OK, last night I checked out the balsa wood I bought awhile back. The packing slip doesn't show what density I purchased but I'm guessing it's more in the 6 lbs/cu range. The stuff is super light. Anyway, I cut a couple patterns out of it and now I understand why you would need to seal it, or make it harder. It almost acts like foam board as opposed to wood. I can't imagine being able to cast a plug made from balsa? It's not for me. With that said, I'm sticking to poplar for now.
After work yesterday I stopped by Sherwin Williams and talked to the sales guy. I left with this -
http://www.sherwin-williams.com/home-builders/products/catalog/multipurpose-waterbased-acrylicalkyd-primer/
Last night I dipped a few raw, sanded poplar blanks. This morning they were dry and ready to scuff up. The product is pretty thick but runs off the blank well and really soaks into the wood and levels out nicely. They may need another coat but I won't know until after work tonight when I get a chance to mess with them again. I will say, I was using a oil based enamel (like in the video) and it works, but takes forever to dry. This stuff might make my life a little easier? I plan to set a couple of the blanks in a bucket of water tonight also to see if they want to swell up or not after a couple days.