Awhile back, a TU poster reported that epoxy has a density of 1.1 while water has a density of 1.0 - so yes, epoxy will add weight to a lure and ANYTHING you do to a lure changes its performance to some extent. I never use more than one coat of Devcon applied at room temperature - which makes for a fairly thick coating compared to most other topcoats. That said, I don't hesitate to use Devcon on a topwater bait because most of them have enough buoyancy to easily overcome the added weight. If that's a concern, I switch to a different topcoat like Dick Nite Moisture Cured Urethane or a solvent based concrete sealer. I don't heat epoxy once it's on the lure because bitter experience taught me that the more gimmicks I try, the more screw-ups I'm gonna have. a little patience is a great asset to crankbait building.
Envirotex Lite is an epoxy pre-mixed with a solvent so that it can easily flow out on a flat surface and expel air bubbles. Flexcoat is a rod guide epoxy and it is formulated to go on thin so that it penetrates the threads on a guide wrap before beginning to harden. All of these are two part epoxies and for most guys, dipping lures in epoxy is a big waste of the product. Flexcoat or another rod guide epoxy can be found at online sources that sell rod building supplies, like mudhole.com or Jann's Netcraft.
All the back and forth about new topcoats here on TU has been great, but I think it boils down to this: There is no perfect topcoat for every lure, for every builder. You have to choose according to the specific performance you are looking for and the specific process by which you want to build your crankbaits. And you have to realize that there are going to be trade-offs regardless of which topcoat you choose - Faster but not as durable; harder to store but more durable, dipable but expensive, durable but slow to cure, etc etc.