About to Start Mahogany Planking - What do I need to Know?

Discussion in 'Wooden Boat Building and Restoration' started by nbehlman, Sep 7, 2014.

  1. nbehlman
    Joined: Aug 2011
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    nbehlman Junior Member

    I am about to start planking my 14ft mahogany runabout. I have completed the diagonal sub-planking (0.25" marine fir ply) and I'm pleasantly surprised at how fair it came out. I have ripped and planed some South American mahogany down to 0.15" thick and I think I'm ready to go. I will be using west systems epoxy to glue the planks down. I plan to start on the bottom at the keel and work my way outward. Does anyone have any tips for me before I get started? Pictures included. Yes that's a small amount of bondo towards the bow. Things got a bit tight. The plan is to encapsulate with epoxy, so I don't have any issues. Wish I had learned about epoxy-based fillers sooner.

    Attached Files:

  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Do yourself a favor and grind out the Bondo. It's ability to stick to wood is limited, it doesn't tolerate vibration and impact well and the area of the boat you have it will see lots of impact. Mix up some thickened epoxy and fair the area out (again). Yeah, I know it sucks, but if this is the only redo, you've gotten off really really lucky. No, a simple epoxy over coat will not "fix" the Bondo.

    As to tips and tricks, well you might want to be a bit more specific. Generally, for looks you'll work from the centerline to the chine and from the seer down to the chine, for your outer layers. Back cut the plank edges, ever so slightly (1/4 - 1/2 a degree), so you can force some edge set into them and plane off the ridge that'll be created. The very slight gap will fill with epoxy, helping seal the end grain too.
  3. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    I agree about the Bondo. It is in a critical area and glue is the only thing holding the boat together.
  4. sdowney717
    Joined: Nov 2010
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    sdowney717 Senior Member

    Auto bondo swells up with water, it really does.
    It gets wet even from rain or water in the air over the years.
    If you patch any rotted wood with bondo, the bondo will get wet, absorb water right through paint and never dry, even good wood will rot and keep rotting.

    I have mixed PL Polyurethane and sawdust to make waterproof fillings on wood. It will swell up so put plastic like a cereal bag on the repair and press it down.

    And I have used black PL Poly roof flashing mixed with FG milled fibers as filler. Even laid down FG cloth in the black PL, works very well. Flexible and tough, sticks great, will not peel off.

    It is the black PL poly roof flashing PL S30
    This is the technical data sheet

    Or simply use some epoxy mixed with 1/32 inch FG milled fibers as a filler.
  5. Joris
    Joined: Mar 2013
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    Joris Junior Member

    I just finished planking mine and two things that were important to me as a first timer were:
    1/ shaping the planks or pieces.
    When you have years of experience one probably knows which traps and which problems are most likely to come up. I clearly didn't... When i started i drew the lines i wanted onto the hull and then checked the planks on top the lines to see if i could get the desired shape out of them. In a few cases i ended up shortening the plank or adding a small strip to the side because the curve i transferred to the plank during dry fitting changed a bit when i clamping with glue.
    2/clamping the pieces
    While vacuum bagging made everything very easy in the previous layers it seemed nearly impossible to position and clamp my 1/4" planks.
    Again, experience. The lack of it apparently can be countered with a healthy combination of stress, creativity and limbs covered in glue but doing a few pieces when you are in company of helping hands can save a lot of work. Also, running to the other side of the workshop for something while the plank comes loose can be somewhat dangerous (depends a lot on how far it was bent and how loose it came...)
    Hope this is of any help,
    Good luck with the build, it looks awesome!
  6. SamSam
    Joined: Feb 2005
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    SamSam Senior Member

    I've often wondered that if Bondo is so bad for boats, why doesn't it absorb water, swell up and fall off of cars and ruin auto body paint jobs? Cars sit exposed to all the weather, regularly drive through rain at hurricane force speeds and endure heavily salted and or chemically coated roads for months at a time. Most cars have a harder life than most boats, so how come Bondo works on cars but not boats?

    I have a fiberglass plug that was faired with Bondo, painted with regular auto enamel and has sat out in the weather for 20 years, but there are no problems because of the Bondo. Some of the enamel has withered away and exposed the Bondo, but the Bondo doesn't change. It's just there, the same as when I put it on.
  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Bondo does work, just not as well as other products. It works moderately well (there's still better products) on inert surfaces, because they don't have as much movement and moisture gain changes as wood.

    Boats have a much harsher environment than cars. A car sees moisture occasionally and it goes away, but not so much with boats. A boat also sees pressurized water, which cars don't see. Vibration is also an issue with Bondo and it tends to "circle out" around any repair areas. This happens on cars too and is a quick way to check for repairs on them. Bondo also tends to crack if applied too thickly. Lastly the prep and paints used on cars is really good stuff, compared to what the average guy uses on his boat. So, with the exception of those (like me) that use solvent based LPU's that cost $300+ a gallon, the physical quality of paint is somewhat less than that used on a car.

    There really isn't a debate on this, simply because enough Bondo has been dug out of both cars and boats to prove its worth in both environments. I'm glad you've had some success with it, but there are exceptions to every rule, though generally, you'll find that Bondo, particularly on wood, just is the weak knee, redheaded step child to other products.

  8. sdowney717
    Joined: Nov 2010
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    sdowney717 Senior Member

    I patched some rotten wood with bondo around the window frames. I cleaned up the area very well to sound dry wood. Used bondo to patch. Well painted.
    It all failed after a few years, it becomes soaking wet, the bondo lets go of the wood. From what I read, bondo absorbs water, so the wood rots right next to the bondo.
    And the bondo shrinks away from the wood, creates a gap and lets in water to get the rot growing. The bondo bond let go, so it was just loosely there like a plug.

    I think the bondo gets wet and does not dry out, so it feeds the rot.
    People will naturally use bondo for patching holes in wood and some are rot holes.

    My final fix was to remove all the bondo, get back to sound dry wood by cutting the rot out, then gluing in new pieces of wood with PL premium which never absorbs water. It stayed dry and rot free. Bondo is not a glue.

    Bondo works fine on metal. Cars are painted with an enamel or waterproof paint system, so the bondo is sealed in. And the backside is a waterproof metal surface.

    The small shrink factor as it cures leaves it pulling off the wood as maybe it is under some tension. PL expands as it cures ensuring a better seal to the wood, like a spring. Epoxy does some soaking into wood fibers so that works. Polyurethanes also soak into wood fibers and stay flexible not hard so the seam dont open up. Epoxy with no reinforcing fabric might crack in the seams.

    Consider that wood moves with temp and moisture cycling, bondo is rock hard. Wood fibers are weakly bonded to bondo by mechanical means, so the wood fibers will just let go created a small tiny gap seam. Water can get in right through paint. What if the water freezes? See lots of problems with bondo and wood and water and rot.

    Polyester resins like bondo, people say they dont stick well to wood, and I agree

    These folks say the right things and mirror my experience of rot developing around bondo.

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