Question about using epoxy 2 different ways with plywood on frame construction

This question is a result of reading a lot of posts about epoxy use and assembly of wooden boats. My boat is a 22 foot Carolina dory, plywood on frame. First method is to simply apply epoxy to bare wood surfaces being joined and then screw in place. Flip the hull and epoxy the inside. Second method is to encapsulate frames in epoxy first. Set them up on the strongback and temporarily screw plywood in place. Get the fit right and cutting done.
When ready to put plywood on permanently remove temporary screws and epoxy the inside of the plywood fill screw holes with thickened epoxy and screw back in place. This means I will be joining a wet epoxy to a cured epoxy surface [ the frames]. Will I get a good bond this way? Ive read yes and no from other sources. If this works I wont have to epoxy the inside of the hull after flipping. I am disabled and really cant work on my knees so i have to think out how I do things. I trust this forums opinions more than others.

 

It depends.

Every brand of epoxy has a typical open chemical bonding window; this is called primary bonding.

After the primary bond window has closed; you must seek to achieve a secondary, mechanical bond. Epoxy is ver strong, but it will sheer off glassy, uprepared substrates easily. Until you don't want it to, that is....he he.

So, let's say you epoxy coat 6 frames from morning, midday, and nite to get 3 coats of epoxy on them. Then let's say you have an epoxy with a 48 hour bond window. And you get two of the frames installed in that 48 hours. It can technically be done without sanding.

The other four frames would require sanding with 40-60 grit paper on all surfaces to create a mechanical bond surface.

I will tell you that I always sand after 24 hours despite my epoxy and a 72 hour bond window.

 

Hello, I've been reading all of your threads about your boat build. I recently learned that Jeff Spira passed away this past spring 2022 and his plans are not available to purchase anymore - so, I am looking to purchase the plan or a copy of the plan I want from someone who has one. Would you be interested in selling me your original 27' Mobile plan? That is the one I want to build for myself.

 

You could precoat the bulkheads and use peel ply. No sanding you just remove the peel ply to the areas to be bonded.

 

did some samples a few weeks ago: this worked well, all broke in the ply: precoated+sanded, precoated+peelply, wet-in-wet

 

Peelply would not be considered wet on wet.

Wet on wet is when you let a layer kick and then apply more.

Using peelply is more like secondary bonding, with sort of a guarantee of getting rid of amine blush.

 

3 samples:
1. precoat & let harden 10 days, sand, fillet-let harden 10days, sand, glass
2. coat & fillet wet-in-wet& peelply, let harden 10 days, laminate
3.coat & fillet & glass in one continuous operation
these were samples of 10mm Okoumee chines (>120°)

 

Wow. So much to unpack here. Not going for all of it.

Let me start off by telling you precoating is done one hour before thickened resin bonds, not 10 days.

why?

Well, the whole point of precoating is to create a primary bond and to avoid sucking the resin from the thickened material.

By precoating 10 days in advance; you have added all sorts of intangibles into the process.

1. The edge to edge bond becomes secondary, not primary.
2. Dirt can enter the bondline.
3. Sanding the endgrain may not sand all of it. Deep areas may not be sanded. Sanding between chines has access issues..
4. Sanding the endgrain may re-expose thirsty endgrain, defeating the entire purpose!

In almost all my work, I fillet and tab at the same time. This means all the work is primary bonding.

Now, for chines, impossible. Chines typically require access to both sides. And one side is painful to deal with.

The proper way to do the chine joint is as follows, using peelply is optional and if done properly should have no effect on quality.

A. Tape the reverse side of the seam to support the thickened resin.
1. Prewet the seam.
2. Allow the resin to kick to sticky for an hour or more.
3. Fill the seam with thickened resin; avoid making it too dry.
4. Allow it to cure. The time is arbitrary.

5. Sand the joint outside or if you used peelply, pull it when ready. Typically the bondline will have shrunk. It is wise to etch the bondline with 60 grit or less in either case. I sand always; even with peelply.

6. Use more filler to deal with shrink. And tab the same day. Tabbing and fillet are always best done as one, but chines impossible as I said. But it is possible to add fillet or do the inside fillets same day as tabbing.

7. For flip side, same.

8. In the times you cannot fillet and tab same day, peelply is okay, but I almost always sand over it or spot sand.

Now about this 10 day wait in the process. That is not done.. All for now, but I could say more.

 

We have different concepts of "precoat", fallguy. What you mean is a saturation coating before the fillet
my definition: "precoat": apply either a flowcoat or 3-5 coats wet in wet, let harden comletely, sand
(for reference: The Gougeon Brothers On Boat Construction book https://www.westsystem.com/the-gougeon-brothers-on-boat-construction/)
when you "precoat" the frames, you will have much more time than 10 days before the hull-skin is joined....
The purpose of the samples was to see, if the secondary bonds are strong enough; they were, they all broke in the plywood.
(& thank you for the education, but I have worked quite a bit with Epoxi...)
solvent-free Epoxi will shrink very little, btw.

 

By the way, you state 3 samples, but only show us two pictures.

Since the precoating was purpose defeated, I'm not drawing much conclusion.

Okay, that was what I meant by lots to unpack. One piece was the breaks were as expected for the tabbing.

 

My breaks are looking all the same. I made "bulkhead-joints" too (Ts 90°, fillets & tabbing both sides), clean breaks in the ply above.
My posts were in answer to the OPs question
"Second method is to encapsulate frames in epoxy first."
& to test the secondary bondings of fillets on "precoated" ply & tabbing on completely cured & peel-ply-ed fillets.
If you want to see all the pics, fallguy, I'll photograph tomorrow everything.
I don't have the feeling that any purpose has been defeated

 

With limited mobility I'd pursue the method that stresses your body the least. This will lead to better chances of the project being completed.
I am usually skeptical of secondary bonds done with cured epoxy but the number of respected builders that trust the bonds to cured peel ply surfaces is convincing.
Any time you can apply epoxy or fiberglass on the bench it saves a lot of fussy work later.

Russel Brown has a cheap ebook on his epoxy techniques and videos on offcenterharbor.com that are worth the small price to watch and read.
The guy is amazing.

 

the pics (correction of my posts above: the chine samples were not made to test secondary bonds, the „T“ joints were. 4 samples with either no or all or some secondary bonds)
to eleborate a bit belatedly:
no peeply used, just wiped with methylated spirits & sanded thoroughly
(peeply would quite likely have given the same results
the chine samples were to test if 2 layers of different width double-bias tape would give the same results as 2 slightly staggered layers of same width db tape. Used some peeply there as I hadn't used any before.
Ply is Okoume, further tests with denser ply would be interesting, but I guess the wood would still fail before the fillet/tape joint

 

Thanks for sharing your research, tane, very helpful.

 

to get our terminology right (after all this is my 2nd language) I consulted the Gougeon book:
your "precoat", fallguy, is a "saturation coat" (for wet-in-wet)
my "precoat" is "prefinishing" (for "secondary bonds"

interesting the comment of the G.Bs. about the "secondary bond": as Epoxy is a glue it bonds very well to itself, provided surface is cleaned & abraded.
tried to peel the double bias:
wet-in-wet & prefinished. Both failed in the top-layer of the ply