Realistic expectations when re-coring deck?

[thinwater]

Thanks for the vinegar tip. Sounds much better than solvents!


Wow! This is outstanding advice! Thank you! Based on the fact that it's very difficult to reuse the old skin, I think I'll take your suggestion and do this repair from below. Does the new "skin" provide any structural support? I'm trying to determine the appropriate cloth to use. I assume standard 6oz is appropriate for this application?

I plan on using balsa. If it lasted for the past 40 years, it'll outlast the boat. Is there such a thing as "marine grade balsa" or can I buy regular balsa and add scores/cuts with a table saw?

Yes, the inside skin is structurally vital and provides much of the strength (the inside is the part of the I-beam that is in tension).

No, 6-ounce cloth is not primarily for structural work. It is, as it is called, finish cloth. it would take a lot of layers and the resin-to-cloth ratio would be poor. You will not use it. 1708 biax is the best material for this job, probably 2-3 layers, but match the existing layup. If there is no liner, you may use 6-ounce cloth as a "finish" layer to provide a better finish, of the 1708 has not laid down well. BTW, 1708 has a thin attached mat layer. Place that up, against the core, for better bonding. Also, alternate directions on the 1708 (it is not symmetrical).

 

[rgranger]

Does the new "skin" provide any structural support?

An engineer could probably explain this better but it is my understanding that the majority of the hull stiffness comes from the thickness of the hull and has less to do with what the hull is made of. Until the mid-1970's, builders just piled on more fiberglass to get the thickness they needed but as the price of materials rose, they adjusted their building methods and starting coring segments of the boat. If the core is mechanically bound to the inner and outer skins, you can get a very thick (stiff) deck that is relatively light and less expensive to build. What the core and skins are made of is less important than the need to mechanically (or chemically) bond the core to BOTH the inner and outer skins. If you have good bonding, then any stresses on the surface get transfered over a larger volume of material and you have the stiffness you need.

Okay you engineers on here... tear this up.

 

[Project_Mayhem]

I have some 6" 1708 tape laying around. Any tips to ensure it's saturated? I've soaked it in resin, let it cure and later cut into it to find that it was not completely saturated

The structural sandwich deck description makes sense. It reminds me of the difference between a small solid tube and a larger hollow tube. When the weight is spread out the hollow one will have less deflection. Or if you have two pieces of thin plywood and you bond them together they become stronger. Put them further apart while still bonded and they become even stronger

 

[jssailem]

• 6" 1708 tape
• I've soaked it in resin, let it cure

Not sure what you mean soaked. The tape is woven. If you pour resin w/ hardner onto it and let it sit till curred, there will be air bubbles and gaps in the woven glass with no resin. This sets up the opportunity for moisture to enter.

You need to pour the resin onto the material. Then dab the cloth with a chip brush to push the resin into the weave. Next roll the cloth with a small metal roller to squeeze the resin through the cloth and roll out any air bubbles. This process will saturate the cloth and force resin around the fibers.

EXAMPLES OF ROLLERS

 

[Project_Mayhem]

Not sure what you mean soaked. The tape is woven. If you pour resin w/ hardner onto it and let it sit till curred, there will be air bubbles and gaps in the woven glass with no resin. This sets up the opportunity for moisture to enter.

You need to pour the resin onto the material. Then dab the cloth with a chip brush to push the resin into the weave. Next roll the cloth with a small metal roller to squeeze the resin through the cloth and roll out any air bubbles. This process will saturate the cloth and force resin around the fibers.

EXAMPLES OF ROLLERS
View attachment 200822

In the past I've mixed up some resin, submerged the 1708 and then placed it onto the boat. I think your suggestion with the roller will definitely help. I've that the rollers get messy very quickly when used on bare fiberglass. I've also found that it likes to pull up the cloth/mat. What's the secret to proper use? Spray it with a release agent? Put a piece of thin plastic over the fiberglass while rolling out?

 

[jssailem]

Roll from the center to the ends. Not back and forth.
Do not over roll.
No on the release agent will contaminate your layers.
No on the plastic will just be a mess on a mess
Just get in there before the resin starts to kick. Do the roll and let it sit to kick.
Clean up the roller immediately before the resin kicks.

 

[thinwater]

I have some 6" 1708 tape laying around. Any tips to ensure it's saturated? I've soaked it in resin, let it cure and later cut into it to find that it was not completely saturated

The structural sandwich deck description makes sense. It reminds me of the difference between a small solid tube and a larger hollow tube. When the weight is spread out the hollow one will have less deflection. Or if you have two pieces of thin plywood and you bond them together they become stronger. Put them further apart while still bonded and they become even stronger

I dislike tape; the edge reinforcement makes a lump.

I think an I-beam annalogy is simpler. The top flange is in compression, the bottom in tension, and the web (core) in shear (harder to visualize, but obvious when a sample is tested).

When saturated, the material (1708 or other) will be come highly translucent. It will be pretty obvious. Not clear, like cloth, but obvious.

Note: you cannot use regular mat with epoxy. The binder only dissolves in polyester resin.

 

[rgranger]

Clean up the roller immediately before the resin kicks.

yup, This is where the vinegar really comes in handy. Keep a small pail of vinegar and dip your roller in there ... soak it well and wipe it clean with a paper towel.

If you find the resin is kicking too fast, chill it first. It will mostly remain unreactive until it starts to warm up.

 

[Project_Mayhem]

yup, This is where the vinegar really comes in handy. Keep a small pail of vinegar and dip your roller in there ... soak it well and wipe it clean with a paper towel.

If you find the resin is kicking too fast, chill it first. It will mostly remain unreactive until it starts to warm up.

I love working with fiberglass in 80-90 degree weather. I can get a lot more work done in a short amount of time.

Overall I think I have a pretty solid idea of how to approach this project. Corrections and advice are welcome
1. Remove lower skin and rotten core
2. Remove deck hardware if needed
3. Order balsa and if needed, more 1708
4. Install balsa: Coat all surfaces with (thickened?) epoxy and apply to bottom of deck
5. Fill in any gaps with thickened epoxy
6. Apply 1708 over balsa
7. Apply lightweight cloth if desired

Will any balsa work or do i need "marine grade" balsa?

 

[thinwater]

Balsa for cores:Jamestown Distributors

• End grain.
• Kerfed and attached to flexible backing.

Notice that the balsa is in finite squares. There are flat spots. You MUST use thickened adhesive to fill the gaps when installing. Otherwise it will not be well bonded, and it may fall on you.

Thickener is a vital part of these jobs, as is learning how much is enough. Do not over thicken; thick ketsup is about right to start.

 

[Project_Mayhem]

Balsa for cores:Jamestown Distributors

• End grain.
• Kerfed and attached to flexible backing.

Notice that the balsa is in finite squares. There are flat spots. You MUST use thickened adhesive to fill the gaps when installing. Otherwise it will not be well bonded, and it may fall on you.

Thickener is a vital part of these jobs, as is learning how much is enough. Do not over thicken; thick ketsup is about right to start.

Ahhh I did not realize that the balsa is attached to a flexible backing. That should make it much easier to install. I'm guessing the end grain absorbs the epoxy and results in a stronger bond?

I use a mini ice cream scoop to get repeatable consistencies with 404. I'm always amazed at how much I need to mix in to get the desired result

 

[thinwater]

Ahhh I did not realize that the balsa is attached to a flexible backing. That should make it much easier to install. I'm guessing the end grain absorbs the epoxy and results in a stronger bond?

I use a mini ice cream scoop to get repeatable consistencies with 404. I'm always amazed at how much I need to mix in to get the desired result

Yes. The but the real reason for end grain is high compressive strength (balsa is hell weak the other way) and high shear strength (balsa and most woods are weak the other way). The purpose of the core is NOT to carry the flexation load, but to keep the two skins in precise location relative to each other, and transfer vertical compression and horizontal sheer loads. It is actually STRONGER than plywood when used this way. Many users don't get this part of the engineering, and think plywood is a good core. it is a terrible core.

The only real weakness of end grain balsa as a core is rot. You must keep it dry.

 

[PaulK]

Might be helpful to read some of the pertinent chapters of this The Gougeon Brothers On Boat Construction book
available free online, though if you get the book you don't have to worry about getting resin in your keyboard.

 

[jssailem]

resin

Makes my mouse sticky...

 

[Project_Mayhem]

Yes. The but the real reason for end grain is high compressive strength (balsa is hell weak the other way) and high shear strength (balsa and most woods are weak the other way). The purpose of the core is NOT to carry the flexation load, but to keep the two skins in precise location relative to each other, and transfer vertical compression and horizontal sheer loads. It is actually STRONGER than plywood when used this way. Many users don't get this part of the engineering, and think plywood is a good core. it is a terrible core.

The only real weakness of end grain balsa as a core is rot. You must keep it dry.

That's pretty interesting that it's stronger than plywood when used correctly

This is the image of the balsa from Jamestown's website. It looks like there's little to no kerf/space between the blocks so the flexible backing would have to face down. In a perfect world the balsa would conform to the curve of the deck and stay like that while I coat it in resin and fill in the spaces between the blocks. In reality, those blocks are going to lose the deck shape in the process. When the blocks are pushed together (straightened out) some filler between them will ooze out. With less filler between them, there will be a void when spread apart to match the deck shape. What's the best way to avoid this issue? Work in small sections? Do a row or two of blocks at a time?

 

[PaulK]

That's why you use resin that isn't too thick. Fluid resin WILL fill the spaces between the blocks of balsa and soak a bit into the wood. This is good because it closes off potential paths for any water that might leak in later. When you remove the rotted core you will likely find the skeletal remains of the resin that filled in between the blocks when the original deck was laid, sticking out from the underside of the deck. You will have to grind/sand these down so the new balsa can conform to the underside smoothly.

 

[jssailem]

PM you posted a question about saturating the fiberglass to soak in the resin. I saw this video adn thought it might help. It shows the guys usign the roller and comments about how to use it and when the glass has soaked up the resin.

 

[thinwater]

In the past I've mixed up some resin, submerged the 1708 and then placed it onto the boat. I think your suggestion with the roller will definitely help. I've that the rollers get messy very quickly when used on bare fiberglass. I've also found that it likes to pull up the cloth/mat. What's the secret to proper use? Spray it with a release agent? Put a piece of thin plastic over the fiberglass while rolling out?

You do NOT submerge the 1708. You pour mixed epoxy onto it, on the worksheet, and spread it around and work it in for a 20-30 seconds with a 3-inch disposable plastic squeege. Neat and fast. Then you roll with a ridged laminate roller, not a regular paint roller, while still on the work sheet to work the air out. After it starts to become translucent, place the section on the ceiling and roll it in for a minute or so, working from the center out. it should be well bonded and fully transluscent. You should not have to add resin, but you may until you learn how to gauge the correct amount. If there is extra resin coming to the top (there should be) it will be absorbed into the next layer; adjust as needed, and the final finish layer may require no resin at all, wetting from the excess resin you squeze out. It will still need rolled in, and the edges may need a few brush strokes of touch-up resin, but generarlly very, very little.

 

[jssailem]

All good descriptive elements of what is shown in the video. They work well together.

If there is extra resin coming to the top (there should be) it will be absorbed into the next layer

Of note the video is being filmed in a work shop where the temp is controlled. Additionally they use a slow setting resin. This gives them time to add additional layers to the project. The extra resin is not yet kicking off, so that the second layer and third of cloth can soak up the extra resin. Once the resin starts to kick off (gel), it will no longer be soaked up by additional layers. You just step back and let it do it's thing. Once it has cured, you can go back and fix any places that are not right.

Have fun and go for it.

 

[Project_Mayhem]

This has been an incredibly informative thread!

@jssailem That video has a lot of little techniques to resolve issues that I've come across in the past. Thank you for posting it!

@thinwater I'll definitely give this a try

When is it appropriate to use a foam roller? They seemed to use it when initially wetting out the cloth and around corners in the video that @jssailem posted