Here is my blog post about end pours: http://www.guillemot-kayaks.com/guillemot/blog/nick/end_pour_vs_end_block

I'll give a quick summary of my conclusions.

The total weight of my typical epoxy endpour is about 3 to 4 ounces. The endpour mix is epoxy plus as much microballoons as the epoxy can absorb and still move. A cedar block of the same size would weight around 1 ounce less, before it is glued in place with thickened epoxy. The glue would add a few fractions of an ounce, plus the end grain should be thoroughly sealed with more epoxy. For a good seal you should keep adding epoxy until the endgrain no longer absorbs resin. This will add more fractions of ounces.

While it is probable that the cedar block is still marginally lighter, I would only saving less than an ounce on the weight of the whole boat. In some applications that may be worth it.

However, a cedar block is a solid chunk of wood. It will expand and contract because that is what wood does, the epoxy seal on the endgrain will inevitably crack, no matter how good a job you did sealing (unless you fiberglass the end, but that adds weight). Over time these cracks will allow water into the wood. With proper care this is likely not a problem, but I don't know how people will care for their boats, so it may be a problem. For example a boat store upside down outside, condensation may accumulate in the ends. Water infiltration may make the cedar heavier and it may promote rot.

I've decided, for the boats I build, that I am willing to pay a less than 1 ounce penalty to assure that 30 years from now there is not a chunk of water logged rotten wood in the ends of the boats I build. I acknowledge that it is not likely to be a problem, but I've decided to spend