Problem with painting plastic.

This post originally appeared on my other blog. However, as it is VBCW and wargame orientated I feel it should more rightly be here.

With the Viscountessa crook I was up early. The previous day I had primed and washed in mild soapy water a box of Airfix WW2 British Infantry, mounted them on some painting boards and left them to dry overnight. Roger so far?

First thing the (0600) I undercoated them with Humbrol Black Primer and left them to dry in the conservatory, weather was warm. When I came to check them at lunchtime I found the undercoat had cracked and upon handling them the it flaked off!

This was a deviation from my usual routine as I was trying out a different painting technique. I undercoat in 'Wilco's' Grey Primer and this has never happened before.

Any ideas as to why this happened? And/or how, or if I can overcome it?

Cheers in anticipation.

Lorcan.