Making bottom of boat scuff resistent

Discussion in 'Boatbuilding' started by 300wm, Dec 10, 2015.

  1. PAR
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    PAR Yacht Designer/Builder

    Though I've suggested wood flour previously as a thickening agent, I tend to avoid this now, depending on the species (flour) used. Softwood in particular tend to suck up a bunch of resin initially, though once saturated and exotherm starts up, the wood particulates release the now thinner resin, making it runny and no longer a "non-sag" mix. Cotton flock and the other usual suspects (fillers) don't do this as significantly.

    I too have tested various mixes, against straight resin coatings and milled fibers, silica and graphite really toughen it up. Decomposed granite (with a splash of silica) is a mix that's very tough too and relatively inexpensive. You can get this rock dust from tile supply outfits. Get it on smooth, as sanding this will test you.
     
  2. 300wm
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    300wm Junior Member

    If it's that challenging to sand, then it's exactly what I'm looking for. I'm pretty sure I read you talking about polyurethane needing a good Scotchbrite rubbing between coats. Does epoxy need any kind of scuffing between coats or do you just sand the end of the three coats?
     
  3. Pericles
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    Pericles Senior Member

    Some years ago, I mixed Portland cement with West System resin to bond expanded mesh onto a cracked concrete block garage wall. The resin required far more cement than I expected before it began to thicken. Even then, the mixture continued to slump & flow down the wall a little, but the wall is stabilised & now cement rendered.

    However, I spread some of the mix on a piece of scrap ply. It flowed smoothly & when it fully cured, it resisted being scratched with a coarse steel wire brush. It's heavy, but it could be a strong bottom finish for a canoe. Use this method to save time.

    http://duckworksmagazine.com/03/r/articles/glass/bottom.htm
     
  4. PAR
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    PAR Yacht Designer/Builder

    Most coatings (epoxy and polyurethane included) will need sanding prior to subsequent coatings, unless you're within a "chemically active window". Some formulators and manufacturers will not tell you what their non sand window is, suggesting you wait until full cure and scuff it up. This is likely a liability issue, but most every coating has one, where no sanding is necessary. As a rule, most homebuilts don't have environmental conditions that will permit repeated non sanding recoating. Suicidal bugs and dust will quickly force you to sand their bodies out of whatever you putting down. Just using fluorescent lights above the work will create dust (yep, actually generate dust). So, my advice is no short cuts without a paint booth, which means scuff prior coats before over coating, regardless of material being applied.

    Yeah, stone dust works good. I use it on leading edges of appendages regularly, typically molding it, so I don't have to do much sanding. Cement has a tendency to suck up a bunch of resin, but release it when the exertherm gets going good and thins out the goo. Wood flour does this too. A bit of silica can help a lot in this regard. I don't think there's any epoxy mixes I make, that don't have a smidge of silica in it, even fairing compounds.

    Given your canoe is already pretty darn heavy, go with something lighter than stone. Silica is tough enough alone, though a real bear to sand, so maybe some Q-Cells for bulk and to ease the sand chores. Not much weight and a tougher surface than simply coating the bottom with resin.
     
  5. 300wm
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    300wm Junior Member

    I did the cinder block test, today, (in the rain...piece of ply spot glued to a cinder block) with the resin, alone, on a 48 hr cure. All it did was put surfaces scratches on it and they weren't even that bad. I dragged it about 30' before I looked at it. What made the biggest impact on the test was dragging it up and down my concrete driveway. It didn't wear through, but the concrete did dull it way more than the asphalt did. Just the resin, alone, will be plenty tough enough for what I want. The bottom of my yaks will never get that kind of abuse, and I think the cinder prolly puts more weight per sq inch than the yak will. I'm very happy with the results.

    If I could ask another question, off topic...what ratio of thinning would I use with clear gloss urethane to get the first coat to seep into the wood, well, without over thinning? This will be for the deck and sides. The exact name of the ply is 'underlayment' and it's 7/32" thick.
     
  6. PAR
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    PAR Yacht Designer/Builder

    If you thin urethane, it depends on the actual vehicle used, though spirits will usually do, some require toluene or other chemical. Check the label. If on raw wood you can cut as much as you want, though much more than 30% is self defeating to a degree, requiring more coats. Penetration isn't as important as you might think. The idea with the first coat on raw wood is to seal the pores, with subsequent coats needing only enough cut to permit it to flow nicely. It also depends on application. Spraying requires more cut, than brush or roller and again, just enough to permit the coating to self level. Temperature also can be a factor.
     
  7. pauloman
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    pauloman Epoxy Vendor

    poly paints want a non porous surface to go on. that pretty much means a primer, often epoxy
     
  8. PAR
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    PAR Yacht Designer/Builder

    I think he looking to clear coat raw wood Paul, but you're right about polyurethanes. I've given up on anything but epoxy primers now. If he's looking to put a clear polyurethane over raw wood, he'd still benefit from a sealing coat, which would be better if epoxy, instead of thinned polyurethane.
     
  9. 300wm
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    300wm Junior Member

    Got it. Thanks. I'm just going to epoxy the entire shell and call it a day.
     

  10. 300wm
    Joined: Jul 2014
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    300wm Junior Member

    A little update, here. I have everything put together except the deck. All bulkheads are in as well. Once I got everything on the outside shaped and sanded, I epoxied it (bare wood...no sealer). I did in in stages where I applied first coat and let cure, then wet sanded with 320 just to knock off the burrs. I then used Stotchbrite pads (it took a few and it took a lot of pressure) to get the shiny, lows. Did second coat and wet sanded with 320, but went fairly flat with that sand. Ended up with very smooth finish. Not going to lie...it took some sanding to get it smooth enough to paint, but the result is exactly what I was looking for.

    On the test piece of wood I used to test the cure and recoat, I used a flat head screwdriver to press into the cured epoxy to see how much force it would take to puncture the two coats. It was quite a bit more than any pebble or shell will ever put against it. Two coats of single stage poly would not have been tough enough. For the extra $60, it is well worth knowing I don't have to touch up nicks that may let water get to the wood.
     
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