I have brought some of my building technics from my high power rocket habit to rc.
One of these technics is the use of epoxy fillers.
First off start with good quality epoxy. I use West Systems myself but there are other good ones out there.(note: do not use fillers with 50/50 mix hobby store epoxy, it just makes a mess)
Colloidal silica- after mixing your epoxy stir in the silica until you have a mayanais consistancy (note- volume will drastically increase so only use small batches of epoxy) This increases the strength of the epoxy and can be applied without worrying about runs. Good for firewall and landing gear blocks, ect...
Micro balloons- this will give you a lightweight fillet that sands easily, Mix the same as the silica. Good for beefing up areas (like rudder mounting) where you want to keep the weight down (note: micro balloons are weaker than colloidal silica so avoid using in high stress areas)