Paint Recommendation for Steel Keel

Discussion in 'Metal Boat Building' started by SeaJay, Feb 13, 2011.

  1. SeaJay
    Joined: Jun 2007
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    SeaJay Senior Member

    I know I'm opening up a can of worms here and have read many threads in t hese forums discussiong similar matters, but thought I'd ask you guys to weigh in on my specific situation.

    As I've discussed elsewhere, I've constructed a hollow steel fin keel (bolts to bottom of composite hull) and am pouring lead into the form. I'd like to get as much of the paint system on before I move it to the boatyard and attach it to the hull. The keel will need some minor fairing. I have not yet decided upon a bottom paint system although the hull has three layers of Interprotected 3000 expoxy coating and the topsides have been primed for Awlgrip.

    My first question is, do I need to decide upon a bottom paint system before selecting the primer and filler system to apply to the bare steel keel?

    Secondly, what is the baddest-a$$ coating to use on steel. Money is no object as I only have about 60 sq. ft. of surface to fair / paint. I just want the most durable BUT also FOOLPROOF system available. I stress FOOLPROOF as I don't have a very controlled environment or a lot of speciality paint experience. If the "best" requires extraordinary application skills I think I'd get a better finished product I went for something that wasn't quite as touchy to apply.

    I look forward to reading your suggestions.

    Best Regards,

    SeaJay
     
  2. CaptBill
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    CaptBill CaptBill

    The key is getting that first initial primer to bond well. another concern is the electrolysis that can occur in the paint system. Coal tar epoxy has been found to still be a good approach. The coal tar insulates the electrolysis and also sticks better than most systems/products on lead. Old School tar epoxy (with new generation epoxy base). I like the idea because then you can go with a copper-epoxy as your anti-foul topcoat (my favorite potential options) with less concern of the lead and copper getting along. Of coarse the same will apply with any antifouling (copper and tin usually)

    http://www.epoxyproducts.com/coaltar.html

    Just a note. The Interprotect 3000 is not in the 'new generation' of epoxy grades. Read the instructions and it tells you to wait 15 mins. to 'kick' before app. It is a fine product, I am sure, just know the 'new gen' stuff is a better choice.
     
  3. michael pierzga
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    michael pierzga Senior Member

    Plenty of first class steel primers...Intenational...Awlgrip.. Jotun ...take your pick then contact their technical representative in your area. . The secret is not the paint, but the surface prep , then following the instructions for paint application, over coating time and film buildup.

    I find the three colour primer application technique best. Red, white, grey. The colour keys give film thickness indication and also prevent you from sanding thru the coating when fairing in. Naturally after fairing , more primer is applied.

    Surface prep for steel means sandblasting . No other way is acceptable. Pay particular attention to the top mating surface of the keel. NO SHARP EDGES on the top mating surface ....as a matter of fact NO SHARP edges on any part of the structure. Paint doesn't like sharp edges. Grind a radius on all edges before sandblasting.

    The most important part of a steel keel mounting is the hull to keel joint . To do this you must correctly mount the keel. Any corrosion which forms in this joint will be a future nightmare.

    A keel is fitted to the bottom of a boat with the same level of precision that a head in mounted on an engine. Perfectly flat, true surfaces that permitt 100 percent surface contact between keel and hull, a thin waterproof gasket, then correct torque on the keel boats.

    This almost always means that the primed, finished keel will be fitted to the hull set in a bed of thickened epoxy or Chockfast to correct for mating surface irregularities or over sized keel bolt holes in the hull structure. . . .
    Do remember to use mold release on the keel and keel bolts. You dont want to glue the keel to the hull .

    Not a difficult job, but since the gear is heavy, not easy either.

    Many times since the keel is a big heavy beast its best to have the fitting done under the supervision of a boatbuilder.
     
  4. SeaJay
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    SeaJay Senior Member

    Capt. Bill - I've heard other good things about the coal tar epoxies. (Just to clarify things, the outside surface of my keel is steel, not lead.) I may have the wrong idea about coal tar epoxy, but a) is it in fact the primer material, or does it go on after a primer? and b) is the fairing done before or after the coal tar epoxy? Also, I noted your comment about the Interprotect 3000. It was applied some years ago but the hull has never been in the water.

    Michael - You are confirming what I've been told about sandblasting. I guess I'll just get set up to do it. The one spot that is problematic is the underside of the keel "foot". I can lift the empty steel form with my forklift, but after it is filled with lead (8,500 lbs), it exceeds my lift capacity. What I plan to do is to put about 3,000 lbs of lead inside, then lift the keel and get primer on the bottom. If I opt not to sand blast this area, what is the next best way to prep the surface?

    Thanks to both of you for the input.

    Best Regards,

    SeaJay
     
  5. michael pierzga
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    michael pierzga Senior Member

    Ya Seajay...the bottom of a steel keel is problemetic. Hoist , blast and paint when new. But As soon as you fit it, the paint film on the bottom of the keel will get damaged. Everytime you haul out or lay aground ...damaged. No easy solution. Thats why I like a steel keel with a lead bulb for toe stubbing . Hoperfully the bottom of your steel keel has plenty of thickness to the steel plating to compensate for natural wear and tear.

    Ive never tried it, but some kind of sacrifical skid gaurd added to the bottom of the keel after priming, might be worth while. Perhaps several layers of fiberglass cloth set in epoxy as a chafe gaurd to restist boatyard handling ? It too would probably get crushed by the weight of the boat when handling the boat at the shipyard and might be a waste of time.


    Luckily bare steel on the bottom of the keel is not such a big deal. Just like a lead keel, Every steel boat Ive every sailed had bare steel patchs on the keel bottom from abuse after a year or two.

    Are the empty areas of you keel....no lead ballast compartments .... oil or antifreeze filled ?.
     
  6. CaptBill
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    CaptBill CaptBill

    SeaJay
    You would want to sandblast/prep as usual then apply the coal tar to the bare surface, if my understanding is correct. This will establish a good isolation from water and other metals found in most paints/antifoul etc. So the coal tar is the primer, basically. Fair next. The coal tar needs to be overcoated with an epoxy primer PRONTO so it fuses with the coal tar epoxy. This is a good practice anyway with all coating systems but especially important with coal tar. An oily residue can migrate to the surface over time and then nothing will stick well (new generation coal tar epoxies supposedly don't have this issue though)

    Supposedly the coal tar and steel work excellent together. The coal tar is very flexible to, so this should aid the grip of the topcoat vs. attempting to stick to bare metal.

    I just ran across the coal tar idea myself yesterday so DO research it for yourself. See how others have used it. From my initial impression this stuff seems like an excellent system for steel. Plus it is 'old school' with modern 'epoxy base' which has it's own appeal.
     
  7. SeaJay
    Joined: Jun 2007
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    SeaJay Senior Member

    CaptBill - I just got off the phone with Bluewater Marine Paint technical dept. They are recommending their new formulation of coal tar epoxy, Marine AC 70 (although they said their traditional coal tar epoxy works fine). Apparenlty the newer stuff contains a different by-product (tar) that is less noxious and easier to work with...may be what you were refering to as "new generation". The sequence is 1) sandblast 2) AC70, let dry overnight 3) fair w/ epoxy putty, let cure overnight 4) 2nd coat AC70, let dry overnight 5) 3rd coat AC70, let dry. In my case, the keel will be primed long before it is attached to the boat, so I'll need to sand the AC70 w/ 80 grit when it is on the boat and I am painting the entire underbody with bottom paint. Ideally you would want the bottom paint to be going on while the AC70 was still in the cure stage.
     

  8. CaptBill
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    CaptBill CaptBill

    Sounds like you have found a winner. The 'new generation' I spoke of was just the epoxy 'vehicle/base' that is added to old fashioned coal tar. Sounds like they are selling 'new gen coal tar' AND 'new gen epoxy' combo. Sounds even better.

    I would still feel alot better applying a coat of regular epoxy after done with the coal tar to prevent the possibility of the tar leaching to the surface though as the final move. Just for a good nights sleep.

    Hope it turns out nice.
     
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