using interior paint on external wall

I have an external plaster wall to paint and and I plan to use a watered down coat first, as the plaster has never been painted.
is there any problem in using interior paint for this first coat, watered down 50/50?

It would be a bad idea - it could end up washing off/peeling off etc. Frost plays help with cheap or wrong paint. Your first coat is your important coat.

I'd water down exterior paint beforeinterior paint I'd use interior paint outside.
Remember the adage - it does exactly what itr says on the tin'.

IMHO interior piant if used outside is not fit for that purpose.