antifouling a steel boat

[stephen_warby]

Hi,

Our boat is coming out of the water for the first time in 3 years (terrible I know)
Short of sand blasting (can't afford this time) how should I go about it. It's antifoul over epoxy over zinc primer. Any clever ideas for that nasty rusty spots I know she'll have?

 

[Guest]

I would start with a real good powerwash, followed by some heavy duty sanding on any rust spots...and extending the sanding out a little bit from the rusty areas, then use a very good zinc type of primer....allowed to dry out properly (two coats) followed by an appropriate number of coats of an epoxy barrier coat over the zinc primary.....then your anti-fouling.

If you are using a cuprous oxide paint.....make sure you use a minimum 6 (six) coats of epoxy barrier coat.. more than that is better. If you are using a non-cuprous paint as in an ablative.....you shouldn't need as many epoxy barrier coatings.

You say you can't afford to sand blast, thats all well and good. I know how much the goodies to recoat your boat properly would cost here in the US as I work for a major marine chandlery company....... But I also know you can't afford to shortcut on your repairs to your bottom

Reality is the cause of all stress!!

 

[PaulJ]

I would agree with most of what you have said but I am not sure about the zinc primer. The advice I received from Jotun when coating my boat was that the epoxy should either go straight onto bare steel or onto an EPOXY primer. I know that it used to be "good practice" to spray zinc onto the steel prior to applying other coatings but as I understand it, the development of modern epoxies has made this unnecessary. I think I would powerwash and sand back as suggested followed by several coats of epoxy (depends what you are using and how thick it is) and then some kind of tie-coat depending on what type of antifouling you are going to use.....

 

[AndrewB]

Be careful with applying epoxy direct over zinc primer. You will find the epoxy doesn't hold too well.

I assume that like me you have an epoxy base which is now starting to fail. What I do to avoid sandblasting is to grind and then wire-brush out all rust spots. It is important to get the rust out of those tiny pits at the base of the spot - beware of burnishing the rust. Then apply zinc primer immediately. Leave it on a day or two, sand it back virtually to bare metal and then build up epoxy primer in the normal way (I use 5 coats), using a filler for a really immaculate finish. The remaining traces of zinc primer seem to inhibit re-rusting better than following the usual practice of applying an immediate epoxy primer.

It is also possible to get zinc-based epoxy primer (through specialist suppliers - not retail). I've tried this but not found it any better than the above.

You can expect rust spots around the waterline if the original primer is failing, but I would be surprised if there is much trouble deeper down. If there is, it may be the result of electrolysis rather than simple rusting.

 

[stephen_warby]

Thanks to all,

I know this always boils down to a lot of work.With regard to the cuprous oxide paint why do I need to put so many layers of epoxy over it?

 

[PaulJ]

Number of coats? I think it depends on what type of epoxy you are using and the build of each coat. What is important is the total film thickness which of course is difficult to measure without specialist equipment. You could do worse than to ask the advice of manufacturer of whichever product you decide to use.....

 

[stephen_warby]

Do you mean "when all else fails, read the instructions?"

Thanks for the advice, it will fun to muck around with the boat for a week or so anyway, get me out of the office.

 

[JEG]

Has anyone tried 'hammerite' instead of all this fancy anti-fouling/epoxy stuff?

 

[vyv_cox]

International's steel primer looks very like Hammerite to my untrained eye. I used it on a keel but achieved very indifferent results. Although it is easy to blame the paint, the truth is that it is more likely that the preparation is the real cause.

I had my keel grit blasted and epoxy primed something like 6 years ago. I then applied about three-four coats of a coal tar epoxy (VC-Tar). The VC-Tar has become rather brittle and some small small areas have now flaked off. However, the majority of the keel is still coated and rust free. The boat has been in the water for all except one winter since coating.

About 10 years ago some colleagues ran a research project on every available method of rust treatment and preparation. Steel plates were sanded, blasted, chemically cleaned, painted with all sorts of phosphates, chromates, zinc compounds and other wonder stuff. Each was then painted at the same time and using the same paint systems, then exposed in a salt spray cabinet and on a raft belonging to the paint testing facility in Poole Harbour. The results showed very conclusively that:
1. Grit blasting is miles better than any other preparation method.
2. Aluminium oxide grit is better than any other
3. Needle gunning was next best, but nowhere near as effective
4. Wire brushing is less good than grinding and sanding
5. Not one of the chemical treatments was worth buying.

This will doubtless bring a flood of postings from supporters of various types of chemical paint. I can only present the results found in objective testing. I can accept that there may have been some small improvements in some of the products, but I doubt that anything earth shattering has emerged. My employer uses thousands of gallons of the best paint systems available, i.e. epoxies, to protect sea-going vessels and structures, for which they employ quite a few paint specialists. These experts continue to advise according to the above.

 

[AndrewB]

Thanks - excellent posting.

Its very helpful to have such a well-informed posting on this subject.

I've experimented with many methods on my yacht with results which seem to be in line with what you say. The phosphoric/oxalic acid treatments are not only ineffective but in time will attack paint overcoats. I found Hammerite totally useless.

Isn't there a difference though between ships and yachts, in that the plate thickness on ships is large enough that the question of really effective recoating doesn't apply? As long as the rust is kept reasonably under control after the original coating, the plate will not rust through in the normal working lifespan on the ship. With the plate thickness used on yachts however, controlling rust effectively is an absolute priority.

The key to successful recoating is to remove every trace of the original rust. One point though on which I possibly disagree is that I would NOT recommend flat sanding, for three reasons. (1) It will never remove the microscopic deep pits of rust that lie at the base of spots. (2) It is too likely simply to burnish the rust. Then even grit-blasting won't always remove it. If the rust does get burnished in the process, that's the one time I use acids, as they quickly loosen the surface again. But all trace must be removed before repainting. (3) If the steel is polished smooth, the primer won't stick to it so well.

 

[Guest]

Re: The priming of steel...

I stand corrected on the use of zinc chromate......I was thinking of Pettits methods for painting metals in a saltwater environment. If you go to http://www.yachtpaint.com you can check out the different priming methods as recommended by Interlux/International paints. They break their website down based on the country you are in so that you can see what is accepted in your home country.

Here in the US they recommend using Primocon as the second step in the priming routine. Then comes the epoxy barrier coats.

The reason I suggested at least 6 coats of the epoxy barrier coat is if you are using the Interlux/International 404/414 epoxy barrier coat. Each coat after drying is only about 3 mils thick.....and you need a good build up of barrier coat to prevent the cuprous oxide in an anti-fouling paint from interacting with the steel of the hull and causing corrosion.

Actually, conventional wisdom is that you never use a cuprous oxide paint on a metal hull. But it can be done with "proper and thorough preparation". A paint like Trilux would be better for a sailboat with a metal hull.

Reality is the cause of all stress!!

 

[castaway]

I read this with interest Ive always suspected that grit blasting was the only way to clear all the microscopic rust particals. If done then its worth using epoxy finishes.
If not, well its not worth wasting fancy materials as its all going to come thro from underneath eventually anyway. Rust never sleeps.

What about Fertan?. Ive used it on several occasions and its a splash it all over material, but does it have any scientific support???

I understand it contains tanic acid and that the old ship wrights kept their tools in boxes with oak chippings in, as this also contains the same stuff. and apparently prevented rust forming. Mind you neutralising existing rust is another thing.

Nick