Woven roving to chopped strand mat in small glass repair??

[Dinghydreams]

Ahoy,

I have a small glass job I need to do on my Bristol 24 sailboat. To strengthen the strbrd chainplate I need to glass in the piece of wood it currently sits on, so the stress is distributed beyond this small portion of the bulkhead. (don't worry, I'm not glassing in the actual chainplate).

Here's the thing--I'm using Don Casey's fiberglass repair book as a guide and I'm confused. He says NEVER adhere woven roving to woven roving, as the bond won't be strong, so he suggests a layer of chopped mat in between the two roving layers.

A few pages later he goes on to say that epoxy resin is incompatible with chopped strand mat. So, what gives??

I am only doing two layers of glass, very small project.

Do I go Woven roving to woven roving,
or,
Woven roving, chopped strand, woven roving.

Any thoughts are helpful

 

[Ron20324]

WELL, AT FIRST YOU QUOTE THE BOOK with logical advise, THEN LATER ON YOU POO-POO THE BOOK. GET THE PHAMPLET ABOUT THE WEST SYSTEM OF GLASS WORK.

 

[Chief RA]

I was taught (CSU) that chopped mat was used more as an inner filler between or under woven but woven adds more strength than mat and a smoother linear surface. With that said: I would suggest using woven layers. As for resin, I would check with the boat manufacturer on that and use what it was built with. I am sure you will get more help. Chief

 

[rgranger]

WELL, AT FIRST YOU QUOTE THE BOOK with logical advise, THEN LATER ON YOU POO-POO THE BOOK. GET THE PHAMPLET ABOUT THE WEST SYSTEM OF GLASS WORK.

Hey man! Did you mean to yell? In case you didn't all caps indicates yelling.

 

[Capt jgw]

With the coarse texture of woven roving if you put two layers together you can't get a lot of fabric to fabric contact. There will be gaps between them. Put a layer of mat between to fill the gaps. The mat itself doesn't add much strength, but it sticks the layers together better.

Most CSM is made with a binder that dissolves in polyester resin. That allows the mat to conform to curves better. It's not that you can't use it with epoxy, it just won't follow curves as well. On flat surfaces it doesn't matter. As long as it wets out translucent it's fine.

 

[Chief RA]

CJ: If you put 2 layers of woven together especially on relatively flat surfaces or small areas and you are having trouble expelling the air pockets you probably do not have enough resin involved. Chief

 

[Chief RA]

DD: Are the 2 surfaces both at the same level or does the plywood plate mount on the bulkhead? Their relative position to each other will be a major factor in whether you need to use mat or not. Chief

 

[Capt jgw]

CJ: If you put 2 layers of woven together especially on relatively flat surfaces or small areas and you are having trouble expelling the air pockets you probably do not have enough resin involved. Chief

Yes the gaps fill with resin but that's not as strong as good contact, and adds weight that's not contributing to strength. That's why they say to use the mat between.
And also why it's used as the top layer under the gelcoat. It prevents the woven pattern from printing through the gel. We've all seen the old boats that look like a wicker basket, they didn't use mat under the gelcoat.

 

[kito]

Just use 1708 biaxial cloth..............

 

[centerline]

regular chopped strand mat is different in that it has a additive on the fibers that dissolve WHEN USED WITH POLYESTER RESIN to help wet out the dense mat and build strength...
epoxy will NOT dissolve this additive and so will leave the fibers encapsulated, but NOT adhered to, even though it looks like it is.....

if you want to use something similar to the process, take some fiberglass cloth and fold it over a few times and cut it on a 45 degree angle, into very short (1/4" or less) pieces..... in 15-20 minutes you can have a lot of it cut up to stir into the epoxy resin... it makes for a reasonably strong filler/build up material for smaller areas.... for large areas that need building, layer it with roving. just make sure you dont get it resin rich. the very least amount of resin needed to wet it out is the perfect amount.
when I have to repair a powerboat transom, and using roving, I use my own chopped strand formula in between the two (or more) layers.... but a person really does need a resin roller to work the layers down into the resin and work the air out of it to do the job quickly without getting it too heavy with resin...

EDIT... its not really MY "chopped strand" formula, but something I picked up from glassers who have much more talent in the craft than I ever will...

 

[Chief RA]

Looks like we did well. Would like to get a response though. Chief