Treat it first as though it were not going to be gilded, but have a paint finish. Clean it thoroughly, if there are any hints of rust hit them with POR15 even if they clean up, as a matter of fact I would use POR15 as a primer on the whole thing. Then top coat with a good enamel, personally I like to use red, it is said to make gold leaf look more lustrous.
After that size & gild. I would use loose leaf because it works into the cracks & crevices better.
Now for the reason for my method...we did a couple hundred metal fence finials and were told by an "expert" that we only needed to prime them before gilding. After a few months they started to rust right through the leaf. We had to redo them all. So even though bronze won't rust, and even if it's inside I treat all metal like I said before I gild.
Also the paint gives a nice smooth finish that enhances the leaf.