Print, sand (carefully - you only want to take off the print ridges, not the lettering itself) then seal with clear varnish or paint the same colour as your plastic.
Once it's fully dry you can pick out the letters and it shouldn't bleed (much...)
I tend to indent the text so I can sand completely smooth and remove any excess varnish.
Hi Bill, thanks for the advice. Will give the varnish a try, hopefully that will seal before painting..