Restoring Metal Roofs Original Finish

Hello. My metal roof I believe is galvalume and going on 16 years of age now. Needless to say it has lost its metal sheen and I can see a small amounts of algae and scale taking hold in some spots. Id like to get it cleaned and was going to get it pressure washed. Is there any recommendation as to what additional cleaners I should use in combination to the pressure washing - to eliminate scale, dirt, etc.? My biggest issue is what I can use to restore and preserve the original metal finish. Can you recommend anything on the market that could accomplish this that is not a paint/color and is essentially a clear coating? Any help would be greatly appreciated.