So here’s what I ended up with. The black is not transparent, but I didn’t do the underside of the tank so I can still put a light under it to shine in from the bottom. I wont be able to see through the tank from the outside, but when I take the cap off, the inside will be lit up which is useful if I want to check my fuel level in the dark especially. Anyway, this is car trim dye called forever black. It wasn’t sticking the best, so after a day of it sitting and still having areas that were not fully covered, I decided to try applying it and blow drying, and it worked very well. If anyone is looking for an easy, fast way to get a black paint job on plastic, apply forever black and blow dry it.