This is why we ask questions. I am a cheap sum*****, so I verify EVERYTHING before making a purchase. I would say its a good mix of blame on not being able to use the entire contents of the can. YOU should have asked, and I believe Monstaliner should state clearly that the product has an approx 1 year shelf life.
Hard lesson learned, but now you will research better before jumpin on a deal before you are ready to utilize it.
I am kind of a hypocrit since i purchased mine in May and haven't done my tub yet.