Brushed finish problem- can you advise?

[Superarnie]

Hi folks, I thought I would post a thread in your Kirkham forum as I believe you're the guys that can help me.

I have an aluminum bodied genuine AC. In fact it is an AC Superblower- it's a follow on to the 1960's cars. It was made by AC Cars in Brooklands in 2001 before they hit financial troubles (again). However the car was never finished by AC and I have the task of completing the build. My question to you all is this;

I have decided to go for the brushed aluminum finish with polished stripes as many of you guys have done. I think your cars look awesome with that finish! I am using a DA sander and have sanded it down to a 400 grit finish and then Scotchbriting the grained look, but what I am finding is that the soldered body joints are visible. As you'll be aware the car bodies are made in many sections and soldered/fused together. I am finding that on each of those joints the welds are visible. Have you guys experienced the same problem and if so how did you deal with it. I'm wondering if I'm going too smooth with the grit and maybe if I finished with a rougher grade I wouldn't see the joints?

Many thanks for your help/advise
regards

Gary