I have been several jacks and bottles .. have used envrotex which is a food safe resin .. works great! I usually put a coat on, wait a day and put another coat on. Sometimes up to 3 coats, depending on how well it took. You can tell how thick it is by running your finger on the inside of the jack (when dry of course), and you should not feel any bumps. It should be smoothe resin.
You can but it online or at most craft stores .. Michaels has it, at least that's where I've found it every time. It comes in a few different sizes, and you get quite a bit out of each bottle, so I would suggest a small bottle, unless you plan on making lots.
Enjoy!