I've finally gotten the seams fair and everything sanded smooth on my Murrelet. Ready to add seal coat and glass.

I have talked to several people who said that this was the stage where they added too much weight to their kayak by adding too much epoxy. I'd like to understand better how this happens and how to avoid it, since one of my reasons for getting a wooden boat was that it was lightweight!

Advice?

I personally believe that 'too big fillets' and 'heavy end pours' and 'mirror finish obsession' are bigger weight problems but.... ;-)

There are different 'schools of thought' on wetting out and glassing these days, so it can be confusing. I was lucky - when I started building canoes there were only two books (Hazen and Moores) and only 'Canoecraft' by Ted Moores had detailed info on glassing with epoxy. Canoecraft/KayakCraft by Moores are still good, IMO.

Most of the pro builders here have detailed info available, and the Pygmy instructions are quite good also.

My two cents worth:
-it's worthwhile doing a practice panel or two with some glass scraps (or even if you have to buy some extra glass). You can glass a section of something like doorskin which is cheap or you may have something in the corner of the shop that will work.
-temperature affects the viscosity (and working time) of most epoxy so practice with that. Pre-warming (a bit) the epoxy makes it wet out the glass more easily but you can't leave warmed epoxy in the mixing container for long - get it on the work surface and spread it out. If you are in a warm shop/climate this will not be an issue at all.
-keep some denatured alcohol and paper towels handy, also spare gloves - don't hesitate to put on new gloves if things get sticky, and keep your squeegee (hopefully you will be using them) cleaned off. BTW, if your gloves don't have a lot of powder, it can be helpful to have 'baby powder' nearby for your hands as sweaty hands don't slide into gloves easily.
-shiny is BAD when you are doing the wet-out stage in glassing; you want to end up with completely transparent glass (full wet-out) with no shiny areas with excess epoxy. You also need to be careful not to get the glass 'floating up' over epoxy - it should be 'tight' to the wood everywhere
-epoxy squeegeed off the boat is not 'wasted', it's a good thing!
-try to arrange for the boat/shop to be cooling after you do the glass work. e.g. don't glass in the morning if the day is going to be hot as it invites outgassing and bubbles - not a disaster but upsetting to see and causes extra work.
-enlisting a 'mixing helper' can make it easier when you are new to the work....somebody to 'man the pumps' if that is what you are using. But you can do it solo- once the epoxy is spread out on the boat you have a reasonable amount of time to work.