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Thread: How much plaster should I shop for?

  1. #1
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    Default How much plaster should I shop for?

    I've got an old, much abused fiberglass canoe I bought cheap a few months ago. I like the aspect of "fiberglass" because I knew that various cracks and gouges would be simple to fix, and the whole thing was hungry for a new coat of paint. There was one exceedingly poor "patch" midship along one gunnel that has been a puzzle for a while; how to cut away a short length of PVC waterpipe that had been encased in fiberglass under the gunnel and remove a flat hunk of PVC sheet adjacent to it. Then inspiration struck; why not make a plaster cast of a section of the interior of the "good " side, cut away the whole poorly patch area, apply the plaster "mold", and laminate a proper patch over the void.

    My question is simply how much plaster to purchase? I know a 5lb bag of dry plaster, mixed with water, does not yield a gob of plaster as large as the original bag. I will need a gob of mixed plaster about as large as a soccer ball.

  2. #2
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    Default Re: How much plaster should I shop for?

    I've never used plaster for that job...

    I have taken a fibreglass mould from a good boat to create a new part for a damaged boat.
    I'd much rather lay in my bunk all freakin day lookin at Youtube videos .

  3. #3
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    Default Re: How much plaster should I shop for?

    I once made a large plaster exterior mold of my Shearwater's bow after sawing away part of the hull underneath to remove E&D's bow centerboard case. I filled in the void where the case had been with modeling clay to simulate the final contours. Forgot how much plaster was required, but I remember having to go out to get more at a builder's store. New fiberglass was laid into the mold using SLOW hardener, then the whole thing was jacked up against the hull to cure. It all worked out very well

  4. #4
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    Default Re: How much plaster should I shop for?

    With your proposed method, you would still need to fair and feather in around the edges. So, if you have to do the finish work anyway, why not just cut out the bad section and laminate a new section in-situ? Then do the finish work.

    Kevin
    There are two kinds of boaters: those who have run aground, and those who lie about it.

  5. #5
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    Default Re: How much plaster should I shop for?

    Cutting away 18" of gunwale to a depth of 8" down the topsides (which have a nice tumblehome) leaves nothing but air to lay the new laminate against. Yes, the edges of the cut require feathering. After three layers of exterior laminate cure, I will add a fourth on the interior, feathering that as well. With careful longboarding one may not even notice that the topsides and gunnel were rebuilt.

  6. #6
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    Default Re: How much plaster should I shop for?

    Quote Originally Posted by Nicholas Scheuer View Post
    Cutting away 18" of gunwale to a depth of 8" down the topsides (which have a nice tumblehome) leaves nothing but air to lay the new laminate against. Yes, the edges of the cut require feathering. After three layers of exterior laminate cure, I will add a fourth on the interior, feathering that as well. With careful longboarding one may not even notice that the topsides and gunnel were rebuilt.
    You could tape on some cardboard, or better yet foam, and lay up on that.
    But,I see your point. Always more than one side to a story.
    There are two kinds of boaters: those who have run aground, and those who lie about it.

  7. #7
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    Default Re: How much plaster should I shop for?

    Do the layup oversize on the good side of the boat, pop it off and trim to fit and join it over the hole.
    -Dave

  8. #8
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    Default Re: How much plaster should I shop for?

    ^^^ Good idea in some cases, however, here the shape of the gunnel (section similar to a question mark with a flat top) would be too much "oversize" for a decent matchup.

  9. #9
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    Default Re: How much plaster should I shop for?

    The plaster will work, maybe but you will have to do something to keep the resin from bonding to it and it needs to be solid against the hull. It has also been my experience that large amounts of plaster can be really hard to mix, one moment it 's white water the next it's hard as a rock. There are ways to mix large quantities but it is a skill and I don't have them. With fiberglass I have used thin cardboard taped up just right, then coated with resin several times on the mold side, built it up on the outside to give it strength being very careful about drips. I would use lots of wax, even leaving a layer on the piece which would be trouble if you wanted to gelcoat it later. the whole thing is kind of micky mouse. Really comes down to how big of a hole and the shape, if curved in two directions you might have to just sculp something and patch it in.
    The other thing about plaster is the air bubbles, good casting plaster is not cheap I make core boxes for my little foundry projects but works so much better than something like plaster of paris but costs ten time as much, and then there is sculpture quality plaster.

  10. #10
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    Default Re: How much plaster should I shop for?

    ^^^ Will lay some polyethylene on the hull (tipped up on beam end) first. The cast will likely have a few wrinkles, but these can be smoothed out after casting the shape.

    The Plaster Of Paris for the Shearwater Bow mold was poured into an 18" x 30" box positioned a couple of inches away from the hull and required a bit of smoothing after removal from the hull. A generous coat of auto wax was used for a release agent. One interesting aspect of the project was to lift the 28' hull off the trailer with a overhead beam loading hoist at the loading dock in the factory where I worked and reverse the hull so that the bow overhung the aft end of the trailer. When the project was complete, the procedure was repeated in reverse. That whole project came together without a hitch. TRUE NORTH proved to sail a bit faster without the original bow centerboard case. One Shearwater had previously been built by Edey & Duff sans bow centerboard and it had been faster than my boat. Bolger once said "anything disturbing the flow of water forward of the maximum beam will slow the boat.

    Still don't know whether to buy a 10 lb bag of plaster or 20 lbs. I want to have enough plaster at the outset.

  11. #11
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    Default Re: How much plaster should I shop for?

    Quote Originally Posted by Nicholas Scheuer View Post

    Still don't know whether to buy a 10 lb bag of plaster or 20 lbs. I want to have enough plaster at the outset.
    It's been quite a few years but I used to mess around with plaster a lot. I bought it in 100 pound bags. Sorry I can't give any advice on how much to purchase... mostly because I never thought about using it in that way. But surely there are tables or formulas that have been published that give yield results. If it were me, I'd just buy the biggest sack I could manage. It also depends on what is available where you live. I just checked at Seattle Pottery Supply... they have 50 pound bags for about $26. At that price, that's what I'd get. Then I know I've got enough and I'll probably have some left over to mess around with on who knows what.

    Jeff

  12. #12
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    Default Re: How much plaster should I shop for?

    25# of DAP branded plaster of paris sells for $19.99 at the ACE I work at. From what you've posted I think you'd need maybe 40# for what you want to do, less if you use fabric as a reinforcement (in a manner similar to fiberglass cloth in epoxy) which would help reduce weight of your casting.

    Don't try to pour too much at a time, that only slows drying and adds weight. I'd build up three or more layers; first two of plaster alone then start with the fabric stuff until it's about 1/2" - 3/4" thick then start adding ribs – thin wood wrapped in plaster-saturated cloth – for stiffening in both dimensions.

    Toughest trick I'd think would be fabricating your mould so it doesn't lock into the hull section you're trying to make an impression of. That's where a more flexible urethane casting resin might come in handy.

  13. #13
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    Default Re: How much plaster should I shop for?

    -Try making it like a plaster cast for broken bones- look on Amazon for plaster cloth rolls- plaster impregnated gauze- make a cast of the good side and use the cast as a mold on the other side- no mixing and pouring plaster.

  14. #14
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    Default Re: How much plaster should I shop for?

    Quote Originally Posted by paulhh View Post
    -Try making it like a plaster cast for broken bones- look on Amazon for plaster cloth rolls- plaster impregnated gauze- make a cast of the good side and use the cast as a mold on the other side- no mixing and pouring plaster.
    Paulhh might have a good suggestion there.
    I use Plaster Cloth a lot in my other hobby, Railroad Modeling, for mountain scenery, etc. It's extremely easy to use, MUCH less messy, very easily moldable to complex shapes and seems to be as hard as cast plaster with the gauze reinforcement built in.

    It could be worth trying. It would cost more, but I'll never go back to mixing plaster in a bucket again for reasonably small areas. You would still need a lattice work of maybe 1" cardboard strips to lay the cloth against.
    Here's a basic primer. You can also use bubble wrap/masking tape lattice work, etc.
    Look on Amazon for good prices and larger sizes. I've build a Cascade Mt. range on my layout that is strong enough with 4 layers or so to lean on "gingerly". My overlay is a resin based plastic material (much less strong and hard than fiberglass) that's the painted realistically (like a museum diorama) and I can put quite a bit of weight on that.

    https://www.youtube.com/watch?v=JxYN-MGtpVY. Seeing as you'll be using it temporarily for a mold, it might do the trick just fine. Although it would be more expensive than powdered bags of plaster you could try a small roll lst and see if it fits the bill.
    https://www.amazon.com/s?k=plaster+c...f=nb_sb_noss_1. The real advantage is that you can do a little at time, rewet the old plaster cloth with a damp sponge or cloth and add on even a day later. You might not feel as stressed/rushed to complete the job and you won't have to worry if you've bought enough plaster sacks because you can mock up the whole thing beforehand (pre-wetting) by merely taping the cloth over your work area. I've never heard of anyone using it in boat building but don't see why it wouldn't work...
    Hope that's potentially helpful.
    DP

  15. #15
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    Default Re: How much plaster should I shop for?

    I have a little experience of using plaster to mould from.It absolutely has to be dry and that may take a couple of weeks.The surface of a piece cast from a hull may be very porous and it would be no bad thing to seal it with a wiped coat of PVA and then to wax that.You could alternatively use shellac as a sealer under the wax.The plaster is quite likely to break when you try to remove it and even then,the repaired section will need some cosmetic finishing.

  16. #16
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    Default Re: How much plaster should I shop for?

    Why did you remove the bow centerboard? Why not just glass over it? (Shearwater curious!)

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