I hope and pray it comes out to your liking 2515R Dude, Take the high road your better for it and you're right the good lord will take care of them.
I hate hearing stories like this from scrupulous contractors with no sense of pride in their work. unfortunately, they are out there but so are...
I would use acid to remove3 the top layer and scrub then wash off should take the residual left on those areas may have to use a wire brush to scrub but that should work but be careful with the acid and make sure to use all the precautions on the label you can find it at most hardware stores.
I made the mod exactly as Slowpoke slim did with the Stanley IH4 muffler on my 2018 3520 Branson. I am very happy with the mod. I have had it like this for about three months now. Data logger completely removed and no issues at all runs great, rpm gauge works too. THANKS SLIM!
I do landscaping...