First the solder joints need to be wiped clean to remove any flux.... I find that wiping the solder joints with paper towel and whiting several times will remove any flux that is still there....
and then you can polish the solder joints by rubbing with a good shoe brush...at least a good size one.......that will shine up the joints to receive the patina...and possibly, you WON'T even need any patina....give a test on the back side of the panel to see how cleaning and polishing turns out.
Clean the excess cement,
Run a wood stick and paper towel around the edges of the lead to remove all the cement,
Brush some whiting against the cemented areas,
Let dry a couple of days,
Brush/polish again.....this action will carry some lead onto the cement and add a grey color....actually I have used sawdust as it seems to work just as well. Using a pointed stick and paper towel will clean the edges of the lead
CAUTION.....during this process you need to VENT any dust to the exterior....and depending on the state/county in which you live, there may need to be an EXHAUST FAN that captures the dust......and, many large studios have a separate and vented room for this total process.
I don't use patina on the joints. Puttying and brushing with whiting will darken both the leads and the joints. Brushing with whiting will take OFF most of the patina if you put it on, so it doesn't make sense to put it on.