AAAh , my area of expertise . I own a leather upholstery and refinishing shop for the last 30 years that specializes in leather designer furniture and business jet interiors in the NYC area . corporateleather.net
A "naked" unfinished leather on a bike seat is not a good thing. Any moisture , rain or body oil will seep into the pores of the leather and stain it unevenly.Water will flash off and dry the leather out and oil will weaken the fibers of the leather ( leather is very porous ,similar to wood as far absorbing oils and water) . Whether the leather was tanned as a veg (vegetable) or chrome ( chromium salts ) tan , it needs to be sealed . A naked or straight analine leather is tanned to soften it and you have a very soft durable leather that actually feels like leather. Most dye systems are a urethane base now. Older systems were a lacquer base that is very brittle ( same as auto paint systems ) . The finish actually seals the leather ( as well as giving it a color , there are no blue cows ) . With a lightly finished leather ( semi-analine ) you have some of the original leather feel with some protection. Heavily finished leathers have lost that "hand " and feel like vinyl ( like modern auto seats) but offer great protection to body oil,spilled drinks ,sun damage ect..). After the finish has been applied there needs to be a top coat applied to protect that base coat , through that you can adjust the gloss/ dullness through additives . Even a pearl flake can be added to the top coat ; these systems are very similar to modern auto refinishing procedures. All decent modern systems are water based .They are usually a proprietary product . The over the counter systems ( Sem auto products ect..) are not advised. PM me and i can talk further , i don't want to post a short story on the entire process.