I use ankor wax from Morris as per the link above on all chassis related to vehicles that I plan to keep! Its not cheap, but you get what you pay for and frankly have you seen the cost or repairing a rotted chassis?!? It goes on easily with a paint brush.

As per my Facebook post I used the Comma Wax Oil. It was recommended to me by Tony Lloyd when I bought my Puck L from him at Eriba Caravan Sales.

I also mentioned in that post that I only needed 2 cans, what I didn't mention is that I had bought 6 cans

I have used waxoyl on my MGB with good results which I have had for the last 32 years, it does pong a bit but the smell soon wears off. I took a splash guard off the inner wing over winter and inside the cavity was still perfect with no rust with a thin film of wax rust proofing was still visible.

https://www.hammerite.co.uk/product/waxoyl/

I used a small wire brush to remove surface rust where the box sections meet the body. I then masked with newspaper and sprayed the chassis ends with Dinitrol. I was surprised by how easy a job it was and happy with the results. At some stage I’ll get under the van and do the whole chassis.