Fairing the Hull Before the Final Layer

Fairing compound on the hull
Low spots have been filled, and the entire hull has been scrape-filled with fairing compound.

After three layers of 1/8″ planking, it’s time to fair the hull.  In some builds, particularly those with a painted hull, you might wait until after the final layer is on.  But with only 3/16″ thickness in the final layer of mahogany, you can see how it’s advantageous to get the hull nice and fair before it goes on.  Sanding through the mahogany to get a hump out would be disastrous.

Look closely at the picture above and you can see a solid patch of the fairing compound at the chine.  Using a batten, I found the curve was a bit flat here, so I added fairing compound with a notched trowel to build thickness.  This sands easily to the shape you want, then fill it with more fairing compound.

Long boards for sanding
Long boards for sanding and a taping knife used for scrape-filling.

This means scrape-filling the entire hull at least two times, with sanding in between.  So I mix up some fairing compound (I use WEST System with 407 filler) and scrape it onto the hull with an 8″ taping knife.  Then I spend some quality time with sanding longboards like the ones pictured at left.  I’ve never been afflicted with “tennis elbow”, but I do get “sanding elbow” at this stage of the boatbuilding process.  That’s why I need young people to help me!

7 Replies to “Fairing the Hull Before the Final Layer”

    1. No. Sometimes you do, but in this case the design calls for a 9/16″ thick bottom and 1/2″ topsides. So I have 3 layers of 1/8″ planking, followed by 3/16″ mahogany. I could plane the mahogany to 1/8″ for the topsides. I haven’t decided if I want to do that, or just have a little margin in the topsides thickness. It really depends on how well the 3/16″ behaves when going around the curves of the topsides hull. That extra 1/16″ of thickness makes a big difference in stiffness. By the way, the deck will only be 3/8″ thick.

  1. This was a very encouraging post for me! I was fretting about exactly the same flat spot on the chine on my model and wondering how to fix it. After seeing your post I got to work with 410 filler, which fortunately is very easy to sand to shape!

    1. I use 407 micro-balloons for these kinds of “structural” filling and fairing because that’s what I was taught. It’s pretty easy to sand, but provides more strength than microlight. You might want to check with WEST System technical support before you use microlight on your full sized boat, Steve. Those guys are very helpful and easy to talk to. I always recommend calling them for any questions about this kind of thing. I’ve consulted them many times.

    1. Yes I did. If you remember, wood is very strong in tension. So the 45 degree diagonal pattern, alternating from layer to layer, really adds strength to your monocoque structure.

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