Look up the Herculiner How to string in the FAQ, and search "DIY Herculiner" string. You will have enough to do all the tub X2, with enough to do a 3rd coat on the high wear surfaces. I even pulled out my heater box and went up behind it on the firewall and had about 3 oz leftover. Very worthwhile thing to do to your tub.
You will also be able to read that your outcome will depend on the prep work you do. "It's all in the prep."
....I'm not smart, but I can lift heavy things.......
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Just an FYI, the amount everyone above has recommended is if you are rolling. If you are spraying, prepare to use about twice as much. I figured on the gallon when I bought the stuff but once I started spraying, I realized that there is alot of waste and soon found myself running out for more. Comes out amazing but definitely costs you more money due to the overspray. Difficult to control with the cheap gun you are forced to use for this process.
I just did mine. I'll throw some pics up but it came out looking great! 1 gallon is plenty. I will say though that the roller they give you didn't do it for me. Me and my buddy used big brushes for the whole deal.
I just started this project today and didnt get much done. Well, I did get all of the seats taken out as well as the carpet so I guess its something but when it came time to take out the roll bar I stripped at least 4 bolts so far and just gave up for the night. Looks like Ill be putting the herculiner down with the roll bar still in now