauler and occasional camper! Im doing stuff like insulation in prep for an a/c unit which Ill be getting from yall soon. But now Im working on getting it all sealed up. Is there anything youd recommend for the underside of the floor? I just had to replace all the ply in the ramp rotted out from a leak somewhere and Id like to avoid that with the floor. Its in pretty good shape but a once of prevention...you know the drill! Any product/process you recommend or just leave it?

asked by: james d

Expert Reply:

Hi James, really appreciate you saying that, it means a lot and we definitely couldn’t do this without great customers like you. It sounds like you’ve got a pretty awesome project in the works, and I can completely see why you’re thinking ahead here, especially after dealing with that ramp. That’s one of those lessons you only want to go through once.

I’ve seen a lot of trailer owners on forums mention fully sealing the plywood with plastic or completely encapsulating it, and while that sounds like a great idea in theory, it can actually backfire. I had a customer share that they tried sealing everything up tight, and what ended up happening was moisture still found its way in but couldn’t get back out. The plywood ended up rotting from the inside out, so definitely something to be cautious of.

A better approach is protecting the underside while still allowing things to breathe a bit. Along with undercoating options like part Tekton 44 or even a Rust-Oleum undercoating, some people also use a marine-style bilge paint with a proper primer first. We currently do not carry the undercoats but they are available online. That can give you really solid moisture protection while still being more forgiving than completely sealing everything off.

Are you mostly using the trailer in wet conditions, or is it more of a dry climate setup?

expert reply by: Heather A

James D.

4/2/2026

Hi Heather! So I've heard the same thing on the trapping of moisture. Sounds reasonable...the underside of the ply floor seems to have been treated with something, probably from the factory. It's holding up so far, so with the thought in mind of trapping moisture I might just roll the dice and leave it as is. To your question: here in TX it's mostly dry, and if raining it's only sitting in the outdoor storage park so the water is probably not an issue. I rarely tow when it's raining but I have been caught in a few storms on the way back from a race. I'm probably overcomplicating this and it's all fine. Eventually the floor will need to be replaced, but hopefully not anytime soon!

Heather A.

4/2/2026

@JamesD It sounds like it should hold up just fine in that dry Texas heat. Those factory coatings are built to handle the occasional rain, so you’re in a good spot. You’re not overthinking it, just staying ahead of things! Now you have me curious about your race car.. what kind of car is it?

James D.

4/2/2026

Oh, nothing fancy. I race amateur with the SCCA. It's a Mazda Miata, or at least it used to be! Spend WAY too much money on it! In the paddock, having a cool trailer is almost as important as having a good racer! :>)

Heather A.

4/2/2026

@JamesD I just looked up some SCCA races and that looks pretty legit! I’ve heard great things about Miatas too, especially how fun they are to drive. Hobbies can definitely get expensive, but they’re worth it for the enjoyment and a nice break from the everyday routine. My family would say I spend too much money at the antique mall so hey.. we all need our outlets!