Sandbox sand is the worst thing you can put in your blaster. It's too soft (won't cut), has too much other crap in it, and it can give you deadly silicosis. Avoid it like the plague!
I'm going out on a limb here to say I probably do more blasting on a weekly basis than 90% of the FABO membership combined. For my powder coating purposes, aluminum oxide is good (but expensive) and glass beads and walnut shells leave too soft of a finish. I use Black Beauty black slag in a fine grit almost exclusively. It cuts extremely fast, costs $16.50 for a 100 lb. bag around here, and lasts at least four times as long as a similar sized bag of blasting sand.
Though I've never used it myself, there's a couple guys on the board who have soda blasting businesses; I don't recall their user names offhand but one is very well versed and the other just got into it. Do a term search for "soda blast" and you should see a recent thread about that very thing.
Good luck and have fun with your new Harbor Freight blaster! Check with a place online called TPTools.com for ceramic tips, better guns, evaporators, etc., to keep your investment in top shape. (There are also several threads on here about blasting / cabinets you may want to read through -- lots of good info in there.)
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Oh yeah, you're the father/son team from my old stomping grounds!!! :-D
You can find fine grit, true blasting sand at Landry Lumber on Sugarhouse Road in Luling ... think it was about $3.00 for fifty pounds.
