Rust prevention

[lathamj]

I am planning on building a barge type boat with steel plate.

I was hoping to use a box steel frame and weld the plate to that frame. I am wondering what is the best method of preventing rust between to two surfaces (frame and plate) over long term.

I am new to the forum and impressed by the level of information. I daresay I will be a regular as my project progresses.

Thanks in advance.

[Dutch Peter]

Serious rust will only devellop in wet surroundings, so keep these things in mind:

- parts open to the elements, continous welding
- inside watertanks, continous welding

For the rest, a good paint system will do the job

[MikeJohns]

Dont do it.

Condensation will form and linger in every nook cranny and paint maintenance in a steel boat is more important inside than out, double skinned areas will corrode.

the RHS (rectangular hollow section) will often rot from the inside and the area between the RHS frame and the hull will rot through. I have seen and reported on many boats that have failed due to this construction method.

Then you have the added fun of re framing as well as replating.
Use T sections of equivalent section modulus. Or even flat bar frames.

Whatever you do dont use RHS.

[FAST FRED]

"Or even flat bar frames."


These are surly the best from a maint point of view.

Remember every 10 years or so the ENTIRE interior will need to be removed , every square inch sand blasted to "water white" and another 7 to 10 coats of paint sprayed on.

Might pay to learn to weld alluminum?

FAST FRED

[Eric Sponberg]

You want to wash the metal surface with Chlor-Rid before painting, and again after priming. Go to www.chlor-rid.com. Chor-rid is a watery solution that combines with the salts on the metal surface and removes the oxygen in those salts. Metal needs oxygen to rust, and so if you remove the oxygen, you kill the potential for rust. It also lasts a pretty long time. We are washing the metal on the Moloka'i Strait vessels with Chlor-rid when necessary to kill the rust. It really works.

Eric

[MikeJohns]

Eric
I had a quick look at the site.
Do you know what the active chemical is? is it just another phosphoric acid rust converter?

I've always have excellent internal results simply using a cold galv primer as the first coat before good mastic epoxy top coats. Some of these jobs are 20 years on and still no problems. Even where there is available oxygen below the paint, once it is used up no more corrosion occurs providing the paint seals it off. This has worked well on shoreside cranes that we maintain too.

We have seen a lot of claims for sandblast free paint systems that have never worked well over the years and I get a bit shy of wonder chemicals.

In reply to Fred
Flat bar gets too unwieldy in large boats you can considerable reduce the mass by using a T for the same strength. Also access behind the frame is ok when it's a big boat the frames are larger too.

[Eric Sponberg]

Mike,

Unfortunately, I do not know what the active chemical is--I moved about a year ago to Florida here, and I think my Chlor-Rid manual is still packed away somewhere. I am still looking for it, and it might be able to answer that question. Chlor-Rid was developed for the US Navy who uses it a lot. It has only recently become commercially available.

Certainly, careful priming and overcoating with mastic epoxy is a proven method that works. As you know, the success is in the care of details--get every little spot in the boat so that there are not tiny bare areas open to the atmosphere.

Eric

[J Ralph]

what would you guys suggest for a steel dragging frame on a boat that is pretty well rusted up.It isn't going to get sandblasted,so is there a coating that will "kill" the rust,or can it be chipped off and painted and be expected to last?Am just curious if there is something new out there to save having to sandblast the whole frame...thanks for any info you guys may have

[MikeJohns]

J Ralph

You'll probably find that underneath the rust scale there are numerous deep corrosion pits, nothing will "kill" this rust, its gotta go.
These should be chipped out till you get bright steel. You may find that a good rotary wire brush will clean things up enough to attack it and finish off with small sharp cold chisel. Hard to say without seeing it. Might be easier to chop out the more corroded parts and weld in some replacements ?

[J Ralph]

thanks for the reply....there is probably130 feet of 3inch square tubing in the a-frame....it's about 10 years old,and it probably isn't feasable to try to chip away that carefully...I was thinking of banging off as much as I can with a hammer and painting it with either zinc cromate(spelling?)paint or the stuff they use for truck bedliners....I can't imagine it would make it any worse but who knows

[PADDLEGUY]

I Have Used Zinc Chromate On Aluminium, After Etching Of Course. But Not Real Sure How Good A Stand Alone Protection It Would Be On Steel. It Goes On Very Thin, And Will Scratch Easily. My Thoughts Would Be Something With A Heavier Tougher Film Thickness. Now, About The Bedliner Coating On Steel. Has Anyone Used It On A Steel Boat? I Think It Would Be The "bee`s Knees" For A Deck Coating... Experience Or Thoughts?

[Eric Sponberg]

Another product you might try is POR 15. This is a thin but rubbery coating, dark gray, that we use in the bilges of the Moloka'i Strait motoryachts to protect the steel hull. If a steel boat is going to rust, it is on the inside and in the bilges. We put on 2 coats and leave it alone. I don't know how well it would work if you overcoated it with a topcoat. Incidentally, POR stands for "paint over rust". I am not sure where we buy it, but you might find it by googling it on the internet.

Eric

[J Ralph]

thanks eric .....I found por 15 on the web and it sounds like just what I'm looking for.After a month in the boatyard west-systeming my rails,front deck and chafing on the hull...extending the cabin,new windows..painting my dragging frame will be an easy chore.Hope to west system the whole hull in the spring...any experence with west system?From what I've seen so far with it,it's amazing.I put a patch up over my prop,and along the garboard and it's been there for over 4 years and looks great. J

[Eric Sponberg]

WEST System, in my opinion, is the best boatbuilding epoxy going. And apparently, you have favorable results.

Eric

[J Ralph]

a buddy of mine just got done west systeming his 40 foot wood dragger and it came out fantastic.He probably has 3/4 to 1 inch thick at the keel and maybe 3 to 4 layers up on the sides.He used bi-axle(spelling?) cloth throughout and he must have used at least 70-80 gallons of resin.His boat is about 40 years old and it has been sister ribbed so many times that there is literally no more room for any more ribs.He did all the work alone and did a hell of a job....hoping mine comes out as good.mine is a 47 foot white cedar planking so I think it should stick alright.thanks for your reply and if you have any advice on doing a boat this size I would be all ears ......thanks again J