I built a steel ketch
back in the late 70's /early 80's.
Protecting the steel was a big issue for me.
No question, # 1 priority is providing a clean surface for the paint to adhere.
This means sand blasting to white metal. An onerous project
by any definition.
After blasting, I painted the interior
of my boat with a two part coal tar epoxy. This stuff was thick as glue and strong as steel and required 12 gallons to paint the inside of a 38' boat. I'm not sure if this is still available to the general public, but the big ship chandeliers may carry it. It's wonderful stuff. In areas where you can't get too, you can just pour it in...I had some areas, which could not be reached by blasting so I simply just poured the coal tax epoxy in there...and sealed whatever gap there was...
I believe a one part coal tax epoxy is also available, but I have no experience with this product.
The outside of my boat was done with a whole paint scheme. I used a company called Devoe. They can provide you with everything you need. This will start of with a zinc primer, several base coats and then the finish coat. Besides Devoe, there are other paint companies, that can provide the same thing, but I used Devoe and found their product to be very good and easy to apply.
I can tell you, that for as long as I owned that boat, almost 15 years, I never had a single
rust problem of note. That boat is still around and still in very good shape, almost 40 years old now.
Once in a while, I would drop something and chip the paint. To patch this area, I would first wipe with a product called Ospho, this will neutralize the rust on the steel, and then I would hit the area with a zinc primer and paint.
When I didn't have Ospho around, I used West System epoxy...marvelous stuff and will cling to anything, including rusty steel. After drying, a quick sand and paint.
The whole objective is to keep moisture, especially salt
moisture away from the steel.
99.99% of steel boats will have rust problems on the INSIDE, where water
, moisture, etc, will find the smallest unprotected steel component and rust away, often out of sight.
When I sold my boat, the new owners had the hull
"sounded" by a surveyor
using an ultrasonic device. Nothing out of the ordinary was found anywhere, but I mention this as a tool you could use or have someone do it for you, if you have concerns about the steel thickness anywhere.
Steel is inherently extremely strong, so a little bit of surface rust is generally not a structural issue.
Finally, to overcome my paranoia about dropping stuff on my deck and chipping the paint, I had my entire deck, cockpit
, etc, covered with " decktread"....this is a 3/16" thick high grip material that can be purchased in sheets
which you can cut to suit and glue in place. I used epoxy glue. The "decktread" can be purchased in a variety of colors.
On my boat I had stainless steel
stanchions welded to the deck. On a long ocean voyage, where I had constant spray washing
over the boat, the stanchion/hull weld would bleed a little rust. Though the s/s stanchions were welded on with s/s welding rod, there were some areas where the paint system allowed some micro sized rust bleeding. This was more a pain in the rear end than anything else, but on arrival at my destination
, I would wash this off.
Like others on this thread, I would sometime use a dremel tool fitted with a metal burr, to clean up a rust spot before using the ospho, etc...
Hope this helps provide some guidance..