The metal 'weathering' products I have used require the parts to be thoroughly rinsed with water. So if used on tur0outs do it before installing them.
The 'weathering' does help paint to adhere so I like to dip brass wire and parts before installing on rails cars I'm building. Same would go for 'weathered' rail.
the nice thing about ME's track is when you lay out a curve, it stays in place. Atlas and Shinohara you have to hold in place or they spring back. Also ME's rail is closer to code 82 vs. code 83 on Atlas and Shinohara. Personally I have not found that to be a problem over the past 18 years. Phil