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Rachel
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Re: What paint grade plywwod to use?

Post by Rachel »

Well it looks great as-is! ;)

I wonder what the experts' view is on MDO for paint-grade bulkheads. I have not replaced bulkheads, so I have not been down this path, but I see it recommended from time-to-time, and it certainly is smooth. What I don't know is if it is overly heavy, not structural enough, or has one or more other negative properties.

I have seen it recommended to remove the paper where the tabbing will go, but then that would apply to any thin veneer/top layer.

Rachel
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Re: What paint grade plywood to use?

Post by Skipper599 »

Personally, I would definitely consider MDO and here's why:

MDO Plywood - The Paintable General Purpose Panel
Characteristics: An EXTERIOR type plywood with a weather-resistant resin overlay bonded to the wood by heat and pressure. This process fuses the molecules of the overlay with the fibres of the wood to form a bond as strong as the wood itself. MDO has all the advantages of regular plywood as well as additional properties. The overlay, which has 28% resin content, resists water, weather, wear and degradation. It has texture that paint can grip with remarkable tenacity. Paint finishes on MDO are up to three times more durable than the same finish applied to ordinary plywood.Ideal for Painted Signs: In many instances, overlaid MDO plywood has proven a more durable sign material than metal. MDO plywood is resistant to the elements with no danger of rust or corrosion.

The DOH uses this material for Hwy Signs - you know, those big green boards with info in white you see along the highways.
I am: Bob of Wight.

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There are good ships, there are wood ships, and these ships sail the sea

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Re: What paint grade plywood to use?

Post by mitiempo »

The MDO sounds good and it will take a good paint finish. Okoume or meranti marine ply will take a good finish as well but the MDO is probably a bit less likely to dent if something hits it. Whatever you do I recommend against any fir plywood (excepting MDO) as it is very hard to get a smooth paint finish without the grain showing through.

I would use the MDO for cabinets and such but I would use a good quality marine ply for any structural bulkheads. Choices would be Hydro-Tek (Meranti), Aqua-Tek (Meranti), or the Joubert Okoume ply for any structural bulkheads.
Here is some information here http://www.westwindhardwood.com/price_marine_ply.php
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Re: What paint grade plywood to use?

Post by mitiempo »

Tabbed and epoxy edge sealed as you are planning longevity should not be a problem. The only issue I can see with MDO in your use is the quality of the other side of the panel really. It will of course have voids unlike marine ply but that shouldn't be an issue for your intended use.
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Rachel
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Re: What paint grade plywood to use?

Post by Rachel »

mitiempo wrote:The only issue I can see with MDO in your use is the quality of the other side of the panel really.
I was thinking it probably had the paper on both sides, but I gather from what you wrote that it's only on one side?
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Re: What paint grade plywood to use?

Post by mitiempo »

Rachel

You could be right - it doesn't state whether both sides are coated or not. http://www.canply.org/english/products/ ... lywood.htm
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Re: What paint grade plywood to use?

Post by mitiempo »

If it is domestic it will be fir throughout. After a bit of research I would pass. It is heavier for one. And this MSDS is an interesting read.
http://www.freemansupply.com/MSDS/scann ... lywood.pdf
Scroll down to hazards.
Zach
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Re: What paint grade plywood to use?

Post by Zach »

I like your layout/design plan.

I have not used MDO, though one of the suppliers I use stocks it, and recommends it for painted walls or wallpapered walls. Its around 20 bucks cheaper than marine grade okume, uses type1 glue and is probably the longest lasting wood based product invented by man. I don't know how well it tabs with fiberglass, and haven't done destructive testing. (IE, glass it and throw it overboard on a rope, and toss chunks in a pile to come back and see how it ages... boil test it with fiberglass...)

What is structural:

As far as the Triton goes, in stock form the main bulkhead is structural, as is the cockpit bulkhead. The reinforcing "stringers" that keep the hull from oil canning in the bow, were the tops of the bunks and the small hat shaped stringer just below the shelf on each side of the v-berth. The lifting force of the forestay is transferred into the knee, that runs to the front side of the chain locker bulkhead. The chain locker bulkhead holds the bow apart, and keeps the v-berth from bowing up and down allowing the hull to oil can.

The main bulkhead keeps the shape of the hull spread apart, the beam takes the load of the mast. The mast is trying to go through the bottom of the boat, while the rigging is attempting to bring the hull sides together. The wood partners under the beam, keep the bulkhead from flexing under load. The downward force of the mast, is the weight of the rigged out, loaded boat. Grab a copy of Skenes yacht design and you can find all the formulas to calculate out loads and what to build.

The hull is stiffened by the berth tops in the salon, running the length to the cockpit bulkhead, the faces of the berth keep the berth tops from moving out of column and allowing the hull to flex. Again the shelf bulkhead is sitting on a hat shaped stringer, and reduces the distance between reinforcements, keeping the hull from oil canning.

The shelves had dividers that were tabbed to the underside of the deck, which were not really up to much, but do shorten the unsupported distance between the bulkheads. Rudimentary hanging knees...

The cockpit bulkhead, keeps the hull spread at the aft end of the salon, but doesn't see near the amount of load of the bow. The stiffness of the aft half of the boat, is in the hull to deck joint, and the "box" profile of the side decks. Your weight, and the weight of a boarding sea is what the cockpit bulkhead see's, but the side decks take a lot of that load too.

The lazarette bulkhead isn't really anything but a partition. The bending force of the back stay, is trying to stretch the stern of the boat into a banana. If you grind the gelcoat off the hull just aft of the rudder you'll see the horizontal stress cracks from this. I think more boats should have stringers running from the transom forward to the bilge so you can crank on more back stay tension for this reason... Dual purpose to keep the cockpit lockers water tight.

All that being said, I would build back to the level and scale of what was existing. The triton is already known to be a strong boat, on the verge of bomb proof. Every additional stiffener, egg crate, stringer adds weight, and uses expensive materials. More important it adds time to the duration of the project... The boat made it nearly 50 years in its current build quality, which is a benchmark to say what works and what withstands the test of time. Going to epoxy, marine plywoods, and doing things by the book will yield a higher quality, stronger boat than what you started with, without going to excess.

Work to draw a line in the sand, at which point the extra structure begins to negatively impact the complexity of the project, the budget, and the use and storage capacity of the boat. Go ahead and buy all the equipment now, and set things in place, mock up locations and build your structural improvements around them so you aren't left with demolition at each stage of new install.

Plywood:

For plywood you can save a lot of time on things that will be glued or resin coated, by sanding off the stack of plywood once when you buy it... and keeping it dry and clean from there on out. It is easier to sand off, as whole sheets production line style than it is to sand cut pieces and not goof up cut edges. Ask how many sheets it takes to get a contractor price. Sometimes its 10,11, 20... it depends where you are dealing. Retail price is almost double the commercial price, and some places if you buy 11 instead of 10, you get the 11th free for the price drop. Figure out about how much you need, buy more up front and hurt once instead of paying 95-130 for a sheet at a time... This goes for all construction materials, then ask the cash price. Grin.

I use marine grade okume for a lot of things that will be painted. Resin coat (add white pigment to the resin) and paint. I like microballoons, or west systems 410 for fairing things that will be painted white but not fiberglassed first. Once the first layer of paint or primer is on, you can switch over to evercoats ultrasmooth with blue hardener. Each coat the material gets whiter, and whiter so you have a hard time burning through and hitting any dark colored putty.

I would stay away from AB fir, or pine faced plywood as they aren't really paint grade... The grain stands up and continues to move around. Okume and Meranti you resin coat once, and fair it once... its slick and there for good. Fir, not so much.

A good white paint for getting a base layer fast, is sherwin williams tile clad epoxy paint, which is as tough a paint as I have found and will fill the grain of plywood, and stand off enough that rounded over corners look fiberglassed. I feel comfortable "Edge coating" the sides of the panel and epoxy coating the back, and tile cladding the front. It's the only paint that I would advise doing that with, everything else resin coat, water wash, sand and paint. It is expensive, but you won't scratch it off or rub through it. It yellows with age however. A close match to snow white awlgrip, is rustoleums white appliance epoxy which lays out nicely on small parts, and its tough...

For inexpensive, easily replaced things I use pre-finished birch for things that don't need any structure and shouldn't get wet. Shelves, dividers, small partitions inside cabinets and things of that nature. Still resin coat them, but make up time and cost on them.

My suggestion is that you cleat out your work, with a 4-5 inch wide piece of 3/4 plywood epoxy/screw laminate to the face of your bukheads and partitions, so that when you put your wood cleat on to catch the faces of the cabinets you can use 1 5/8's square drive stainless screws, with either epoxy and have everything backed up by screws.

A 1 inch cleat screwed or glued to a piece of half inch plywood is strong, but the above when glued together is bomb proof. Butt the 3/4 plywood to the line where you want your cleat. Put your cleat, on the edge of the 3/4 plywood. Screw into the cleat, and not the plywood when attaching your face frame. Put it all together dry, with screws mark the good screw holes. Take it all apart and butter everything up with either epoxy or 5200. You can put the face on with #1 head square drive trim screws, shoot or brad nail it in place and putty over them or counter sink and plug leaving the square drives in place. Pre-drill your holes into your cleat. I use pine for most of my cleats, but juniper (super light weight) and mahogany (better screw holding and rot resistance) work just peachy.

West systems 610 is more expensive, but much easier to apply and spread if you are going production line style on assembly. Two beads down each cleat is minimal squeezeout for cleanup.

On complex pieces, I build them first out of 1/2 inch birch plywood or 1/2 inch MDF and use them as templates for a router to run along. Sand the edges and cuts smooth with 40 grit and fix any burns or low spots with bondo. Get the templates perfect, and route out. Clean up the edges and corners with a rabbet plane, bent neck chisel or sandpaper on a block. Spiral up (For hand held router) and spiral down (for table router) carbide cutters with bearing guides don't tear out or burn very much. I cut close (1/2 the radius of the cutter from the line or template) before making a pass to finish it off.

It's 15 bucks for a 4x8 sheet, and your labor goes to getting the perfect scribe, perfect fit... and you can add back or take away where ever you need to get it perfect.

Zach
1961 Pearson Triton
http://pylasteki.blogspot.com/
1942 Coast Guard Cutter - Rebuild
http://83footernoel.blogspot.com/
Zach
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Re: What paint grade plywood to use?

Post by Zach »

You are welcome...

I don't have any pictures off hand, but I hope this sketch helps.

Screw lamination, you hold the plywood doubler up to the bulkhead in question and screw it to it dry... Don't go through the other side, so short screws. Make a reference mark, back out the screws, mix glue. Spread it over where the plywood is going and hit the same holes, it goes back in the same spot. Put on your wood cleat on the front edge, dry first and then glue it. Wipe up all excess glue with a squeedgee and rag wet with acetone.

Now you have a cleat that is held on to the bulkhead with longer screws, 1 5/8's instead of 1 1/4's, or just glue, and your face frame can take load against the plywood, the cleat being there to add glue surface and make it easier to pull down the face. End grain plywood doesn't like screws, and it is a lot easier to do a nice job installing a face frame, where the center of the cleat is an inch and change off the wall instead of exactly 1/2 inch.
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1961 Pearson Triton
http://pylasteki.blogspot.com/
1942 Coast Guard Cutter - Rebuild
http://83footernoel.blogspot.com/
Zach
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Re: What paint grade plywood to use?

Post by Zach »

Update:

I have had a piece of MDO plywood overboard for two months in salt water, and then two months in fresh water.

It has taken on a bit of water through the end grain, the faces have stayed mostly intact. The very outermost edges of the facing has swelled a bit.

I also put a piece of high end exterior grade A-B faced plywood along the same string, with a piece of okume marine grade plywood.

The advantech pine has a few layers of the plywood that have softened and swelled.

The marine grade has turned a bit grayer than it started, but looks and feels the same as when it went overboard.

Glass work on dry MDO:
Epoxy and 17 ounce cloth sticks nicely. I did a but joint between two pieces, one layer thick and did not glue the end grain together. Put it in a vice, twisted and pulled the other piece the glass strands began to break and delaminate in tension and shear and gave way at before it lifted from the MDO, or the MDO's face tearing. I might not build a structural bulkhead out of it, but I would be comfortable using it pretty much anywhere else.

I'll glass another chunk of MDO and throw it overboard, and see if the facing softens, and the test behaves differently after its been wet for a while.

Zach
1961 Pearson Triton
http://pylasteki.blogspot.com/
1942 Coast Guard Cutter - Rebuild
http://83footernoel.blogspot.com/
Hulukupu
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Re: What paint grade plywood to use?

Post by Hulukupu »

Hi Zach,
I like your experimental approach and appreciate reporting back to those of us watching this thread.
Thank you!
David
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