To mat or not to mat, that is the question

[waterbaby]

I am in the process of attaching two flotation chambers to the exterior side of the transom of my old fiberglass boat that I have completely rebuilt. Thus far all of the 'glass work has been straightforward. All the work has been done with epoxy for it's superiority to polyester in strength and adherence. If the flotation chambers break loose the boat will take on water (not a good thing). There will be minimal room for bonding ( 1 1/2 - 2 inches) in one direction so the bond HAS to be good.The bond will be wood to wood and wood to 'glass.
One "expert" says that mat should ALWAYS be the first layer due to it's superior bonding and waterproofing qualities, another says that epoxy is not compatible with the binder in mat and the two should NEVER be used together, another person has a third opinion and so on. So far the mat seems to work well with the epoxy (good wet-out and strength when cured although it has not been stressed and the binder seems to dissolve but so far the work hasn't been in a critical area. Now it will be critical and I don't want to sink.
Who is correct? What is the real truth about using mat? What should I use?

[Landlubber]

Waterbaby,

CSM (Chopped Strand Mat), is available in different styles to suit different applications. CSM is made up of 2" long glass fibres, randomly splattered to form a cloth. It is normally used where an emulsion binder is applied as part of the manufacturing process of the csm. This binder is soluble by the polyester solvents (styrene), and dissolves when the resin is applied. It holds the little suckers in place on the mat whilst in use.

It is also available in powder bonding, same story basicaal as above for your purposes.

It is also available without polyester binders or powders, this is the material that you would be using when using epoxy resins for your boat.

The csm is the base product that is used, as it does flatten and allow cloth materials to bond better, we use 300gsm and then 900gsm woven rovings in alternate layers when making trawler style boats.

Hope this helps a bit.

[waterbaby]

Thanks for the quick response and helpful answer. I have been looking for CSM without the styrene binder or the powder. So far, all I can find that definitely fits those criteria is Evercoat brand. It only comes in 34" by 38" pieces, if you order more they just send more pieces, until you buy a roll and I don't need a roll. I just need about 5 yards at least 50" wide to keep from having to piece it. Where else can I buy epoxy compatible CSM? Does continuous strand mat share the problems and would it work?
Thanks for sharing your knowledge,
waterbaby

[waterbaby]

Thanks for the quick response and helpful answer. I have been looking for CSM without the styrene binder or the powder. So far, all I can find that definitely fits those criteria is the Evercoat brand. It only comes in 34" by 38" pieces, if you order more they just send more pieces, until you buy a roll and I don't need a whole roll. I just need about 5 yards at least 50" wide to keep from having to piece it. Where else can I buy epoxy compatible CSM? Does continuous strand mat share the problems and would it work?
Thanks for sharing your knowledge,
waterbaby

[Landlubber]

Thats OK

[the1much]

you put mat on first,,(when building from the mould),,,
and mat on last (on exterior work),so that the "pattern" of your roven doesnt bleed through,,,with mat you can get a smoother finish.,,,and mat is ONLY as strong as the fibers are,,it doesnt bond any better then the strength of the "fiber" thats doing the "hanging on" bonding,,,,,,does that sound right LL?

[Landlubber]

Well, yes and no, the csm is, correctly stated it does not bond any better than the firbe, but what happens physically is that the 2" csm fibres get into the woven rovings and create a better bond than the rovings themselves would have done in contact with a smooth surface. The csm is "force fed" into the rovings on consolidation, without the csm, I believe, the rovings would not be in such intimate contact with the last layer.
It is very important to use the csm after the gelcoat and before the woven rovings, as it will surely produce pattern without it. We use two or even more layers on large flat surfaces, such as the topsides on a 60 footer. There is no specific reason to finish with csm thou, as long as the roving finish is acceptable for the end result. In engine rooms and lazarette, we are screeding with mocrobaloon filler anyhow to produce a more acceptable finish for the end user.
As the market for our boats expands, customers become more unsympathetic towards raw glass finishes, it is only a money problem.

[the1much]

we used ta have to gel coat the whole inside in our yachts,,,,man what a headache that caused us