Bonding Aluminum/Gelcoat and Perfection

Discussion in 'Fiberglass and Composite Boat Building' started by missinginaction, Mar 3, 2011.

  1. missinginaction
    Joined: Aug 2007
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    Location: New York

    missinginaction Senior Member

    Well, I'm getting ready to go back to the yard and finish my old Silverton.

    I have a question regarding my hull/deck joint.

    The rub rail that I'm using is the original extruded aluminum that came with the boat in 1973. Since this boat didn't see lots of use the rail is in remarkable condition for it's age.

    So I have an aluminum rail that will cover a plywood deck finished with Perfection over epoxy/glass and gelcoated fiberglass on the topside of the hull. The original builder used a strip if 2" fiberglass tape and placed it under the rail covering the joint. This worked pretty well but the boat did develop some leaks over the years.

    I could duplicate this method but I'm wondering if I might be better off to prime the aluminum, deck and topside with Sika 206 G+P and then backfill under the rail with Sika 291 LOT and bed the screws into the deck and topsides.

    There is a 2" x 2" mahogany cleat at the hull deck joint.

    I'll post a couple of photos to give a bit more clarity. My goal is an absolutely watertight hull deck joint. Cost is secondary.

    Thanks for any advise you can give.

    MIA
     

    Attached Files:

  2. mark775

    mark775 Guest

    Is that water I'm looking at in the picture? Is the plywood getting wet? First thing, I'd take measures to get it dry. Then I would examine how the glassing they did originally failed (Was the joint flexing it too much, did it just come unbonded, did it fail at screw holes? Joints like this that are just caulked don't have a good record.
     
  3. michael pierzga
    Joined: Dec 2008
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    michael pierzga Senior Member

    Important to waterproof the end grain of the plywood. Take a sander...prepare the end grain wood , then coat...saturate ... with epoxy.

    I Assume the sheer clamp joint...deck to hull... is sufficiently strong without fibeglassing the joint ? ...if not sand a half inch radius on the deck ply edge ,then tape the joint with 6 or 12 oz "biax" tape and epoxy.

    When mounting the rub rail you will puncture this newly waterproofed sheer clamp joint when driving the rail fastening into the ply end grain. These fasteners are going to present a water intrusion source.

    It will be very difficult to make them watertight. The first time your rail strikes the dock , the fastener will be strained , move in the wood , break the waterproof joint and it will...leak. No easy long term solution to mechanical fasteners.

    Epoxy bonding the fasteners can be used. Review the Gougeon brothers chapter on hardware bonding. http://www.westsystem.com/ss/bonding-hardware/

    Also... Sika or 5200 develops a tenacious grip on properly prepared surfaces...you may be able to remove the mechanical fasteners, after bonding the rail with 5200 or Sika, then plug the empty fastener holes with epoxy with a cosmetic treatment of the rail holes.
    Rail fasteners are very difficult to make waterproof over the long term
     
  4. missinginaction
    Joined: Aug 2007
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    Location: New York

    missinginaction Senior Member

    Thanks for the replys guys......

    Perhaps a little more information is in order here.

    1. Thats water allright, but....that plywood has multiple coats of epoxy sealing the endgrain. As it happened we had to move the boat in the yard last fall. The storage cover was of course removed for the move and it rained. Not a problem. I happened to take the photo after the rainstorm.

    2. The aluminum rail is designed so that the support screws go into the plywood from the top (deck) and the topsides, but at the topsides the fasteners do not enter the plywood endgrain. The screws actually pass through the fiberglass at the hull deck joint and end up in the 2" mahogany cleat or clamp that reinforces the hull deck joint.

    3. The original 2" fiberglass tape simply lifted here and there from the ply deck and fiberglass topsides. The rail wasn't damaged and I do not believe it moved I just think that the bond failed over the years.

    If my idea of using SIKA alone isn't worthwhile, perhaps I could prime the aluminum and the deck topside area with Sika 206 G+P and then fiberglass the joint. 206 G+P is a great adhesion promoter with Sika products, I'm unsure if it would increase the bond between aluminum and epoxy. My limited experience with bonding aluminum with epoxy alone hasn't been good.

    Based on the imput from you guys, I'm leaning towards the tape joint. That may be the best solution but perhaps I'll get some ideas on how to bond it more securely.

    Thanks,

    MIA
     
  5. mark775

    mark775 Guest

    I know what I'd do but I hesitate to input. I'd rather see some advice from Parr or somebody else more knowledgable on this but, for what it's worth, I'd grind down to bare glass over six inches or more on each side and then glass, then cover with trim bedded in Sika or 5200. I actually like a softwood trim for here because it is sacrificial rather than transmitting energy to the substrate (aluminum is really bad for that when meeting a piling). Maybe a little more thorough advice will come. Sorry I don't know more.
     
  6. michael pierzga
    Joined: Dec 2008
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    michael pierzga Senior Member

    Gee...when abraded, Sika adheres tenaciously to epoxy and the primer tec sheet says its suitable for metal. You should be able to generate a very good bond. I normally have to remove sika flexed aluminum tracks from their substrate with a blow torch and wedges. The bond is so good that the sika bond overpowers the substrate strength.

    http://207.250.200.229:8080/1/doc?id=68299

    Its difficult to judge your project from a picture. It you would like to tape the joint, go for it...with epoxy and biax tape . Naturally before you tape the joint you must first fill and fair the joint with high density epoxy filler.
    Be aware that fiberglass tape will not conform to the sharp deck edge radius in the picture. The sharp radius must relieved...rounded over ....half inch should do.

    Its the fasteners penetrating the deck and hull that will take attention to detail to prevent water ingress.
     
  7. missinginaction
    Joined: Aug 2007
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    Location: New York

    missinginaction Senior Member

    Thanks again for the replys..

    I think that a roundover bit on the ply edge and some fiberglass tape will do the trick. I can hit the gelcoat with some 40 grit disks just to rough up the area on the topsides under the rail where it wont show. After the tape is down I'll still back fill with Sika and bed the screws into the cleat thats behind the joint.

    Thanks again,

    MIA
     
  8. michael pierzga
    Joined: Dec 2008
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    Location: spain

    michael pierzga Senior Member

    Also...I imagine that your rub rail extrusion is a 90 degree angle ? How will you allow for the rolling bevel at the sheer clamp ?

    Its typical to dry fit the rail.. then disassemble and re bed the rail or hardware into an epoxy thickened bog with mylar packing tape used as a mold release.

    for max service life ...0ne hundred percent rub rail to sustrate contact is the goal.
     

  9. missinginaction
    Joined: Aug 2007
    Posts: 781
    Likes: 76, Points: 28, Legacy Rep: 512
    Location: New York

    missinginaction Senior Member

    I was thinking of backfilling the rail with Sika to deal with the radius at the deck edge. The thickened epoxy/mylar packing tape is a great idea.

    Thanks,

    MIA
     
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