Do you spray this sizing stuff through a mask or something to define your pattern?
I retired as a Master Sign Painter, using brushes, around the early 2000's. I should say I was bouncing back and forth producing painted lettering and computer graphics at that time. Eventually computers took over all my lettering and graphics tasks. Still today, some applications are still done the "old School" way. Simply because the result cannot be achieved any other way.
"Gilding."
It was just one of the tasks you did as a sign guy to satisfy a customer. Once learned, the application was no big deal. And it isn't. I'm sure with information and direction, most modelers can pull this off and do a good job with it.
As a sign guy, I applied gold leaf and foils to many materials, mostly vehicles. Fire trucks and rescue wagons, certainly boats and yachts and show cars/hot rods. Glass windows and doors. Crown moldings and 3D sign letters. I even gold leafed a helicopter of all things!
What you're leafing doesn't really matter if you think about it. The choice of the method of application and leaf choice may change but basically it's the same process
Plenty of stuff on line and U-Tube, I would start there. research is important.
Size can be sprayed and brushed. General lettering, most sign guys simply brushed out the font or character with a soft quill. Normally, we used half hour size. Time was important because we were doing this for money.
My largest account that used gold leaf on their fleet of trucks was Boars Head. Today they use gold leaf vinyl. Yes, gold leaf vinyl but it has limitations.
The gold leaf on my Avenger I simply brushed free hand. I could have made a template and sprayed the image, but I have brushing abilities and the template wasn't necessary because of this.
Gold leaf is expensive but there is imitation gold leaf that does the trick and costs much less.
There are many choices in leafs and foils for different looks and uses.
Be nice to see a bit of leaf applications on models.