I use a blow dryer to make sure the masking is completly seated, especially in the crease and indentions. The paint seems to creap in those area if the masking in not placed firmly.
If it is the purple piece spur gear (middle). I have done everything to keep the screws from backing out but not even lock tight worked. I used a nut to hold it but it rubbed against the transmission. I recommend getting the steel spur gear 52 or 47 and throw that thing away.