Shrinking spot colour

[fi0D45]

Hi,

I'm preparing a tiff file to print using a UV printer. I am creating a white spot colour behind the text because it will be printed on clear glass and also black glass however when it prints the white spot colour (behind the text) shows slightly out the sides of the letters. How do I shrink the spot colour so that it does not show? 

[c.pfaffenbichler]

Filter > Other > Maximum

[fi0D45]

Thank you