How should I protect my steel hull from rust?

[Wotname]

Coal tar paints are not universally available (or are restricted to industrial uses) in some places due to it's reported toxicity and potentially it is carcinogenic.

Otherwise it is good stuff!

[MicHughV]

True, for many years coal-tar was used to treat timber pilings, etc along the waterfront in various places, where it got a bad rap....justified or not I could not say.

I do know that it is still commercially available.

[skipperpete]

Wottie’s 100% correct, tar epoxy is ugly stuff. Yes it works as advertised, bullet proof, practically impossible to over coat and finally, very very hard to blast off if it needs to be removed. It gets as hard as glass and at an hourly abrasive blasting rate, hugely expensive to remove….. chlorinated rubber was another of the big money earner paints for gritblasters because …. It was rubber!!, same as the hoses the grit came through and the boots the operator wears. Yes it definitely works on timber, trawler owners were in the habit of getting the hull whip blasted then sprayed with 2 coats of tar epoxy and one of Jotun Vinyguard while the tar was still uncured. Great paint if you appreciate the potential consequences and do the best possible prep blasting.

[Bowdrie]

In the old days, for both steel and wooden fish boats, it was not uncommon to heavily coat the wood/steel bilge/keel with a non-hardening tar and then pour in cement and trowel it to leave a smooth easily cleaned fish hold.

[MicHughV]

I am retired now, but am a marine engineer by profession.
I've been involved in countless port and harbor projects where the steel sheet pile used to line the edges of a terminal are ALWAYS coated in coal-tar epoxy. Same goes for the steel tie-back rods that support the sheet pile.
Bollards and cleats that hold a ship's mooring lines are also coated in the stuff.
Same holds true for various and sundry ship loading ramps.

Almost every tug in the world is coated with the stuff.

In Florida, there are many, many coastal communities along the ocean, many protected by coal-tar coated steel bulkheads.

Same goes for many marina bulkheads.

Railroad track wood sleepers are usually always coated in the stuff.

Etc, etc, etc.

Not sure where exactly line gets drawn here ???

[RaymondR]

I discovered that one of the problems with coal tar epoxy was that because it was thick and hard rust could start underneath it and progress until serious metal loss had occurred whilst remaining invisible under the coal tar coating.

I discovered this when careening and observed a rust pit which turned into a large hole when I dug into the pit with an awl and a large hole appeared. Fortunately it was just above the waterline and amenable to my emergency repair by blind riveting a piece of 1/8" aluminium liberally coated with Sikaflex over the hole.

One of the things I learned from all those years of steel boat maintenance was that designing a steel boat with longitudinal stringers should be a gailable offense under any proper justice offence.

[MicHughV]

Back in the day, nautical architects that took a stab at designing steel sailboats, didn't really understand the material they were dealing with, so steel boats ended up being designed as wood boats....ie....frames, longitudinals, etc. Add in the fact that, that most earlier designs were hard chine, which added yet another bar to the mix.

The reality is that steel is so strong, and the welded shape is so strong, that frames, longitudinals, etc are not really needed, except maybe for the keel area, etc.

Hull plating thickness is another variable. While 1/8" plating has ample strength, invariably 3/16" plate was used as it was less prone to heat distortion while welding.

I could have easily removed 1,000 lbs of weight from my boat had I know then what I know today.

[skipperpete]

Originally Posted by MicHughV

Back in the day, nautical architects that took a stab at designing steel sailboats, didn't really understand the material they were dealing with, so steel boats ended up being designed as wood boats....ie....frames, longitudinals, etc. Add in the fact that, that most earlier designs were hard chine, which added yet another bar to the mix.

The reality is that steel is so strong, and the welded shape is so strong, that frames, longitudinals, etc are not really needed, except maybe for the keel area, etc.

Hull plating thickness is another variable. While 1/8" plating has ample strength, invariably 3/16" plate was used as it was less prone to heat distortion while welding.

I could have easily removed 1,000 lbs of weight from my boat had I know then what I know today.

Absolutely correct about construction and frames. The requirements for commercial boats were well established over many years of boat building and I suspect that there was some sort of correlation to the very old techniques and scantlings of carvel timber construction. The steel plating , frames, stringers and bulkheads were checked for compliance during construction by a qualified AMSA surveyor ( not a yacht surveyor) even the lengths of stitch welds were checked and for bulkhead integrity these were fully welded hence the “hungry cow “ look of so many commercial builds.
At some point the steel hull yacht designers realized that “eggshell” design was immensely strong and with computer cut plating all that was needed came down to a wooden frame to support the precut shell plating for tack welding. All that remained to do was chainplates, bulkheads and tabs for the furniture.... no pesky hidden places under frames or stringers where the grit blaster couldn’t reach and no burnt paint along stringer runs from stitch welding.
RaymondR is spot on about the hidden corrosion beneath tar epoxy, that’s why the prep had to be of the highest quality..most of the civil structures that were coated with tar epoxy were abrasive blasted off site under perfect conditions and painted within 30 minutes of blasting , often much less, by pro painters with airless guns with wet and dry film thickness inspections throughout the process..... that’s the main reason the coatings lasted so long. Oh, I forgot to mention that tar epoxy was the cheapest of all the industrial coatings. Micaceous iron oxide, chlorinated rubber, epoxy zinc and vinyl were all more expensive but remained the favorites in the shipping industry.

[MicHughV]

Too true...
When I had my boat blasted and painted, it was done by "pro's". A 2 man team. I helped where I could, which was mostly adding sand to the hopper or sucking sand out of the keel cavity while inside
They would blast for around 20 minutes, then stop and paint.

All told, blasting and painting my 38' hull and deck took a full 5 days to do, from sunup to sundown.

To add injury to insult the whole operation was done at the height of summer, wearing full protective gear. It was a merciless .

[Bowdrie]

For a lighter note on preserving steel.

https://www.youtube.com/watch?v=BjqURwMNbH0&t=10s

[cgood]

Owned a steel hull for 15 years.
Get the bible.
Metal boat maintenance, Scott Fratcher

[Yojimbo]

Corten Steel, specified as A588. Someone please correct me if I'm mistaken. My experience with A588 [Corten is a manufacturer's trademark name] has been architectural. Also referred to as "Weathering Steel" is produced to rust to a certain extent and then cease further oxidation. Similar to the oxide surface of uncoated aluminum.

Seems that removing the rust surface of A588 would defeat its purpose and promote further oxidation, further reducing the steel thickness. If the above assumptions are correct would it not be wiser to use a coating system that would adhere to the oxidized A588 surface and allow the steel to perform as it was designed to do?

My library is in storage so I don't have access to my metal boat building texts but iirc there was a designer/naval architect in the 70s/80s that promoted the use of A588 for just this reason.

[gulfislandfred]

Epoxy Tar for steel!

tar/epoxy products are the standard industrial coating for steel that will be immersed in salt water.
If you have access to a wholesale paint distributor, you can get the industrial product for a lot less money than other epoxy coatings (the tar part is a lot cheaper ingredient than epoxy resin, although most coatings have a lot of cheap filler in them, even if they look like resin)
Tar Set is one brand. I'm told that Rustoleum make an epoxy tar product.
It hardens into a surface like hard rubber.
I use epoxy tar on cast iron sailboat keels, both fixed and swing types. When I fish for rockfish, here in the Salish Sea, I sometimes rub my keel on the rocks a bit.
At the next haulout, I can see where the keel touched rocks. With epoxy tar, the surface will be smooshed (to use the technical term), but, the tar is still on the metal.
Epoxy tar takes bottom paint well. It's hard to get it perfectly smooth. Not something I worry about, but there are instructions in the literature to thin and spray it, so I think it could be thinned and brushed on in several coats to make it smooth.
I have used epoxy tar over the black oxide that results from phosphoric on rust, with good results. I rinse where the acid has been, to be sure there's no acid left on the surface.
Like most epoxies, tar epoxy sticks well to a damp surface, but I would do my best to dry the surface if possible.

[P3sailor]

If you don't want to deal with some rust, your best option is to take your boat to the desert. Steel boats will rust, never seen one that doesn't have some rust stains somewhere, it's the nature of the beast and a salt water environment expedites the process.

[Tillikum]

One comment: steel is porous and absorbs moisture from the air. Whenever I paint a steel or iron surface I always play a torch over it to heat the surface enough to drive the moisture out; and remain hot enough to keep it out until I get the coating on. When the pores of the metal are not full of water they can fully absorb whatever coating is being applied, which IMHO can only mean better adhesion.

I've had people tell me this is just condensation from the torch gases etc. All I can say is pour some molten babbit metal into a cold bearing shell and see what happens. Just don't have your face over the pour because the moisture in the pores of the bearing shell will vaporize with explosive force blowing the molten bearing metal onto the ceiling, if there is one.