Lining a tank

We just took our gas tank off and the condition looks awesome but it old. Since most gas tanks rot from the inside out, I want to line it before we put it back on the car.

I have the liquid but have been told that I need to remove any gum, sludge, varnish that has accumulated over the years. This was a running car until this spring.

I went down to UAP to but such liquid was told it wasn't available.

Is there any place in Canada where I can buy this or is it a non issue ?

Thanks
ian.