i read the process Kreem recommends for cleaning a tank and i didn't like it either. I think it's way too involved and requires buyying more of their products and it's all too much work.
But the Kreem coating itself is excellent. Even if the tank is very rusty inside and you don't remove any rust, the Kreem will coat and seal and prevent that rust from flaking off.. and your fuel lines will stay clean for years.
I know because thats the situation i found my tank in. Look inside and thick rust is everywhere but a thin white coating of Kreem keeps it where it is. There are metal plates soldered to my tank where it rusted completely through.
But my petcock screen is immaculate when i remove it...
My tank cap was very rusty inside. (i actually took it apart so i know) The outside was clean. The cap's vent holes would clog up once in a while and engine performance was erratic.. sometimes the bike would slow down for no reason. Since i took this cap apart and sandblasted it performance is better everywhere. I'm not joking.. i can't be 100% sure this cap vent alone caused erratic fuel delivery but i'm pretty sure..
So, you may want to get a new cap. If i was a gambler, i'd bet you whatever you like that if you open that cap up, it is packed with flakes of rust. Sticking a wire into the vent holes will not help it.