rust on a steel boat

[rkane]

The steel yacht that we've bought has a few superficial rust patches on the topsides. The hull (inside and out) is in pristine condition and we really want to get on top of these before they get any worse. We've had lots of conflicting advice about the best way to deal with them - does anyone out there have any definite, tried and tested advice as to whether we should rub them back to bare metal, whether we should treat therm with a rust preventer and about the best type of paint to use. We're planning to repaint all her topsides a new colour so we aren't tied to any paticular paint but at the momnt they are done in International Perfection.

Thanks in advance!

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[tcm]

rubbing down to bare metal and painting with ...anything straight away - is a good option. But I think your subtext is correct - it's a good sign if the hull inside and out is good - inside being the area where (several mates) have discovered problems. Word on the street is that steel hulled boats rust from the inside out.

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[oldsaltoz]

G'day rkane,

You should find some good advice here:-

<A target="_blank" HREF=http://www.yachtpaint.com/intropage.htm>http://www.yachtpaint.com/intropage.htm</A>

Hope this helps. . . . . . .

<hr width=100% size=1> Old Salt Oz /forums/images/icons/cool.gif Growing old is unavoidable. However, growing up is still optional.

 

[Birdseye]

I'm not an expert, so what follows is what i would do rahtre than a recommendation.

as I understand it, a bit of light tightly adhered surface rust isnt a problem . what you have to remove is any flakey rust and any salt and moisture. so lots of washing prior to wire brushing / angle grinding is called for.

next issue is to ask yourself why the rust is there. is it simply because the decks have high wear, or because there are lots of sharp angles where paint can crack, or areas where the anchor damages paint etc. do your best to get rid of as many of these probs as possible. some areas you might have to plan to put down some sort of protection as well as paint.

then go along to your industrial paint supplier - the sort of place where people making heavy external use steelwork get their paint. they have to put up bridges etc without re-painting for years, and absorbing a degree of flexing. they will probably advise an epoxy primer and poly top coat (they did with me), and it will be way cheaper than the diddly little yacht paint tins. you will have to add some non slip.

finally, recognise that you have swapped osmosis for rust. there is no permanent prevention so you will be at it again in a few years or shorter.

incidentally, the british steel challenge boats had stainless steel decks because of the problems of keeping rust at bay on a well trodden area.

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[dignity]

In my opinion it is very important to get back to bare shiny metal, removing any trace of rust, then treat with a rust preventer, then a high quality primer do not leave the bare metal surface exposed for long because the metal soon begins to rust.

I've just had a shell shotblasted, the company stated ideally the metal should be primed within 2 hours, easier said than done when you've got a 50ft hull!

Hope this is of some use.

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[AndrewB]

Agree with everyone else that if you can possibly do so, go back to bare metal. No 'rust converter' will provide more than a temporary fix - the rust will keep working beneath. And I've tried many, coming with impeccable guarantees both on the label and by people from forums like this; as well as had them applied by so-called 'professionals'.

In my experience the best tool for cleaning off small patches of rust is a heavy duty wire-brush attachment used on an angle grinder (good protective eyewear is absolutely essential). That will remove the rust without too much good steel. Beware 'burnishing' the rust, to a blue-grey colour which is distinct from the steel itself. If you do this, it must be angle-ground off. In any case, chipping and angle grinding will be needed at first if the rust is heavy. Don't over-polish the steel though or the paint won't be able to grip.

Make sure you are not upwind of any GRP boats if you angle-grind outside. The tiny specks of steel fly up and stick to their decks, then go rusty creating a mess.

Once the steel is quite clean then use epoxy quick-dry primer ASAP, to match what was probably there originally. In many ways a zinc-rich epoxy primer would be better, but these are not quick drying and more difficult to use. Five coats of primer, with a filler (e.g. Interfill 400) to smooth out the surface. Finally the undercoat and topcoat of your choice.

 

[burgundyben]

from my limited knowledge tcm is right, metal boats rot from inside out, I spose what you do depends on time and money, you could angle grind, quick splodge of paint and wait till needs doing again or go the whole sandblasting etc epoxy etc route

the only thing I can add is alluded to in one of the other posts, if you go for sandblasting you definately need to paint it asap, within a couple of hours, definately the same day as it will be bright orange by morning, some sandblasters can do aluminium spraying after blasting which is fab and gives you much more time to sort primer and paint, I've restored a few Lambrettas and always go for a gentle sand blast and alumimium coat, my first one has now been done ten years, used lots rain or shine and looks as good as when done, was painted with epoxy etch primer then polyurethane primer and topcoats.

<hr width=100% size=1>Sod the Healey - I think I'll buy an E-Type.

 

[vyv_cox]

Fully agree, Andrew. Colleagues of mine conducted research into rust removal and corrosion prevention corrosion for a tanker fleet. Very much summarised findings:
1. Grit blast to SA 2.5 using aluminium oxide grit. SA 2.5 effectively means that there is no visible rust remnant, perfectly flat, grey finish.
2. Epoxy prime immediately
3. Epoxy intercoats and top coats - almost as many as possible. Painting specs for our offshore structures call for something like three or four intercoats and two topcoats
4. No chemical preservatives, neutralisers, etc had the slightest effect in controlled conditions.

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[Birdseye]

Rust cannot continue "working away" underneath a continuous airtight solid paint coating - chemically impossible uless you have pinholes in the deck. The significance of the grit blasting is much more about providing an effective key for the paint system. The trouble with thick rust is that it traps air / water underneath the paint, you get expansion and contraction loosening the bond of the paint then cracking then water ingress etc.

The same problem applies at edges and joints. Your car doesnt rust in the middle of a plain panel - it rusts at seams, and screw holes etc. You need to get rid of as many changes of section, penetrating holes, weld spatter etc on your deck as you can. Just look where the rust is already occurring and you will see what I mean.



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[AndrewB]

Think we will have to disagree about this.

Rust on thick steel seems able to trap sufficient air and water well beneath its surface to maintain what seems like a self-perpetuating reaction. Yes, if the paint seal is perfect in theory eventually the oxygen will be exhausted, but in practice the seal is broken by the newly developing and expanding rust. Actually a good epoxy coat will hold up very well for quite a long time, and the only clue to what is happening beneath is a slight raised area which can be hard to recognise for what it is.

Rust bores down into tiny pits in mild steel, and will leave a rough surface. Grit blasting is best because it gets right down into those pits: angle grinding can't, and wastes a lot of good steel. But grit blasting is terribly messy and involves removal of all surrounding fittings. That's why I recommended wire brushing as a compromise on small spots.

Neither an angle-grinder or wire-brush should over-polish unless applied for too long, or are very worn, but this can certainly happen if someone attempts to smooth out the area with heavy duty emery paper, rather than correct the residual roughness with filler.

While you are right that seams and screw holes are vulnerable, so is anywhere where water can collect, whether these are in the middle of panels, on stringers, at joints, in the bilges or anywhere else. Behind fittings is another troublesome area on many steel yachts. Indeed, it seems like the harder a place is to access, the greater the rust problem.

I've owned an aging steel boat for 10 years, and worked on a number of others, and appreciate the pattern of rust development does tend to vary, depending on the quality of build and particularly the grade of steel. So a strategy that has worked well on one boat may not be best on another.

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[vyv_cox]

<a continuous airtight solid paint coating> This has yet to be developed. There is always some porosity in coating systems. Plenty of evidence that this is so, from the Forth Bridge to the average car. Rusting may be delayed by a variety of methods like phosphate primers, zinc-rich layers, multiple coats, but the moisture will always get through.

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[Anchorite]

The problem is 'superficial rust patches': most of the answers seem to imagine
large areas to be treated SA 2.5 etc + endless coats of paint. Usual prob is
different: how to treat a small area where you require microscopic amounts of
2 pot. I rang International some years ago to learn they have no answer - except
use Interfill. I definitely wouldn't smudge some single pot over a 2 pot finish: when
you come round to repaint, 2 pot won't stick over 1 pot (but the opposite is fine).

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[AndrewB]

Interprotect is International's 2-pot epoxy primer. But I've not found it good for touch-up jobs. Much better is Blake's epoxy primer, which most chandlers also carry. It will last several months after first opening, provided the lid is well resealed (be careful not to drip the curing agent around the lid, as it will clog up and eventually prevent the lid being replaced properly. This paint is quite compatible with International polyurethane top coats.

Several other marine paint manufacturors also make epoxy primer. Beware those where the ratio of paint to curing agent is high (e.g. 10:1) because it is hard to get the mix exactly right in small quantities. And that is always crucial with epoxy paint.

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