Not really sure which thread to put this on as we don't have a 'just plain weird' category. I often wash my cars on the same day, within a few minutes of each other thus same atmospherics, temperature, products etc. Yet my classic red Brera nearly always smears, my steel grey (silver to all but the pedants) very seldom does.
I know not to wash in brilliant sunlight as this increases the likelyhood, so does the heat/light make a difference? Is red hotter than silver? Does light reflected make a difference? The other thing that occurs to me is it could be something to do with the length of time it's in contact with the car, if it takes longer to run off does it leave a smeary deposit? The coop hasn't got a flat/straight line anywhere (ok the cut outs) but the Brera's not exactly boxy either.
Any ideas?

What do you mean by "smear"? Are both cars similar in terms of the "starting" surface, i.e. both recently clay barred, polished, waxed? Hand or MOP-ed? By "classic red" do you mean "flat" (non-metallic) red whereas Coupe is metallic? What products are you using? Two bucket method? Same cleaning method?

I think the Coupe is telling you to get rid of the Brera

Are we talking about water marks? Add some rinse aid if we are. It's just the soap/mineral content in the water being left behind as it evaporates

I find something like this useful: Car Shampoo

Thanks guys, it must be some deposit/residue from the shampoo but why more on one than the other, their starting surface must be similar as i've been using the same products on them for years, both non-metallic paint. We are in a hard, chalky water area which would have more deposits but doesn't explain why one not the other.

I'll try a different shampoo though i've had different ones over years with no real difference, getting rid of a few italian money pits isn't a bad idea, only problem is apart from loads of petrol and tax the Brera hasn't cost me anything whereas the coupe mega bucks.