Rust reformer on cast iron - will epoxy stick?

[ohthetrees]

I'm in the middle of dealing with paint failure on my cast iron keel. I'm in Panama, and they don't have sand blasting in this yard, so I'm looking for a "good enough" approach until I can do it right in a more fully equipped yard with proper sand blasting.

More here:


Keel paint failure. Hull is fine. How to proceed? - Cruisers & Sailing Forums




The approach I went with was to grind down to bare metal where ever there was signs of rust or failing paint. There is some texture to the casting (which had been covered with a layer of fairing compound), and some crevices where grinding and wire brushes couldn't get every trace of rust, so I decided to treat with rust converter. The only brand I could find locally here in Panama was Sur Corrostop.


The plan from here is to use a 2 part epoxy primer (5 coats), then underwater primer over that and all the areas of the keel that I didn't grind back, then bottom paint.


The problem is the rust converter. It has left a shiny residue that I'm suspicious of. I'm worried that it will act like a release agent and prevent the epoxy primer from bonding properly to the metal. I've included photos of some drip marks left from the rust converter that illustrate the sort of residue it leaves behind.



What do you think? Am I better off leaving the rust converter in place, or going through some sort of solvent/mechanical process to get it off? Remember this whole thing is supposed to be a stop gap solution for 1-2 seasons until I can do a total redo of the keel with sand blasting equipment.

 

[greeny]

I've epoxy primed and epoxy filled on top of similar rust converters over the years. Like yours, the ones I've used leave a shiny surface. the instructions have said that it should be left in place as a primer and painted directly over. I tried that and the rust still came back over the next couple of years, under the primer and filler. The paint didn't fall off, it just rusted underneath and bubbled and showed through. I think the best you can hope for is that it make it cosmetically good and will slow down the rust until you can get at it by blasting etc.

 

[ohthetrees]

I've epoxy primed and epoxy filled on top of similar rust converters over the years. Like yours, the ones I've used leave a shiny surface. the instructions have said that it should be left in place as a primer and painted directly over. I tried that and the rust still came back over the next couple of years, under the primer and filler. The paint didn't fall off, it just rusted underneath and bubbled and showed through. I think the best you can hope for is that it make it cosmetically good and will slow down the rust until you can get at it by blasting etc.

Thanks for that input. I'm glad that at least this rust converter is like others you have worked with. Bought in Panama, I wasn't really sure what I was getting. I guess I'd be happy if it makes it two years.

 

[Tranona]

Not sure the expense of epoxy is worth it in these circumstances. International Primocon ot Jotun Vinyguard smlmarinepaints.co.uk/primers/single_pack/vinyguard if you can get it would be a cist effective way of keeping the rust at bay. Epoxy really only works if you can blast clean.

 

[sawduster]

We ground a cast iron keel back to the metal with power tools, used fertans then overcoated with a zinc rich epoxy primer. Well adhered and no bubbling after 3 years so going well so far.

 

[dankilb]

I think the OP is right to ask this question. Once going to the effort of treating the keel at all - I personally don't think having a layer of some sort of rust-coverted oxide (I'm guessing?) between the metal and the primer is the best solution.

Of course the rust converters claim to create a primed surface, otherwise nobody would use them. But having epoxy coated both blasted and converted cast iron surfaces, I think there's a big substantive difference between how the blasted surface (keyed and clean) feels compared to the converted substrate (variously shiny, sometimes with an obvious film layer from the product, perhaps with black dust you have to wash off, etc. etc.).

Don't get me wrong, converters do a good job of the converting part (as does simple phosphoric acid) - but personally, I would mechanically remove the top layer and key the substrate before paint.

As posted above, a zinc rich epoxy primer will do a good job (note, though, specified to apply direct to metal, not necessarily to a layer left behind by a rust converter).

 

[GHA]

there is an argument for single part like primocon. Sand blasting/epoxy works well on steel hulls, but that's nice clean pure metal, your keel will be full of contamination, not much will work long term. Could be just do the best you can with cheaper paint & patch up when bits fail.
Phosphoric acid is good to clean up & convert rust it sees to a harder more inert iron phosphate or something. Fair chance you'll find some phosphoric acid locally in brick cleaner or something, just ask around or read a few hundred labels down the diy store.
But don't get too sad when there are some rusty streaks next liftout..

 

[greeny]

All interesting stuff to me because I'm in the process of overhauling the swing keel bearing and pads on my "new to me this year" Jeanneau Sun 2K. The ballast is provided by a large cast keel plate set in the hull. Easy enough to blast or grind the bottom of it to remove rust but mostly not easy to do anything other than to scrape and sand up the inside of the keel slot. After that it'll be rust converter and paint. It'll suffice for a couple of years and then I'll do it again. The only other way is to drop the ballast plate off the boat and blast it all over and that's a step too far.



Keel out

 

[neil_s]

For a couple of seasons, International Primocon will be good. Just put plenty of coats on. I use it on my cast iron drop keel.

 

[ohthetrees]

Great stuff guys, lots of food for thought.

 

[rotrax]

As posted above, a zinc rich epoxy primer will do a good job (note, though, specified to apply direct to metal, not necessarily to a layer left behind by a rust converter).

I agree.
We had a paint failure on our steel Hartley 32 in wellington, NZ. The stanchions are fixed into welded sockets along the deck. The welds for these sockets were forever breaking out in rusty bubbles. The welds were cleaned by grinding and a needle gun. Fertan was used, followed by a local - and very good - zinc rich primer.
It failed, the rust was even worse.
Reading the tiny letters on the tin of primer I found it clearly said 'Do not use on surfaces that have had an acid rust converter applied.'
The welds were cleaned again and straight epoxy applied before painting.
No further trouble.

 

[penfold]

All interesting stuff to me because I'm in the process of overhauling the swing keel bearing and pads on my "new to me this year" Jeanneau Sun 2K. The ballast is provided by a large cast keel plate set in the hull. Easy enough to blast or grind the bottom of it to remove rust but mostly not easy to do anything other than to scrape and sand up the inside of the keel slot. After that it'll be rust converter and paint. It'll suffice for a couple of years and then I'll do it again. The only other way is to drop the ballast plate off the boat and blast it all over and that's a step too far.

View attachment 146790

Keel out
View attachment 146794

Drop it off, have it blasted and galvanised; protected by decent paint the galvanising will last well beyond your lifetime.

 

[ohthetrees]

Drop it off, have it blasted and galvanised; protected by decent paint the galvanising will last well beyond your lifetime.

That sounds like a USA sort of operation, not a Columbia or Panama operation! Will regular primers and bottom paints perform well painted over galvanizing?

 

[penfold]

That sounds like a USA sort of operation, not a Columbia or Panama operation! Will regular primers and bottom paints perform well painted over galvanizing?

I was replying to the chap with the swing keel; given the materials etc. you have at your disposal I'd accept that it's going to blister and direct efforts at keeping things as cheap as possible, if even to the point of using one-part paint if a decent metal primer is available rather than the expense of epoxy.

Galvanising can be painted over once it is allowed to weather for a month or two, although an etch primer is advisable.

 

[arcot]

Not sure the expense of epoxy is worth it in these circumstances. International Primocon

Primocon
is not recommended if epoxy is going to be used as an overcoat.

The epoxy will peel off.
Not mentioned on can label
But in manufacturers website.

 

[Neeves]

Drop it off, have it blasted and galvanised; protected by decent paint the galvanising will last well beyond your lifetime.

I was replying to the chap with the swing keel; given the materials etc. you have at your disposal I'd accept that it's going to blister and direct efforts at keeping things as cheap as possible, if even to the point of using one-part paint if a decent metal primer is available rather than the expense of epoxy.

Galvanising can be painted over once it is allowed to weather for a month or two, although an etch primer is advisable.

On painting galvanised steel

HOW TO PAINT GALVANISED STEEL

Jonathan

 

[robtoujours]

As I want to Coppercoat my cast iron keels I asked Coppercoat directly.

They said NOT to use any rust converters but to apply epoxy coatings on the bright steel immediately.

Then to wait at least a week before application of CC (which is also an epoxy product).

I believe it is also important to have warm dry weather and even use a heat gun etc to make sure there is no moisture before coating.

 

[Tranona]

As I want to Coppercoat my cast iron keels I asked Coppercoat directly.

They said NOT to use any rust converters but to apply epoxy coatings on the bright steel immediately.

Then to wait at least a week before application of CC (which is also an epoxy product).

I believe it is also important to have warm dry weather and even use a heat gun etc to make sure there is no moisture before coating.

I have done this. You need to have the keels blasted and then coated immediately with the first coat of epoxy. I used Hempadur. Do it on a warm dry summer day - I did mine in June 2020 and took 3 days to do 6 coats then left 5 days before CC (in one day) and a further 5 days before launch.

 

[MisterBaxter]

In the WEST epoxy manual the recommendation is to rub epoxy into bare metal with wet and dry paper, to ensure really good adherence. That doesn't help with the quick and dirty fix though... Maybe just sand over the shiny finish to give a key then on with the primer as the OP suggests.