Hi. If you haven’t done anything further on the bridge, May I suggest:
1. if you are happy with it now, spray it with a clear matte varnish (Testor dull cote, VMS matte, Krylon clear)
2. decide if you will use powders, chalk, pan pastel or paint
3. small piece of sponge, dipped into dark umber color paint and blot the paint off as though you are dry brushing OR if using oils or enamels use a small paint brush and create some dots
4. depending on if oil or enamel take order less spirits, dampen a brush lightly pull down from those spots creating streaking, if chalks, powders or pan pastels blend with a 1/2” flat brush to create streaking
let it sit a day and come back to it with a critical eye, meaning use a camera and take pictures, then review it, adjust to your liking. If you like it, spray it with a clear matte varnish as in step 1. You can always keep adding or removing any additional weathering but you will have saved the base.
Finally, there are some great u tube videos out there on rusting. Maybe plug in searching for how to rust a bridge. Have fun