How should I protect my steel hull from rust?

[Mick of 42]

I've just bought a 40 foot Blue water monohull.
I had a plastic boat last time and wonder where I can get good advice on protecting my steel hull from corrosion.
Any tips out there?
Prep and epoxy?
Prep and cold gal?

[skipperpete]

there are many paths to take depending on how bad the rust is and where it’s located. In the worst case scenario it’s necessary to crop and replace the plating in the area where the wastage has occurred but there are some fairly good prep techniques and treatment systems involving local abrasion, rust converters and epoxy coatings. Abrasive blasting on the external plating is the best option if available, either a full blasting job or spot blasting but not easy to find yards that allow this procedure.

[Kettlewell]

Abrasive blasting on the external plating is the best option if available, either a full blasting job or spot blasting but not easy to find yards that allow this procedure.

Might want to talk to smaller commercial yards, like ones that deal with fishing trawlers. Some will take an interest in your boat and they have the equipment and the knowledge to deal with steel, like they do every day.

[sailorboy1]

I didn't read the OP saying he had rust, just how to protect.

Zincs and electrical types of protection I believe are the choices

[MicHughV]

I owned a steel boat for over 10 years.
I had a big 12" x6" x 1" thick sacrificial zin bolted to the hull.

Never had any issues with rust or corrosion, other than patching up nicks on the deck from time to time.

Most steel boats rust from the inside out. If the steel has been properly sand blasted and painted, regular inspection and some maintenance will serve you well.

You can buy a metal thickness gauge online for around $300. Press this against the hull and it will read the metal thickness.

You don't say much about your boat....age ...etc...so difficult ascertain what your concerns are.

[Icarus]

Originally Posted by Mick of 42

I've just bought a 40 foot Blue water monohull.
I had a plastic boat last time and wonder where I can get good advice on protecting my steel hull from corrosion.
Any tips out there?
Prep and epoxy?
Prep and cold gal?

To keep a steel boat in good shape...don't neglect scratches or tight spots..stringers can be a problem...make sure any water can drain into the bilge..limber holes..

Industrial Two Part Epoxy metal primers, high build coatings and polyurethane top coats work well and last for years.

Timber on metal on deck is not a good idea...

A die grinder is helpful

Electrolysis, incorrect wiring and neglect are your enemy

I built a dozen steel and alu yachts...if they were built correctly they last a very long time

[skipperpete]

Originally Posted by sailorboy1

I didn't read the OP saying he had rust, just how to protect.

Zincs and electrical types of protection I believe are the choices

The OP didn’t need to say he had rust, it’s a foregone conclusion that he has, it’s probably just not visible yet. Inside stern tubes, chain lockers, under stitch welded stringers and frames. Everywhere a stainless steel item is welded to a mild steel hull there’s a battle to keep the corrosion at bay.

[MicHughV]

A point that often escapes most people is that seeing rust in of itself is not a reason to run for the hills.
While unsightly, it's not necessarily the end of the world.

Various google searches will demonstrate than even if left completely exposed, a 1/4" thick plate (around 6mm), can last for 50-70 or more years.

A sonic sounder is your best friend here as you can measure the thickness of steel plate in any location you want.

Lastly, even if a 1/4" thick steel plate was rusted down to half it's thickness, 1/8", you would still have plenty of residual strength left in the material.

There are many modern products available that can clean and protect steel.

Various studies on rusting steel plate have been done.
I've seen such testing grounds, where dozens and dozens of steel samples were left out in the open with various paints or other treatments were done.
These samples were continuously monitored and measurement over the years.
Metal loss in steel plate is well understood and documented.

Steel boats often get a bad rap, when rust stains, etc mar the hull or elsewhere, by people that know little about the subject.

For anyone with globe girdling in mind, a steel hull is often the # 1 choice.

[Mick of 42]

Originally Posted by MicHughV

I owned a steel boat for over 10 years.
I had a big 12" x6" x 1" thick sacrificial zin bolted to the hull.

Never had any issues with rust or corrosion, other than patching up nicks on the deck from time to time.

Most steel boats rust from the inside out. If the steel has been properly sand blasted and painted, regular inspection and some maintenance will serve you well.

You can buy a metal thickness gauge online for around $300. Press this against the hull and it will read the metal thickness.

You don't say much about your boat....age ...etc...so difficult ascertain what your concerns are.

The boat is circa 1990 Corten.
There is some scale inside and it was recently scanned and overall the thickness is good.
There are just so many methods recommended.
I have purchased a Torrcoo Blaster to remove the scale where possible and then will use a rust converter.
I have had good results with structural steel (not on boats) and cold gal paint.
It seems to prevent further rust and doesn't seem to chip but I haven't used it in a salt water application.
Good old red lead paint seems like another option.
Thoughts?
Thanks for your comments everyone.

[hpeer]

Sooo many methods, sooo many opportunities opinions. I find sometimes I am successful with most anything, sometimes almost nothing seems to work.

A lot of my approach tends to be geared toward the conditions, of the boat, of weather, my schedule.

I buy 6 packs of POR, they are 3 or 4 oz in size. I keep Ospho on board. I will treat the area with Ospho, then clean that up, then treat again, until I am not making good headway. Then paint with POR, 3 or more coats. This is a rather convenient method, it is a one part paint, the small cans are about right for smaller jobs. POR will need a UV top coat, but that can come when it makes sense.

Nothing is perfect. Just keep at it.

[Mick of 42]

Originally Posted by hpeer

Sooo many methods, sooo many opportunities opinions. I find sometimes I am successful with most anything, sometimes almost nothing seems to work.

A lot of my approach tends to be geared toward the conditions, of the boat, of weather, my schedule.

I buy 6 packs of POR, they are 3 or 4 oz in size. I keep Ospho on board. I will treat the area with Ospho, then clean that up, then treat again, until I am not making good headway. Then paint with POR, 3 or more coats. This is a rather convenient method, it is a one part paint, the small cans are about right for smaller jobs. POR will need a UV top coat, but that can come when it makes sense.

Nothing is perfect. Just keep at it.

Sorry, I live in the rest of the world and I'm not familiar with "POR" or "Ospho" are those proprietary product names?

Thanks for your comment.

[skipperpete]

Originally Posted by Mick of 42

The boat is circa 1990 Corten.
There is some scale inside and it was recently scanned and overall the thickness is good.
There are just so many methods recommended.
I have purchased a Torrcoo Blaster to remove the scale where possible and then will use a rust converter.
I have had good results with structural steel (not on boats) and cold gal paint.
It seems to prevent further rust and doesn't seem to chip but I haven't used it in a salt water application.
Good old red lead paint seems like another option.
Thoughts?
Thanks for your comments everyone.

Corten is a difficult alloy and despite the frequently voiced claims that it’s highly corrosion resistant, this has not been my experience having owned one Corten yacht and been involved in repairing 3 others. Something to watch out for is plate thickness, Corten hulls were often built with 3 or 4 mm shell plating due to the high strength of the alloy but that left a very narrow margin for wastage plus it’s a bit different to weld, you need to be aware that while mild steel rods will weld it Ok, you should go for a compatible electrode. We can’t get redlead paint any more in Australia but you’re right about it being a good primer...... the world has changed a lot since the days when it’s use was commonplace. There’s a rust converter that was used with great results, a milky looking liquid that turned black after it had cured, best applied to a slightly damp ( but not wet) surface. It could be overcoated with practically any primer and was about the best of all the offerings on the market (phos acid and tannin based ones were definitely less efficient), here in OZ it’s still available at the local Bunnings hardware store. I think the US destroyers are built using thinner hi tensile steel rather than thick heavy mild steel so it does have a good following.

[hpeer]

Ospho is just phosphoric acid. The phosphorus replaces the O2 in IronOxide changing it to a black brittle and inert material. The reside can mostly he brushed and washed off. That which cannot can be primed. In heavy areas use multiple coats and give it time. It is sold in many guises often described as a rust CONVERTER. It can be spray, liquid or gel. Just look for Phosphoric Acid in ingredients.

POR (Paint Over Rust) is an American product, it is a very thin one part urethane. This is a technique used when working with less than PERFECT surfaces.

The general gist here is to have a primer that is sufficiently thin to penetrate into the wee cracks and crevices to seal out any O2. A variety of products will do this.

AMERON sells a thinned watery epoxy for this express purpose. But any penetrating epoxy will do. Or you can further thin a penetrating epoxy or sufficiently thin a regular epoxy. West System advises to work the regular epoxy in with a wire brush. They are all trying to do the same thing, encapsulate the converted rust so that no O2 gets in.

I find directions and instructions between manufactures of Rust Converts inconsistent. Some say wash only, some say wash and brush, some say do nothing. After a massive paint failure the most in-depth explanations I have read universally state that NO coating is 100% waterproof, all will eventually pass some water. The problem is worse when there is osmotic pressure across the coating. In short, residual acid under the paint will increase osmotic pressure and cause premature failure. My case was startling. Once burned and all that.

Epoxy’s are 2 part and you can mix as much as you need. West System pumps are helpful. But it is messy and you need thinner.

Another primer series high content aluminum dust in a very thin one part urethane. This is sold in the USA as Pettit RustLok and is widely available. There are other brands. Others and I have used it on props. You end up with an aluminum finish, which can be difficult to keep a top coat on. Comes in quart cans. If the top is not ABSOLUTELY PRISTINE you will never get it off again. We have reverted to transferring it to Ball jars, or canning jars with 2 piece tops.

Back to POR, same as the rest it seeks to keep O2 off the underlying steel or residual. Same as RustLok the cans are VERY difficult to reopen. Because they sell it in 4oz 6packs I can store it indefinitly and then use a small can for a project. Much less waste and mess. Decent results.

ALL urethanes want a minimum of 2 coats, I frequently do 4. A PITA (Pain In The A).

I find I can extend urethanes working time considerably by storing air tight and cooling. So I put the lid on loosely and then put a latex glove over the can, maybe use an elastic (rubber band, hair band) to seal the glove over the can. Between coats I stick the disposable brush inside another latex glove. I then put them in the fridge near the condenser and pray it does not spill. I can frequently get a week of storage out of a can this way, sometimes more.

[s/v Jedi]

It is called POR15 and it’s very good. Won’t come off your fingers either.
https://por15.com/collections/3-step...ventive-system

[hpeer]

Surface Prep
IMHO, being no expert…

Another thing to consider is how the pain/epoxy will adhere to the steel. All these coating adhere Mechanically, that is to say there is no chemical reaction between the coating and the steel. Phosphoric Acid IS a chemical reaction but is NOT a coating.

For paint or epoxy to adhere there need to be sufficient surface area and roughness. The rougher the surface the larger the contact aeea. If the surface has over turning edges, like a curling wave, the coating can get underneath and be locked in. I believe is why West Systems recommends wire brushing through the epoxy, it forces the coating into the crevices and increases the mechanical strength of contact.

So anything you can do to roughen the surface is a plus. Run a bit or 40 grit paper over it. I think the Roto Blaster does a good job because it makes little wave like curls upon impact. A grinding wheel lightly applied. A flap disk, coarse grit, slow speed and moved quickly. I would NOT use a wire wheel, its finish is too smooth.

The time when I would use a wire wheel is when applying a coat of epoxy to force the relatively thick epoxy into the existing roughness. Just enough and no more.

Ameron told me to be careful to not apply the paint too thick. The Rep said the paint shrinks while curing, if too thick it builds up enough pressure to either destroy the bond to the metal or to lift off the underlying coat. I believe I have seen this effect.

This may be why a lot of the urethanes stress to apply in THIN layers. I know that POR, a urethane, if too thick as in a puddle, will actually foam. I suspect that is off gassing during cure getting trapped in the thickness of the paint.