Stringer to Transom and Hairline Hull Crack Repair

[KD8NPB]

KD8NPB: So, epoxy it is! But I can't find LTM 3610. Can you source it? I did read where chopped strand mat is not compatible with epoxy resin. My guess is that it is hard to fully wet out with epoxy, but I am not sure. I did read where Knytex X-Mat is compatible with epoxy. And I have successfully used it with epoxy in the past.

How can you vacuum bag an object that you only have access to from one side?

Exact nomenclatures vary by manufacturer, I use Vectorply material a lot, so I use their part nomenclatures for fiberglass. What you're looking for is a lateral & transverse material, e-glass, 36 oz directional glass with 10 oz/yd nominal of stitched chop on the backside.

A pure lateral and transverse material could be used, but the chop layer is really needed to decrease the possibility for interlaminate shear.

Some chop strands can be used with epoxy, but mostly the answer is "no". The chop that comes prestitched on to the backside of material like BXM1708 or LTM3610 is OK because they are not treated with chemistry that is specific to UPR/VE resins. Rolled chop-strand mat is usually treated with an emulsion to hold it together, which is styrene soluable. The problem is that epoxy has no styrene in it. Stitched mats with chop stitched to the back is stitched mechanically together...there is no chemical emulsion.

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In order to make things cheap and easy on you;
Buy some 5:1 epoxy based on what temp you're working at.
Buy some 1708

Follow the 3 x layers of BXM1708 that the other user recommended.
Cut the 3 layers identically, BUT.

The middle layer, rotate it by 45 degrees before you cut it.

That way, you have the long fibers in 0 degree plane with the stringer, as well as some going 45 degrees.

Effectively what you would have is 6 layers of material.
a thin layer of stitched chop
+/- 45 degree long fibers
a thin layer of stitched chop
0 / 90 degree long fibers
a thin layer of stitched chop
+/ - 45 degree fibers

Don't worry about the vacuum bagging, it took me months to get good at it, and I made a lot of garbage before I did.

 

[E350]

Fallguy: I never would have understood about the 45 degree without your question. Checking for understanding is always appreciated. Thank you for asking!

Some chop strands can be used with epoxy, but mostly the answer is "no". The chop that comes prestitched on to the backside of material like BXM1708 or LTM3610 is OK because they are not treated with chemistry that is specific to UPR/VE resins. Rolled chop-strand mat is usually treated with an emulsion to hold it together, which is styrene soluable. The problem is that epoxy has no styrene in it. Stitched mats with chop stitched to the back is stitched mechanically together...there is no chemical emulsion.

Ok, most of you are going to say, yeah well we all know that. But I didn't, so thank you! I assumed that mat is mat and that the mat on the back of the X-Mat is the same as my roll of regular mat. Styrene is a polyester-only product. Good to know. Nerds are welcome.

 

[fallguy]

Turn the biax fabric if you want.

I did on my transom; not sure it is absolutely needed here.

 

[Blueknarr]

The beauty of bi-axial:

If FG is pulled directly in line with the fibers, is will resist 100% of the fiber's inherent breaking strength. When pulled perpendicular, it will have effectively zero strength. Pulling at an angle yields strengths proportional to the angle. In bi-axial the strength of the two layers is added together. As the relitive strength of one layer decreases the strength derived from the other increases. However, the bias strength is slightly less than the parallel strength.

Orienting the FG yarns 0/90 to the crack places half of the yarns in their strongest direction. BUT half are contributing nothing. And if any sheer loads are involved the perpendicular yarns won't be at their optimal orientation.

Bias or +-45° tabbing is more able to absorb sheer loads than 0/90° tabbing. 45/90/45 makes incredibly strong light tabbing, but is rarely worth the effort to obtain. Three layers of bi-axial is plenty. Alternating strand orientation will enhance the strength of an already sufficien joint.

 

[E350]

Ok, hopefully I will have time today to get in there with a flap wheel on a 4" angle grinder and really clean up the existing fiberglass.

I will use epoxy 4:1 because I have it, so should I still paint the bare wood with polyester and let it cure before I lay in the epoxy wetted fabric? Or if I paint it with epoxy resin, will it absorb sufficiently?

 

[Blueknarr]

Absolutely do not use polyester to prime wood for epoxy!!!!!

Use epoxy to seal up the wood. It bonds better to wood than polyester does.

You will rarely want to change resin mid project.

If you don't prime raw wood, it will absorb resin; sucking it out of the FG cloth.

 

[E350]

Just now I found a Panda Express fortune cookie fortune in my truck. It says: "DON'T FORGET TO SAY, 'THANK YOU.'"

Thank you!

 

[Blueknarr]

Welcome

You don't have to wait for full cure. Just until it gels up thick enough to prevent further absorbsion. Waiting for full cure is good too.

 

[E350]

OK I got out the 4" angle grinder with a new 40 grit flat wheel on it and got serious. What do you guys think? Is it ready for layup?

 

[Blueknarr]

That looks better. I hope I'm just seeing dust every where.

That is an area where the factory guys don't always lay smoothly, leaving lots of lumps and bumps. A little gelcoat left in the deap valleys is acceptable.

Don't try to lay all the new in one go. You will move the 1st piece of cloth out of place while laying the 3rd. Then both of them with the 5th and 6th. This is how/why it comes lumpy from the factory.

Don't try to use one continues piece of cloth. It won't tuck and fold as well as the factory team thinks. Overlap a few inches and stagger the joints.

 

[E350]

That looks better. I hope I'm just seeing dust every where.

Don't try to use one continues piece of cloth. It won't tuck and fold as well as the factory team thinks. Overlap a few inches and stagger the joints.

I did even more sanding after your post.

I am a dumba$$ so I tried to use continuous pieces of cloth.

This was the hardest glass work i have ever done because of the humps and valleys and corners. I will have to hole saw the exhaust hole after it cures. I will have to cut out the drain holes with a vibrating saw and maybe add a lower and upper layer of glass.

Pictures tomorrow.

 

[Blueknarr]

A dremel tool will be your best friend for redoing the limber (weep) and exhaust holes.

If you have more glass to lay. Just do the horizontal (parallel to stringer) tabbing and a diamond shaped patch at the apex of the stringer transom joint. When those set do the vertical sides at the transom.

You can test lay dry glass strips. If they lay down easily; proceed. If they won't lay down; try a different pattern.

 

[fallguy]

Dryfit!

Use thickened epoxy to smooth things out before glassing.

 

[E350]

 

[Blueknarr]

Not bad.

I see some concerns, but could be afectations of photography. White areas which could be flash glare or high lights, amine hazing, air trapped in resin and definitely some gelcoat under repair.

Wash the amine (byproducts of the A and B epoxy chemical reaction) glaze off with hot soapy water and a scrub pad. The amines will interfere with the bond of the next layer(paint).

I would hand sand with 60 grit to remove any flesh hooks. Then 80 grit to smooth things just a bit. I strongly believe that bilges should be smooth enough to wipe up any oily water with a paper towel.

I'm not concerned with the gelcoat left in the depressions. You did extend the glasswork forward onto an insufficiently prepared surface (gelcoat). Let sleeping dogs lie (this time). Gelcoat inherently has less adhesive and cohesive strength as structural glass. It is a weak link in the chain from original glass thru to repair. I think you will safe this time, but might not luck out next time.

Prep and paint. The over whelming majority of gelcoats do not adhere to epoxy. Some special formulations will.