Tamiya masking tape

Best tape going. I have NEVER had paint bleed under Tamiya tape as long as the edges are properly burnished down. You sure you don't have some oil or grease on the surface? (possibly from handling the parts?)

positive. I think because the surface is textured, I don't know, but it's a right royal pain in the arse!

That would certainly do it. Put a line of Future down the edge of the tape. The Future will bleed but it's clear and will seal the edge.

Definitely the best tape going - Eduard are swapping their precut masks to this material and that must say something as to the quality! Definitely the textured surface is contributing to your problem but Scott has put you on the right track with the future sealing!

Are you cutting a new edge before you apply the tape? Even tape kept out of the elements will have dust and other imperfections in the adhesive along the edge, which can allow it to bleed. I learned this the hard way on my R/C cars (Thank goodness for pin-striping tape).

what I'm doing now is putting the tape down then using liquid mask and cutting along the edge, seems to seal the texture without harming the undercoat.
Thanks chaps