Fill or paint first

[BC Mike]

My boat has a cast iron keel that I have ground down to bare shiny metal. There are a few casting blemishes, also it is not quite thick enough forward to make a propper foil. Should I paint first or fill first? The paint and tar epoxy folks tend to side step the question. I painted the keel last time with POR 15 and it adhered well to the smooth areas but not so good to the rough bits. I suspect a bit of moisture was trapped. Duraglas is an epoxy/bondo type filler that I think will work and there are several tar epoxy paints. Both of these use a hardener and both are recommended for under water.
Michael

[delmarrey]

The problem with cast iron is that it is very pourous. Once salt has pregnated into the iron it's there to stay. If it's pitted, you'll never get that stuff out. But you can seal it up!

If you want to smooth it out for perforance. You can fill and fare it with the West Systems epoxy and fillers or any other epoxy, for that matter.
Any pits you may want to sand blast to get as clean as possible. Then make sure they are real dry by using a propane torch.
Once the whole keel is clean and dry and at 70 degrees or above you'll want to get a coat of epoxy on there right away so the moisture doesn't absorb back into the iron. Once you get that first coat brushed/rolled on you can take your time doing the fill and faring work at your leasure.
The main thing is to get it sealed without moisture!

[GordMay]

Is there any point in treating cast iron with phosphoric acid ('Ospho') after blasting/cleaning, and prior to filling/fairing and painting ?

[delmarrey]

Being a water softener and food additive I would tend to shy away from it. It may leave a residue that may resist a good bond of the epoxy.

As a cleaner I prefer dry solvents like acetone or MEK. Or even some of the new industrial alcohol based formulas, but stay away from triclor's for health reasons...................._/)

[delmarrey]

If you were to thin the first coat of epoxy down to a watery substance using MEK, it would soak into the pours a lot easier creating a better seal and bond..............._/)