Question about tank

[2fat2furious]

So I took my tank in to a transmission shop to have them clean it and seal it (supposedly), and now 6 months later I take my tank out to try to figure out why it wouldnt send any fuel. (its been on jackstands for a while and it had a brand new Fuel sending unit) upon emptying it had about a gallon of rust gas come out followed by nice big pile of rust and theres a lot more in there that I can not get out. So my question is, is this my fault for letting truck sit with gas in the tank for 3 months (I didn't fill it right away and only put about 3 gallons in as needed) or did the transmission shop just not seal my tank?