Scott,

No. In fact, it's better if you don't. It uses less epoxy and has a better glass-to-epoxy ration (which makes it lighter and stronger) if you don't. In general, the weave should only be filled on the outermost layer.

Laszlo 

To add to Laszlo's comments a little...

Really the only reason to fill the weave of the fiberglass is because you will be sanding the boat to prep it for varnish or paint.  If you didn't fill the weave, you would end up sanding through the 'glass itself trying to get a smooth surface for the varnish.  This is the reason you don't have to fill the weave of the interior fiberglass, unless of course you wanted to paint the interior of your boat.  

If you were to fill the weave of the interior, all it does is add a lot of extra weight to the boat without adding any strength.   

Thanks Casey and Laszlo for your help!  I cut some fiberglass on the bias and added it to the bow and stern.  As a first-timer I am amazed at how well the fiberglass will conform to the hull.  The bias cut material hugged the radius like a charm.

If I understand you Casey, once the fiberglass has been bonded to the hull with the first layer of epoxy, any additional epoxy will not add significantly to the strength of the hull.  It will only add weight and smooth out the weave? 

Scott 

Another reason to fill the weave is to protect the glass fibers. When they stand up proud, they'll be the first thing abraded when bumped by paddles, anchors, lunch boxes, etc. Unless of course they hit your knee, then it's your skin that goes first.

So there's times when smooth is good, even if not painting or varnishing. Filling the weave on an interior layer adds weight without strength. On an exterior layer it adds weight, but also provides abrasion resistance.

Laszlo 

I have heard of people filling the weave on the interior where their heels will be hitting the bottom of the boat.  Unfilled fiberglass can be pretty abrasive like L. said.  

Scott, if you were to let a thin sheet of epoxy cure on some wax paper or something similar and then peeled it off later, you would find that it is extremely brittle and easy to break.  The fiberglass itself provides the strength while the epoxy provides the means to attach it to the wood and the abrasion resistance, water proofing, and nice base for varnish or paint.   

Re the above science: Pretty much. Actually, glass is very stiff and very brittle.  Epoxy is the exact opposite: flexible and tough.  Yes a thin slab of it will crack with finger pressure, but the same slab of glass would crack WAY sooner.

In a 'glass/epoxy composite, the two opposites complement each other in a beautiful way--so beautiful that trees have used the same basic concept for years, with some success, in a product known to the trade as "wood". 

Because the plastic is flexible, most of the stress of the load is concentrated in the glass, very little in the plastic.  This is why the laminate is so stiff.  But not brittle, because the plastic in between the fibers flexes easily, and the fibers are very thin and can bend way more than a solid piece of glass would.

Anyway, all this science and 3 bucks will buy you a cup of over-roasted coffee from a trendy chain store.  Doesn't help you to build boats at all, but it is fun to know "how they work".

Thanks for the info on the science of fiberglass!  I used some "on the bias" fiberglass last night and it covered the bow and stern segments very well.

Scott

I'd go with what Camper said.  ;)  Though a thin sheet of pure cured epoxy is pretty darn brittle.  

I was thinking about it. You're right, epoxy is brittle when you bend a thin slab of it. I decided I should have said it thisaway. Epoxy is stiff and brittle relative to other plastics, which is why it seems brittle in the shop. It is stiff compared to polyester, which is one reason it makes a higher performance composite with 'glass than polyester.. But it (and polyester) is flexible and tough compared to glass. It is this last comparison that makes epoxy/fiberglass composite such a great material. Anyway I'm glad Scott is pretending to pay attention to all this philosophy while actually getting his boat done at the same time. Maybe I should get out there in the garage this weekend and try to figure out how to get that aft deck stuck to the boat somehow, or at least barbecue some more Dixie cups while trying.