The interior was blasted with aluminum oxide, or silicon carbide. Rock hard media, that has to be completely removed
I poured a 1/4 cup of simple green into the tank and filled with with almost boiling water and 40 nuts of various sizes, then used rubber palm gloves the shake the tank for about 2 minutes.
Dumped the soap-water into the bucket and got plenty of grit laying at the bottom. The brushes are used on airless paint sprayers. Available.
Media grit gets stuck in ALL the treads of the caps and nipples. Where the larger softer brush is good for ports, the smaller hard-bristle brushes knock grit out of the threads the best.
Rinsed and shook the tank about 20 more times. The grit diminished to a few flecks left at the bottom, but each rinse still produced media-flecks.
I filled the tank with water last night and will pour the contents through 3-layers of paint filters to see what's left. If I find any grit at all, I'll rinse them again. Any media that finds it's way out of the sump will travel directly to the oil pump.
If you use copper lines with the nuts for sealing, be sure to make the copper flare wide, because if you wrench the nuts down, they'll reduce the width of the flare to where the nuts won't completely tighten. Don't force a fit.
I was getting media-grit with every rinse, so I've opened all ports and have 120 psi of water pressure directed at every angle a nozzle will reach. There were a few bright metal winding fragments before, so I'm getting down to the jurassic layers.
And with these findings, I'll need to re-think the gas tank cleaning for por-15.