Ignoramus26696 wrote:
I made this boat trailer guide:

http://igor.chudov.com/projects/Trailer-Guide/

I need it because we frequently go boating on a river with very strong
current, which makes keeping the boat aligned over the trailer
difficult during retrieval.

The base is made of 3/8" thick hot rolled mild steel. The bolts are
half inch bolts.

How do I paint it to prevent corrosion?

Personally I would degrease it with white spirit, apply a coat of red
oxide primer, then two coats of a decent quality topcoat such as
Rust-Oleum or Hammerite. I think both Rust-Oleum and Hammerite say you
can apply them without a primer, but I don't. I find they adhere better
(especially to aluminium and zinc) if you use a primer. I've also used
something called "pylon paint" in the past, but I don't know its
official trade name as I didn't buy it myself. It's literally the stuff
the electricity company use to paint pylons. I think it might also be
known as "cold galvanising paint". This is very weather resistant and
you can use very thick coats, but the surface finish is rough and it
looks ugly. Good if you don't view the paintwork too closely! Whatever
you choose, remember to repaint the item as soon as you see rust. This
is where most people go wrong.

Best wishes,

Chris