You nailed the colour in one Jazz, l knew I'd seen it somewhere before :-)

I'll give the M5 bolts a try. I have to admit I'm using M6 because that's what I've got to hand and it looked ok. I'm guessing the advice was originally for mounting rails.

I'm going to paint the steel before mounting the rail. A paint layer is quite thin and should be fairly uniform. I'd guess the errors in the steel swamp any additional errors the paint might cause.