Phosphoric Acid vs. Sikaflex

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Ronald_M

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T'will soon be time to chip off flaking paint and scrape off loose rust from my Hunter's cast-iron (?) bilge keels. I believe the keels are bedded in with Sikaflex. I've had the idea of treating the bare metal with phophoric acid (Jenolite or similar), followed by some zinc based primer (Hammerite?) and a coat of epoxy-based paint. Are the keel to hull joints and their sealant likely to withstand all of this?

TIA for any advice.

Ron.
 
I had trouble getting Jennolite and Kurust comes in 250ml largest size for £12. I bought Vactan - google it for details - got mine through an ebay shop for £16 delivered for a litre. Following surveyors recommendation I then gave keel a coat of underwater primer (they used to call it keel primer) which is silvery. Antifoul on top of that. Didn't disturb anything between the keel and the hull.
 
Jenolite seems to bit difficult to get. But should be available again by November 2007.
However take a look at Jenoseel That's available in a wide range of quantities up to 205 litres
 
I don't think epoxy will work on top of anything but cleaned, dry bare metal. Certainly not anything like Hammerite or Kurust. Not sure about the phosphoric acid though. I used VC epoxy tar on the keels after cleaning off with an angle grinder with grinding disc on the worst bits and wire brush in the not too bad bits. I still get the odd spot or bubble to touch in each year though.
 
Sikaflex will survive phosphoric acid, well it has on my boat. Also, you can paint over it reasonably sucessfully, as long as there is not too much.

But as Kenjohnson says, dont use Jenolite or a paint like Hammerite under epoxy, that is a recipe for disaster. Grind or wire-brush off the rust back to bare shiny but slightly roughened metal. Then there are zinc based epoxy primers such as Skipper's Epozinc which could be used. But just a straight epoxy primer - ideally five coats - should serve fine.

However, unless you are really pernicky, it would be a lot cheaper and easier to use a conventional (non-epoxy) system on a cast-iron keel. Most people do. There have been a number of threads about this in the past on this forum, which you might turn up with a search.
 
Did the same job on a Hunter 272 some years ago. Ground keels back to bare metal and then several (can't exactly remember how many but 5 or 6 comes to mind) coats of Primacon 3 keel primer. Then normal antifoul. Result was good with no problems over next few years. Good initial preparation is the main factor. Did not disturb the keel/hull bond at all.
Hope this helps
 
Phosphoric acid will clean steel and/or cast iron
BUT even nutralizing the acid with a stong alkali results in trouble.

I've been there. Built a steel boat. Tried that on small parts which rusted twice as fast as nornal after the acid treatment.

Wire brushing, grinding and best of all grit blasting will clean to so called >white metal< which has the tooth to take the zinc rich priner. The spray on primers used for vehicles tend to absorb moisture. Zinc rich primers are expensive. I've found 3 coats of Primacon to be excellent .
 
wouldnt worry about phosphoric acid and sikaflex - but you are still unlikely to end up with keels that never blled rust through.

problem is that most keels are fairly poor quality iron castings full of porosity. salt water gets in the holes and no amount of washing / blasting / chemical treatments ever gets it all out. that said, careful work can get a reasonable finish.

personally having tried epoxy on one keel and chlorinated rubber on the other, the rubber won hands down. its what is used on big ships.
 
Phosphoric acid is much more effective, if the surface can be heated. It probably has buffers in it, to prevent it etching holes in the steel, so, if heated, it may work better diluted. It also tends to block the surface with debris, so the surface needs to be cleaned from time to time.

I am a bit surprised at the experience of ianabc, as this acid was used by the car trade to both etch the surface of metal, and to form an iron phosphate coating to prevent rust creeping under the painted surface. However, that was the days when cars rusted like hell!

Jenolite used to supply special primers that absorbed any acidity left after treatment. I do not know if these are still available, and whether they would be suitable for underwater use.

I usually grind first and rinse with boiling water when finished, so that the surface dries quickly. Not going to help much if used on thick steel.

Deeper rust has more iron and less oxygen and takes longer to dissolve.

Philip
 
Thanks for all the advice, folks.

It seems that everyone has their own favourite treatment process - I shall have to find mine!

But it is good to learn that Sikaflex appears to have the "grunt" to stand up to all kinds of abuse, phosphoric acid included.

Thanks again,

Ron.
 
Can't comment on Sikaflex I'm afraid but I would stay away from any kind of acid or rust converter on the keels as good anticorrosive primers will not work well over the top.
Your keel is cast iron, not steel, and as someone has already said it will not be good quality or a good finish.
Preparation will be very important and the one thing that really determines how long before you have to do it again!
Dry abrasive blasting is best and grinding second. Both with primer applied as soon afterwards as possible.
Wire brushing comes a poor third. It makes the rust look nice and shiny but doesn't remove it and corrosion will still be going on. It might, however, be the only way to get into the surface pits.
After cleaning brush off dust and apply a good zinc rich primer and work it well into all the surface imperfections. You will need at least 2 coats to cover properly and either more or an intermediate to build a water proof barrier. Antifouling after that.
You will never stop rust coming through on a cast iron keel but you can reduce it considerably and stop touch ups being an annual job.
Now for the commercial! We have what I consider to be the best anticorrosive primer. PM me if you would like details.
 
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