Quick prep you can do for border areas, is lay down a layer of tape prior to brushing on promoter. Lay down the sealant, form as necessary, then peel tape up, away from the formed sealant. It will leave a nice clean edge. I’ve probably put down a few hundred gallons of everything from B-1/2 to B-12 over the years. Worked with some people that were simply artist with sealant application over the years. Their stuff looked like it had been machined in place.