I went to a 190SL tech session over a year ago that demonstrated how to properly tape and paint hub caps.
The session was excellent. I paid close attention as I wanted to paint the hub caps to my Pagoda. It was a very tedious task. Literally hours to tape, trim with razor blade and paint.
I put off doing the task due to the time it would take. I decided to order the inexpensive hub cap stencil from Authentic Classics. It cost around $45 and thought it was worth a try.
The results were very good. Below is what I did that worked well for me.
Remove paint from hub cap. Prep area to paint with 800 grit sandpaper and clean entire hub cap with paint thinner of your choice.
Using 1/4 " 3M 471 flexible tape tape the outside edge of hub cap. Add 1" tape to 3m tape around hub cap. Cut hole out of news paper and tape to hub cap with only area to paint showing.
Place stencil on hub cap. Paint hub cap. I had a can of paint from Automotive Touch up that was a good match for my car. Paint shows a little greener in photos than actual.
NOTE: The only way to have good paint lines is to not move the stencil and keep pressure on the stencil while painting at all times. If you move the stencil at all you wont have good results.
I used one hand to paint and one hand to hold stencil movement free. Don't switch hands!!! If feels a little awkward but you can do it with deliberate painting strokes and pressure on the stencil.
I did all four hub caps in a couple hours. May not be concours but plenty good for my driver Pagoda.
Photos below.