Cast iron keels, epoxy filler?

[sailoppopotamus]

The standard advice for cast iron keels that are rusting is to sandblast down to bare metal, apply a two-part epoxy primer immediately, fill and fair with epoxy primer, then several coatings of the epoxy primer again to build thickness.

My local yard suggests a different approach. They sandblast down to bare metal, then apply a thick layer of epoxy filler onto it, effectively encapsulating the keel in epoxy filler. Then primer, anti-foul. I've seen a few boats done this way and their keels are close to perfect. Yet, despite going through pages and pages of forum threads, I haven't seen anyone suggesting this. What gives?

I'm quite sceptical, since the various epoxy manufacturers suggest that bare surfaces should be prepped with primer, not filler. Generally speaking, I also fail to see why a product intended for a different purpose would be superior to one specifically formulated to act as a primer. Yet, judging from the keels I see done this way, it does seem to work.

Why is nobody else doing this, apart from maybe cost of materials and adding thickness to the keel?

 

[Chiara’s slave]

I find P38 has better adhesion to metal. I’m shortly going to be repairing a ding on our XOD, probably start with that, then epoxy over the top.

 

[dankilb]

Adhesion would still be better over a ‘primer’ coat of epoxy. It may not say that on the TDS, but if you do a test area and try to abrade you’ll see a pre-primed coat of fairing compound adheres better. If you apply the filler neat you don’t take advantage of the viscosity of epoxy (laminating resin or paint) on such a porous substrate.

Otherwise our Jeanneau keel had a layer of fairing compound from the factory that lasted well for 25+ years so it’s an established method.

 

[sailoppopotamus]

Adhesion would still be better over a ‘primer’ coat of epoxy. It may not say that on the TDS, but if you do a test area and try to abrade you’ll see a pre-primed coat of fairing compound adheres better. If you apply the filler neat you don’t take advantage of the viscosity of epoxy (laminating resin or paint) on such a porous substrate.

Otherwise our Jeanneau keel had a layer of fairing compound from the factory that lasted well for 25+ years so it’s an established method.

If I understand correctly you are saying that the epoxy primer followed by epoxy filler will adhere better than neat filler, which is actually exactly what the TDS suggest (International specifically advise priming a bare metal surface before applying filler). Which is what I thought is the canonical approach, but not what the yard does (as for the porosity, I think the idea is to use a knife to push the epoxy filler into all the pinholes).

I've taken an angle grinder to the whole keel, I saw a relatively thick layer of white under the antifoul, not sure if it was filler or epoxy primer though (1988 Jeanneau boat).

 

[dankilb]

If I understand correctly you are saying that the epoxy primer followed by epoxy filler will adhere better than neat filler, which is actually exactly what the TDS suggest (International specifically advise priming a bare metal surface before applying filler). Which is what I thought is the canonical approach, but not what the yard does (as for the porosity, I think the idea is to use a knife to push the epoxy filler into all the pinholes).

I've taken an angle grinder to the whole keel, I saw a relatively thick layer of white under the antifoul, not sure if it was filler or epoxy primer though (1988 Jeanneau boat).

Ours is a 1989! I felt the white layer was a bit thick for just primer - maybe a very high build…?

I’m (pleasantly!) surprised to hear the instructions agree. I always ‘prime’ with epoxy based on instinct! It’s more a matter of penetration - and therefore adhesion - on a microscopic level, although I don’t claim to know much about how these things actually work! I guess the filler forms a chemical bond with the paint better than a mechanical bond with the substrate?

So yes your understanding is correct. I personally would prime on the basis that you’ve gone to that much effort - but if it’s a matter of acquiring a full 5L tin I’m sure you’d be fine with the boatyard’s method.

 

[vyv_cox]

Two issues I can think of. First is that bare grit blasted iron should be primed within 20 minutes. I cannot see filler going on within that time frame. Second applies to any epoxy when overcoating - the second coat, or filler in this case, needs to go on while the first is still tacky. Gelshield instructions are clear on this, I did five coats in a day, quite hard work.

My keel was primed, then faired, then Coppercoated. That was more than 15 years ago, still pretty good.

 

[sailoppopotamus]

Two issues I can think of. First is that bare grit blasted iron should be primed within 20 minutes. I cannot see filler going on within that time frame. Second applies to any epoxy when overcoating - the second coat, or filler in this case, needs to go on while the first is still tacky. Gelshield instructions are clear on this, I did five coats in a day, quite hard work.

My keel was primed, then faired, then Coppercoated. That was more than 15 years ago, still pretty good.

Just curious about your comment on the tackyness of the Gelshield -- where did you get that information? Looking at the TDS they state the overcoating time at 23C as 3 hours - 6 months, with sanding required if the upper bound is exceeded. From that I would assume that it would still be fine to overcoat at any time within that period.

Though I plan to use a different epoxy primer, the instructions are similar, and I would be delighted to be able to do it in a day instead of driving to the boat and back several times!

 

[neilf39]

I used international watertite epoxy filler after I had mine sandblasted. Coated the whole surface and faired. Was all fine on lift out after a year. Used as the basis for coppercoat.

 

[DownWest]

I find P38 has better adhesion to metal. I’m shortly going to be repairing a ding on our XOD, probably start with that, then epoxy over the top.

P38 is polyester based, so porous, as I found out when trying to cap something for pressure testing. Maybe not the greatest keel sealer? Epoxy on top would then seal it , but I would use something else to start with.

 

[Chiara’s slave]

P38 is polyester based, so porous, as I found out when trying to cap something for pressure testing. Maybe not the greatest keel sealer? Epoxy on top would then seal it , but I would use something else to start with.

Aware of all that. It just sticks like….. so if you have a hole to fill it does the business. Epoxy filler and primer on top, as I said.

 

[vyv_cox]

Just curious about your comment on the tackyness of the Gelshield -- where did you get that information? Looking at the TDS they state the overcoating time at 23C as 3 hours - 6 months, with sanding required if the upper bound is exceeded. From that I would assume that it would still be fine to overcoat at any time within that period.

Though I plan to use a different epoxy primer, the instructions are similar, and I would be delighted to be able to do it in a day instead of driving to the boat and back several times!

I did mine more than 20 years ago. At that time that is what the instructions said. As far as I remember I did all five coats in a day but but it might be that it was two days. Plus a coat of hard antifouling to finish off.

 

[Pasarell]

Filling before priming is a lazy way of doing the job, but will usually not show any defects until well after any warranty period is over.
Cast iron is difficult to clean properly before coating as it traps water and salt in the many pits and pores of the surface. Ideal situation is to blast then pressure wash a few times to remove as much salt as possible, then sweep blast before starting coating but this isn't a practical approach for most.
Realistically blast, dedust if it was a dry blast, or dry thoroughly. Hair drier or heat gun is good for this. Then prime by stippling well into all the pits and voids. There is no chance of a filler giving complete coverage into pits no matter how much "pressed in with a knife" and the uncoated areas will be bridged over trapping nicely corrosive moist air that will start corrosion immediately.
2nd and subsequent coats of primer as per TDS and ambient conditions. I suspect the variation on over coating times and coat on tacky instructions for Gelshield are due to the 2 versions. One contains solvent (Gelshiled 200?) and the other is 100% solids. Certainly the 100% solids version will not have an overcoating time of 3 months!
Fill after at least 2 coats of primer then prime again over the filler to seal it. The filler is never as water proof as the primer due to the microballoons.
Always worth taking advice of the experts - the people who designed and tested the coating procedure. Less worth listening to the people who have a vested interest in doing the job quickly using as few materials as possible!