It won't mark your tunic if you polish it properly. Then when you have achieved a suitably dazzling bull, take a clean duster in one hand and grip the belt from the inside so that you are able to draw its edges past the cloth with the other hand. Repeat as often as necessary to get any stray polish off the edges and do the same with the cross strap.
Do not dream of using Klear unless you enjoy being ridiculed and are content to cough up for a new Sam Brown.
If you have access to a good regimental cobbler (I can recommend the garrison cobbler in JHQ Rheindahlen) â most can do a magic treatment known as soft waxing (I know â sounds like something the wife would do to her legs). It looks the business, lasts years and is truly minimal maintenance.
Otherwise there is no alternative to old fashioned spit and polish. Use Kiwiâs finest dark tan (not Parade Gloss). Beware the nicotine finger!
you could go to your local saddler or tack shop, they have enough advice on polishing leather for a degree course. They will also sell you the kit to do it yourself, or if your lucky they may work on it for you.