Repainting my lifting keel and grounding plate

[jcpa]

The antifoul and epoxy primer coat on the cast iron Grounding Plate and Lifting Keel on my Kelt 8.50 are bubbling up, and I am about to clean it all off with a nail gun and repaint. It is 6 years since I last did this, which was about the expected lifetime, given that I probably didn't achieve 2.5 surface preparation. However, some colleagues are suggesting I should apply Fertan to the bare metal first before any primer, and use Hempadur or VC-tar2 rather than Interprotect. I would be very grateful if others could give me any experience they’ve had with these products.

I will be doing the work outside (in Gosport) in the coming month, with likely air temperatures down to 5C. I will use some plastic shrouding, but I cannot keep the atmosphere warm and dry. I am not keen to do much pressure washing (e.g. to remove excess Fertan) - it will only make it all wet again. I will obviously want to apply some primer as soon as possible after using the nail gun (/needle scraper), and before any afternoon dew-fall occurs.
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[DownWest]

Doing a bit then dry it with a heat gun before appling primer and a length of B&Q tarp (as your plastic) hung from the gunnel to keep the weather off?

 

[zambant]

Investigate a primer / rust converter called Vactan.
Used it on my Jaguar 22 and it worked well......

 

[JimC]

In 25 years of owning twin-keeled yachts with cast iron keels I've tried a lot of primers and treatments for rusted iron. The best I've found is Rustoleum 769. It's intended for application at low temperatures.

https://www.rust-oleum.eu/products/view/catalog/769-damp-proof-rust-primer/

 

[jcpa]

Thank you for your various comments, and I am sorry for delay in responding. I was hoping for a larger sample of comments, particularly on (etching) pre-primers and one-part or two-part epoxy primers. But first I must describe my concerns in a bit more detail.

The boat is now ashore; I have built a tent around the grounding-plate and keel area; and I have started to remove the old epoxy coats from the grounding plate. She did not come ashore last winter, but I antifouled her between tides on a drying grid – and saw the paint on the grounding plate was bubbling off! When grounded, the keel is fully retracted, so I could not see its condition, nor apply more antifoul. Properly ashore and supported on a stack of blocks, the keel could be lowered – which showed it was bubble free – with just some small rusty areas on the tip and leading edge - which are vulnerable to abrasion when grounding (as on drying moorings)!

At this point I realised that I have never had to strip the whole keel back to bare metal (as I am now doing for a second time for the grounding plate); it has just needed some patch repairs. And while I had primed the grounding plate with 5 coats of Interprotect 2-part epoxy, I had no idea what the previous owner had used on the keel, so for my patches I used 5 coats of 1-part Primocon. This seems to have performed better than the generally preferred epoxy option.

I have discussed these things with some marine paint suppliers, and an explanation may lie with a poor bond of relatively hard/rigid epoxy paint to the shiny surface of my (over prepared) cast-iron grounding plate, as against a softer but more pliant bond achieved with the 1-part primer. Certainly, while now stripping the grounding plate, the cast iron surface under where the epoxy has bubbled up is generally smooth, shiny and rust-free. Areas between the bubbles are sometimes poorly bonded, and are easily lifted away as a rigid skin, but elsewhere are still firmly attached and need to be ground away. Any patch treatment would be a “hit-and-miss” exercise.

The paint suppliers don’t recommend rust-converters (the resulting compounds are generally water soluble, giving a possible moisture path between the underlying metal and the overlying primer), but etching pre-primers are an option. Owatrol CIP has been recommended to me, which is a 1-part paint, Alkyd resin based, and can be overpainted by 1-part or epoxy 2-part primers. Maybe this is similar to the Rust-Oleum recommended by JimC.

As water barrier primers, the suggestion is that VC-tar products are better suited to fast craft, and that Hempadur is slow to dry in cold weather. Interprotect has had most support, but Primocon might not be as prone to coming off in sheets – and might be more suited to patch treatment.

If anyone else has anything to add to these thoughts, I would be very glad to hear them.

Thanks all

 

[Seajet]

I've tried Fertan and VC tar on my ( originally galvanised ) mild steel keel, in fact I think I've tried all treatments in 41 years.

Didn't think much of either.

THE one I rave about, head and shoulders above the rest and much the same after a season as when applied, is

Dulux Metalshield.

I know Dulux sounds a joke but this stuff is brilliant, industrial.

It's one part but must go over the special one part Metalshield primer.

Doesn't like other paint underneath, it pickles so prepare to as bare metal as poss.

I find a good source is Brewers Paints.