Cleaning solution for por15 lined tank

I have a 1981 gpz 550. I noticed the other day that my fuel line had black particles in it. After cleaning the carbs up nice, I took off my petcock and there was a bunch of black sludge on the filter. I cleaned the petcock up and tried to get as much sludge out of the tank with the remaining fuel in it.

I sealed the tank with por15 a few months ago and just put fuel in it for the first time the other week. I am not sure if the sludge is from the fuel I put in, the por15, or some accidental paint overspray. 

I want to clean the tank out to remove the rest of the sludge, but am unsure of a solution that I could use that will not damage the por15 seal. Will vinegar damage the sealer? Suggestions? 

I'd drain and filter the fuel into a clean can.  Then look inside the tank using an inspection mirror and flashlight to make sure your sealing job covered 100% of the tank surface.  Assuming it did, you should be able to use a little gasoline in the tank, shaken around really well then dumped out, to remove the contamination.  

Are you getting black sludge again? Or is it different color? Mine is big 5.8 gal KZ tank but it took me quite some time to get all the rust and contaminants out from the tank. Follow by several rinsing cycle with garden hose with high pressure nozzle.

Prior to coating, I let my tank get treated with Evapo-Rust several times, making sure each surface area had enough and ample time to get covered to remove the rust. Maybe it's was an overkill, but I had it on my both side, both top and bottom and finally, front and rear of the tank. I had about 3 gal of this stuff from prior to use, and I dumped all 3 gal into the tank. Because of my work, took about two weeks to get everything done but you don't need to soak it this long. I took this long because I don't have a garage or any covered area so I had to clean everything and put everything away once I finished working on something, because of my work schedule, and I keep finding pin hole at bottom of the tank. I'm hoping that pin holes are all sealed up with POR-15.

Inspection mirror helps specially the top of the tank, which wasn't too bad in my case. Water is heavier than gas, so it usually sit at bottom of the tank. However, condensation will make it way to the top of the tank, which I don't really have to worry because of where I live.

I haven't put any fuels into the tank as I'm still assembling the bike. I'm working on electrical now. After I get this solved, I'll put on carb and work to get it sync. Hopefully bike starts, even running uneven so I can get like a start point.

Nessism, thanks for your response. It appears to have coated the entire tank. I tried using fresh gas to clean it out by swishing it around but when I rub my finger on the inside of the tank through the fuel level sender hole.