I'll just ditch the vinyl dye as it's not giving me a satisfactory result.
I guess the results may vary widely due to many environmental and human factors, and I'm not prepared to take that risk and refine my techniques, when more foolproof methods are currently available.
It's more foolproof to use a proper primer rather than use vinyl dye as a primer substitute, though not everyone has easy and affordable access to such supplies.
I am very pleased with the Tamiya + Ironlak combo, after 2 coats and 12 hours later, under the same conditions as the vinyl dye.
Heck, I did 2 coats on a scrapped piece of Longshot without any prep work (straight on with original paint still there), and although has not fully cured after 12 hours, the paint adheres really well to the plastic and quite resistant to scuffing (hard rubbing with thumb, minimum transfer onto white paper with furious rubbing, resists scratching with a discarded piece of blaster shell).
I wanted to make sure that the folks inside the graffiti supplies store were right when I asked them about if the paint would stick well to plastic.
They said, "It should be fine, as the paint sticks to train seats."
Of course, this is just a preliminary observation, while the paint is dry but still in its relatively plastic stage.
I'm not sure how it will toughen up after some time, however, primer should be used to guarantee that the paint sticks well.
Rust-Oleum Universal All Surface Paints?
This shop posts.http://www.aallpaints.com.au/shop/index.php?cPath=70&osCsid=2bb3f7c38e2ff801fb2093ebb889c435