The matt spray i purchased from Halford. I taped the area off, put the decal on let it dry and then put the spray on. It started to bubble and react straightaway, when it dried it was a different colour to the tank anyway so I've removed it all.
I'm currently looking for a larger decal one that will cover the indentation.
Sorry to hear about the problems with your project, HH.
Vinyl decals need to be of the correct material to be paint and/or petrol proof! Any decent decal maker SHOULD be able to produce something if you can provide an image with correct sizes and let them know it will be lacquered or may be subject to exposure to fuel.
The obvious alternative is, of course, to stump up the inordinate cost of a genuine Triumph item!
Good luck with it - hope it gets resolved satisfactorily.
Regards,
Adie