The rust killing phosphoric acid was left for at least 3 days on the engine. Mainly due to me having other work but a quick check shortly after the application revealed that the solution had dried on the engine and wouldn't cause any damage.
A rotary wire brush in the electric drill soon removed the black oxide and revealed an ideal surface to apply paint.
Gloss Black Engine Enamel.....just like a Vincent Black Shadow !
Time for a cuppa !