Donald, next time try this trick. Before you start buildng get a panel scriber, and rescribe all of the panels,around the windows,and wiper blade areas. Lightly making about 2or3 passes. This will help deepen those and make it easier to compensate for paint and clear buildup. Allowing your knife follow the path more Easily.
I also found a new style of knife at Walmart in the stationary section which wraps around your finger, Essentially putting the blade at your finger tip.It is made by fiskers an cost about $3.00.