preparation of interior rust on steel hull

Discussion in 'Metal Boat Building' started by owensp, Oct 26, 2010.

  1. owensp
    Joined: Oct 2010
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    Location: Ireland

    owensp Junior Member

    I have some interior rust problems to sort out. Can anyone give some advice on the preparation of the metal for repainting. The rust appears in the anchor locker, on parts of the ballast-insulation boundary, under the heads and in the engine room.
    I have been told that the way to deal with this is to chip away at the rust, and then paint with a suitable primer, covering this with a polyurethane paint finish.

    Is this correct ? Any advice on this would be appreciated,

    Regards

    Peter Owens
    Galway , Ireland
     
  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Is it scale or surface rust. If it is scale, you may have a structural problem. Removing the visible rust will be the least problem. Sandblasting and needle scaling are the two fastest and most complete ways to remove rust.
     
  3. owensp
    Joined: Oct 2010
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    owensp Junior Member

    Hi gonzo,

    i think its surface rust , surveyor (survey done a few weeks ago) never mentioned anything about scale and didnt think that there was structural problems. Perhaps you could explain that one a bit more. also some of these interior areas are difficult to get to and i thought that the following procedure would be enough:
    chip/grind/scrape off surface rust, then use phosphoric acid to convert any rust in pits etc, wash with water, dry and paint with primer. can you explain more about needle scaling?

    thanks
    Peter
     
  4. NZ_Shipwright
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    NZ_Shipwright Junior Member

    Hi , Dont hit anywhere with a grinder as you dont want to reduce plate thickness. Use a chipping hammer to remove scaling then use a wire cup brush fitted to your grinder.This should have an outcome of returning the steel to a gunmetal colour. Then Phosphoric acid , clean off after a couple of hours then wire brush and apply the acid again. Clean and paint with your paint system.
     
  5. owensp
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    owensp Junior Member

    Thanks NZ_shipwright, thats the most definitive answer i have got so far. A few questions to add: Do you use neat phosphoric acid or dilute. By clean off do you mean wash with water?

    Regards

    Peter Owens
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    If you are painting it with one-pack I would recommend use of Penetrol as a primer, even straight over light surface rust. Much better than fish oil preps. It is very effective stuff based on my experience.
     
  7. NZ_Shipwright
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    NZ_Shipwright Junior Member

    Hi, Yes sorry clean with water. Unsure about dilution ratio but it comes at around 40-50% from supplier already to go. Its called "Rust and scale remover" any fishboat guys over there will point you in the right direction. Dont breath the fumes it scars your lungs.
     
  8. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Phosphoric acid should be diluted to about 15% with water. Always add acid into the water slowly to prevent explosions. Fish oil is technology from the twelve century. Penetrol is an oil and not compatible with any modern coating.
     
  9. Landlubber
    Joined: Jun 2007
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    Landlubber Senior Member

    from the Dulux protective coatings sheets:

    What Are Rust Converters?
    “Rust converters” are acidic solutions (usually tannic acid and/or phosphoric acid based) that are intended for direct application to rust-covered steel to convert the rust (hydrated ferric oxide) into inert, insoluble products.
    So Why Mechanically Clean When You Can Convert Rust Easily?
    The suppliers of these “rust converter” products claim that there is no need to remove the rust, potentially saving the applicator a great deal of work in removing rust mechanically. These suppliers also make the claims that the conversion products adhere tightly to the steel to form a protective barrier that prevents further corrosion and also provides a suitable surface to paint.
    But, according to Standards Australia’s “Guide to the protection of structural steel against atmospheric corrosion by the use of protective coatings”, AS/NZ2312:2002, “There is a considerable amount of published literature which refutes such claims, stating that where any conversion may occur, complete penetration of rust and reaction with it is unlikely.”i Standards Australia declares that with some ‘rust converters’, only a colour change occurs, whilst in the case of phosphoric acid-based rust converters the phosphoric acid has little or no reaction with hydrated ferric oxide under normal conditions, and that unreacted acid can become trapped beneath subsequently applied paint.
    The Standard states that the use of a ‘rust converter’ with any coating systems quoted in AS2312 Table 6.3 (coating systems for direct application to hand-cleaned and power-tool cleaned rusty steel) is not recommended, and will detract from their subsequent performance.
    How Well Do The Conversion Products Adhere To The Steel?
    Rust converters are not film formers – they cannot seal or bind porous, loosely adhering rust on steel. Rust converter suppliers claim is that wetting agents in the rust converter will penetrate into the rust and that the conversion products firmly adhere, but this does nothing to reduce the porosity of the converted rust. The conversion products may be harder and more cohesive, but anything that was loose before conversion will still be loose after conversion. The porosity of the surface also inhibits complete removal of any residual (unreacted) acid.
    And herein lies a problem – how do you know how much “rust converter” to apply to a rusty surface? Assuming that the solution can penetrate 100% into the rust (unlikely), how can one calculate the correct spreading rate, or measure degree of conversion? Too much “rust converter”, and you will have residual acid on the surface; too little, and not all the rust will be converted. Residual acid is not visible or easily detectible, so it is difficult to tell if you have rinsed off all the excess acid.
    Conclusion
    The protection of steel against corrosion depends very much on the degree to which steel has been cleaned of corrosion products and profiled correctly to take a high-performance zinc-rich primer. There are no quick and easy shortcuts to achieve this. Surface preparation must be by means of abrasive blast cleaning or power tool cleaning to achieve the appropriate standard (Please refer to Dulux® Protective Coatings Tech Note 1.1.2.)
    i Standards Australia AN/NZ2312:2002, “Guide to the protection of structural steel against atmospheric corrosion by the use of protective coatings”

    ....now what I do

    Try a product called Fertan, I have used it for years , it is certainly a simple product to use, the other alternative is to use proper blasting, to white metal, sort of not likely in your case.
    It is a phosphoric acid type material, but it uses different technology to straight acid. It takes up to 2 days to completely work, turning rust to a black substance that can be washed and then painted.
     
  10. NZ_Shipwright
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    NZ_Shipwright Junior Member

    Hi, We have tried different potions over the years Fertan is one of them. We get the sales reps from all over telling the story about how "their" new product will work better. There is absolutely no way a "rust converter" like these will give you long term results. Exactly what Dulux are saying . There is no easy simple way regardless ,if some one tells you this just walk away. Fertan and any other rust converter is a lazy way of hiding the rust, long term results require a bit more work , unfortunately.
    It does not make sense to put a poly vinyl/water based rust converter under zinc
    primer and 2 pack epoxy .
     
  11. michael pierzga
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    michael pierzga Senior Member

  12. apex1

    apex1 Guest

    Fertan (ferrum/tannin) I used for preparation of th hollow spaces in oldtimer cars, with outstanding positive results, already in the early 80ies.
    It provides a corrosive resistant thin film on the metal, and holds the next coat of spray wax perfect.
    But it is not a so named "rust converter" and it does not make sense to use it in such application.

    Blast or needle to bare metal and go for epoxy primers and coatings. That is the only lasting method in such case. Have done it since decades on our museums ships, with rather good results.

    Regards
    Richard
     
  13. Landlubber
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    Landlubber Senior Member

    You are right of course Apex, there is only one way to do a job, and that is properly.
    Sandblasting to white metal is the proper way, however sometimes that is simply not possible because of circumstances, and the Fertan treatment will certainly hold the condition till the job can be done "properly".
     
  14. NZ_Shipwright
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    NZ_Shipwright Junior Member

    Hi, Apex you would improve your results on your Museum ships if you followed our procedures. By applying the Phos after needle gun, buff with cup brush . Repeat twice. Apply zinc primer. The Phos etch'es the steel gives a great base for zinc also. Just using a needle gun then priming is a waste of time as you are just hammering the rust in to the plate , Phos then buff it out/off is the go. Done correctly like this lasts a heap longer than any other method apart from blasting.
     
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  15. apex1

    apex1 Guest

    Perhaps you would improve your results when applying our system?:)

    That was not right that I said "blast or needle", you are right. I should have added, that the needle job can need further treatment. A wire brush for example, when Phosphor is not the choice. When done with the right equipment, you can do a clean job with needle guns though. (not hammering the rust into the surface)

    We blast to SA 2,5, then primer and coat. Needles are useles on hundreds of m² steel. But we use it on small areas.
    In the case we discuss here, it is just a small area and the needle and etching will get a sufficient result, agreed.

    Richard
     
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