I think you would get better results taping up the lettering/numbers and using a scalpel
The blades are far sharper and thinner than that Stanley blade your using. Also have you thought about this . . . . spray the numbers/letters the colour you want, leave to cure correctly. then cover with plasti-dip. trim the letters/numbers. then spray the rest . . . peel off the plasti-dip . . . Wey-hey