Hi Dakotah,
I used Monstaliner on my build and can tell you how much I used. My build is 5'x5'x10' minus the curved areas, as mine is a Grumman(sort of). The total square area is 163 square feet. I was hesitant to start with only 2 gallons. According to another builder on this site and also the info I got from Magnet Paints (Monstaliner), 2 gallons was cutting it to close. I ordered 3 gals and had some left over but probably less than half a gallon. If your surface area is less than mine it may be possible to get by with 2 gallons. I think Monstraliner is great. I would use it again. The one negative is the cost and the fact that there is no way to save the extra. After mixing, it has to be used. Measuring and mixing part of a gallon is a bit tricky. I wouldn't do it with anything other than a half a gallon. Easy enough to divide the two parts exactly in half, any other division and you are asking for trouble. Even if you only mix part of a gallon I don't believe the unused part stores for very long after you have opened the cans.
You have not mentioned if you are fiberglassing and epoxying your build. I would not recommend applying the Monstraliner to bare wood and the joints in the wood need to be glassed.
Hope this helps. If you have any other questions let me know.
I have a couple posts that show the finished results of Monstraliner if you are interested.
viewtopic.php?f=40&t=65851 Introduction
viewtopic.php?f=50&t=65860 Build