Update, so mild steel flanges were pretty average, so after 10 hours of flappydisk, wirebrush, hydrochloric acid and now phosphoric acid I am at this point.
Midpipes will be ready today and hopefully on, going to clean up Hsv mufflers for resell or to sit with stock exhaust.
Cats were the worst due to the rust getting in the mesh conatruction on the flanges, literally falling off after 10 hours of hcl acid bath.
Bought some metal polish and clear coat high heat spray to seal but can't decide which to use, probably the spray.
Anyone else wants to remove rust, hydro acid is cheap and works really well without much labour but metal will flash rust after drying. Phosphoric acid is apparently better but harder to fund especially concentrated. Grinding the flanges with a 60 grit flappy probably saved heaps of CID waiting time but is labour intensive as with the wire rushing which is pointless unless your at end polishing stage.
Why? The more I kept going the more I wanted to eliminate rust and see if polishing and sealing was possible. Wanted to acid wash concrete as well to paint so had a perfect spot to do it.
The Acid is hazardous so be careful and do at own risk.