Having contrasting borders and handrails on your boat looks nice, but it sure makes a lot of extra work. If you are paying a painter, they will charge a lot extra for this. The coachlines in between panel and border can be done in three different ways.
We went for the old fashioned way, which was to paint the coachline before the rest of the top coats. We marked out the coachline with a pencil on the undercoat, then painted it twice the proper width, feathering the edges. At this point it looks like this.
Note the red handrail paint is also feathered down. Then you stick masking tape where the final line would be, covering the coachline paint. Then we masked the edge of the red handrail. Then paint your panel and border, edging over the masking tape. Then when all is dry, pull off the tape and hey presto you have a sharp coachline.The professional painters at our dock do it another way. Just mark a single line for the centre of the coachline. Carefully paint the panel and border, meeting at the line. Then mask up either side of the finished coachline and paint it in, covering the other paints where they meet.
Lastly there is the Phil Speight way, which is to mask over the proposed line and paint border and panel, then unmask the line and mask either side, then paint the line in.
Each to his own. I don't think our way was best, in terms of the finished line, but it was the easiest and quickest.
As to the borders, its a good idea to do them a little less than the width of a roller, so once masked, its easy to paint. Our bottom borders, by the gunwales were too narrow to accommodate a full width roller so we cut our roller down, leaving a stub to run along the gunwale.
That made it really easy to stay on line and we flew along.