What is Reddit's opinion of 3M Cavity Wax Plus Aerosol Spray, 08852, 18 fl oz, Self-Healing, Corrosion Protection, Non-Hardening, Chipping, Peeling, Cracking?

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Professional sandblaster and powder coater here.

For an indoor gym, you will not have any issues until long after you've stopped using it. You are fine. Unless you live on the coat, salt air is harsh.

If you wish to go overboard, you could coat the inside with a product like this.

It's a waxy oil that is meant to coat the inside of inaccessible places to prevent rust. Usually in automotive stuff. It sprays through a long straw, and has a 360 Degree fan. I have some sitting on my desk now, its good stuff. And not terribly priced when compared to getting a professional coating done.

Cavity Wax

Won’t 100% eliminate spreading because you cannot stop it from both sides of the panel without a proper repair but will absolutely slow it down.

There should be drainage holes on the back side. You will use cavity wax with the long wand and get in the channel and hose it down.

3M Cavity Wax Plus, 08852, Self-Healing, Corrosion Protection, Non-Hardening/Chipping/Peeling/Cracking, 18 fl oz https://www.amazon.com/dp/B0741FHDPF/ref=cm_sw_r_cp_api_i_28NN5KAKBGVSTY7CFZFC

3M Cavity Wax Plus Applicator Wand Kit, 08851, 360-Degree Nozzles, Corrosion Protection, Collision Repair https://www.amazon.com/dp/B07B4DFTXF/ref=cm_sw_r_cp_api_i_YDJXS6APH98019558PJR

I am of the opinion that this is not an issue worth worrying about. The amount of rusting that will happen is minimal and unlikely to matter inside your lifetime. Unless you don't seal it well and live on the coast, then all bets are off.

As to answer your actual question, I would suggest leaving some access holes in strategic locations based upon your design. And using a product to coat the inside of the tubes. I would suggest 3M Cavity Wax. It is a wax like spray. You attach long tubes with a 360 Degree spray at the ends. This coats all surfaces with a wax-like coating to limit oxygen exposure.

If you're looking on amazon, the Wax is about $23 USD, and the wands are about $20.

If it helps you any. I run a powder coating/sandblasting shop for a living.