Hi Dave,
Yes, I know of several traditional finishes that can give them a more uniform or homogeneous appearance...
The patina of the rust is actually aesthetically pleasing to some folks, yet the "dry look" and lack of uniformity is often not as becoming, nor does it arrest the oxidation process from further degradation the metal.
I would suggest doing test areas to see if you like the look. Some methods can "darken" the metal in a fashion that is not desired by some.
Flax oil (linseed) as well as other "drying oils" are a wonderful treatment method. Note, this is not the type of oils you purchase at big box stores and most paint centers, but actual food grade pure oils.
The next are traditional mineral paint ($$$), lime washes mixed with oil and even
milk or tempera paints can work as well. All will age differently depending on slope of roof and location. The all age in a more traditional way...slowly over time give the "aged" affect that some like.
The oil alone method is the easiest to facilitate and reduce as desired.
There are acid wash methods, and other similar methods to above that I won't get into, as they are simply not for the DIYer.
Regards,
j