How to Finish Mahogany Bright?

Discussion in 'Wooden Boat Building and Restoration' started by adt2, Apr 9, 2012.

  1. adt2
    Joined: Sep 2009
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    Location: Magnolia, Texas

    adt2 Senior Member

    Next question on the ever-growing list: My new skiff will have a painted hull and bottom, but I'd like to finish the trim (gunwales/inwales, false stem, and transom) bright to show off the expensive Mahogany I am using there. What is the best way to do that?

    Some background info: I am planning to cover the bottom and at least partway up the hull with cloth and epoxy (I have not finished researching this project yet, so I don't have more details right now). The skiff will not be outside for extended periods; it'll be stored inside and trailered to the lake when we want to use it. I would guess it'll get used eight, maybe ten times per summer, say 8 hours per day max.

    So, I'm wondering - do I cover it with light-weight cloth and epoxy? Epoxy only? Varnish? Also, any special treatment required where the drain tube exits the stern (I assume sealing the wood inside the hole before installing the tube)? What about treatment where the cloth on the bottom and/or sides meets the corners at the transom? I.e. if the transom gets no cloth, do I just cut the side/bottom cloth off flush at the transom?

    This will be my first fiberglass job (any my first boat, too), and I really don't want to ruin all the hard work building the boat by finishing it wrong.
     
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  2. rasorinc
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    rasorinc Senior Member

    If you want to cover the hull with glass and epoxy a 6oz or 7oz cloth is more then adaquit to insure that no water will get to the hull. If you want scraping protection for running a ground while beaching then a 6" to 12" fiberglass tape along the keel will provide protection. Use Dynel or Xynole cloth for this as they are made for scrapping protection. To finish your Bright work use a water based stain only, then cover with 2-3 coats of epoxy. Lightly sand and then apply many coats of marine spar varnish that has UV protector in it. What makes it look great is many coats of marine spar varnish.
     
  3. adt2
    Joined: Sep 2009
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    adt2 Senior Member

    The purpose of the glass and epoxy is to waterproof the hull and provide minimal abrasion resistance. The thing may occasionally be beached at the small swimming island in the middle of our local lake, but otherwise the only thing the bottom will ever touch is the trailer.

    The plans call for tight-fitting planks with some kind of caulk or other sealant between them, but I'm making an executive decision to cover the bottom in cloth and epoxy instead. I like permanent construction. It's a lapstrake hull with three really wide planks on each side; I'll probably only glass-and-cloth up the first plank (to seal the plank-to-chine joint and the first lapstrake joint), and then just use epoxy from there up. I think. (My epoxy plans are not yet fully baked.)

    Any thoughts on how to handle brightwork?
     
  4. rasorinc
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    rasorinc Senior Member

    See above for Bright work Yes there are times I sign off by posting an incomplete answer while I look something up then come back and complete the post.
     
  5. adt2
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    Location: Magnolia, Texas

    adt2 Senior Member

    Did you edit that post, or did I just nod off right in the middle of reading the original? In either case, thanks for the input.
     
  6. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Lapstrake builds don't take cloth very well. You can 'glass the garboard if you like, but the brunt of the damage occurs along the lap edges, where there will not be any 'glass, so what's the point.

    If you're going to use a sealant in the laps, then use polysulfide if the boat is to be moored or berth often. If it's a trailer boat and will be dry sailed, then use polyurethane. This assumes a traditional lapstrake build and not a glued lapstrake build.

    Mahogany is a beautiful wood without stain. If you stain mahogany, it loses one of it's wonderful characteristics, which is the color shift the silica content produces. This wood has a pretty coarse grain which should be filled. The easiest way is to use wetdry paper and mineral spirits. Stroke with the grain and very quickly you get a slurry of mahogany dust, mixed with the spirits. Spread this around with a plastic applicator, filling the grain as you go, then let dry. Next using a clear finish (polyurethane or varnish) cut at 30% with spirits (first coat only) cover the whole surface with a thin, even coat and let dry. After a very (very) light sanding, apply a 20% cut. When dry again very light sanding, with straight finish (as the pores are now sealed), which may need to be cut slightly (usually less then 10%) to improve flow, depending on application method.

    There's absolutely no reason to use epoxy on bright finishes. It just adds to the complication and difficulty maintaining the finish. Epoxy is effective in only two situations, as an adhesive and as a wood stabilization treatment (encapsulation). Other than these two instances, you don't need, nor is it desirable to use epoxy, particularly under clear finishes.

    Now if you do encapsulate, then naturally, epoxy under the finish is necessary. Simply put, unless you coat every square inch of planking with epoxy, especially the end grain, then you're not really getting a lot of benefit from epoxy. As a coating, it's only effective in encapsulation techniques and can be detrimental if used otherwise.
     
  7. adt2
    Joined: Sep 2009
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    adt2 Senior Member

    PAR - Thanks for the tips. This is Af. Mahogany, not Honduras...does that make a difference in your application instructions? I wasn't planning on staining it; I want it "au natural" but I assume it needs some kind of protection. Plus I want to seal the end grain (which will be hidden under the lapstrake plank ends) and the bottom where it meets the bottom planking.

    The glass and cloth will only be on the garboard, and only for waterproofing the chine joint and abrasion resistance on the bottom (not the hull sides). No cloth higher than the garboard plank. I am gluing the planks with thickened epoxy.

    Does your sand & stain routine a) apply to Af. Mahog, and b) apply if I'm not planning to stain the wood before sealing?
     
  8. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    There are approximately 40 different sub species of mahogany. No, it doesn't make a difference, they're all coarse grained.

    What type of lapstrake build is this? You seem to be describing a glued lapstrake build. In which case every plank should be encapsulated in epoxy (not completely necessary on some builds). A glued lapstrake can be identified by the lack of internal ribs, floors and other traditional structures. There's still internal supports, but typically built into the furniture.

    Maybe it would be easier if you told us what the make and model of your boat, possably some pictures.
     
  9. adt2
    Joined: Sep 2009
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    adt2 Senior Member

    It's a Macomber 15, recently shown in the Small Boats issue of WoodenBoat. The plans call for traditional lapstrake construction with a bead of "sealant" between the planks (sides and bottom). But I was more into the look of the boat than the "traditional construction methods" used to build it. Hence the epoxy glue between all the various pieces.

    When it's all said and done, each plank will be completely encased in epoxy, inside and out, both edges, both ends. Planks are 3/4" Cypress - another reason for encasing in epoxy (Cypress absorbs water and gets heavy). Everything else - all backbone members, transom, gunwales, inwales, etc. will be Mahogany. I thought the bright trim set against the painted planks would be good-looking.

    Plan info and images here.
     
  10. philSweet
    Joined: May 2008
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    philSweet Senior Member

    If the mahogany is all solid wood and not ply or veneer, you probably won't be able to glass it because of its shapes- too awkward. Use a deck or bright-work epoxy. They are a bit more expensive (like double), but worth it. I'd use a good four or five coats on all exterior bright-work and anywhere subject to abuse. (on my boats that means everywhere). Then start spraying on the varnish. You want to get quite a few coats on and then you can add a few each time you tinker on it. Try to get it into the teens by the end of the first year. Give yourself plenty of mahogany to work with so the stuff doesn't disappear on you after you've wooded it down five times twenty years from now. Mine sat out in the tropical sun for a dozen years and I eventually gave up and painted some of it.

    The alternatve is to use a fast build treatment such as Sikkens Cetol Marine Natural.
    Its not quite the spanking finish that twenty coats of good varnish will get you, But its one heck of lot less work and from ten feet away, few would know the difference. Goes on with a brush.

    Where I lapped my transom with glass to protect the plyplank edges (3 X 10oz tapes), I just masked this area and painted it to match the hull and left the rest of the transom bright. Use a good fineline flexible tape- about $12 a roll:mad:
     
  11. adt2
    Joined: Sep 2009
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    adt2 Senior Member

    I am planning to encapsulate the entire transom (solid mahogany) with epoxy. You say above that "...epoxy under the finish is necessary." I don't quite understand the science there (i.e. how does the finish - the varnish - beautify and protect the wood if it's installed on the epoxy?) but I've read it enough times to know it's the accepted way to do it. Do I just roll on a couple of thin coats of unthickened epoxy and then proceed with the varnishing technique you mention above? Or do the mineral spirits sanding thing first, then epoxy, then varnish?

    I'm just a little confused about what order things should get done in. I'm planning to do a few practice runs on some Mahogany scraps I've been saving, just to make sure everything turns out okay.
     

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

    Epoxy under a clear finish is necessary, if using encapsulation techniques, which you say you are. It's not encapsulation if there's not 100% epoxy, on every square inch of surface, every edge, every hole, etc.

    Epoxy isn't necessary under a clear finish, for the finish to be effective at protecting the wood. In fact, the presence of epoxy makes care and upkeep of a clear finish, more difficult.

    Varnish (or polyurethane) protects an epoxy coated surface, by preventing most of the damaging UV rays from changing the molecular composition of the epoxy. It's this epoxy sensitivity to UV damage that you're addressing with clear coats. Varnish offers UV inhibiting additives that help greatly in this regard, thus protecting both the wood and the epoxy itself.
     
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