Treating mildew on oil based paint before repainting

Thanks for the help everyone on my previous query on where I went wrong with painting In Thailand. It is all in the quality of paint and preparation. (I knew about the preparation and tried but but the painters just defied and ignored me.)

I used a common medium priced oil based paint (JBP/Columbia) over their special (silver tin) anti-fungal primer/undercoat on lots of wooden doors and window frames and wooden window shutters.  It went on very nicely and had good cover.  But In only two or three years I have originally painted once, repainted once and mildew is everywhere yet again.  

OK. So I buy the best quality premium paints next time (TOA etc as was recommended)  but what preparation do I need to do to guarantee the mildew does not come back again. 

Do I have to sand back? If so, how far? Do I need to re prime with a different brand of anti-fungal undercoat before topcoating again?  (The JBP anti-fungus primer  undercoat definitely didn't work) 

My ideal would be to be told some sure fire mildew killer chemical that I can use first and then wipe over with turpentine before simply repainting with a better quality finishing coat? But maybe I'm an optimist......555  I have put a lot of work into it so far and it is disappointing to see mildew everywhere I paint oils on wood, twice already in 2 or 3 years on this new home and also everywhere else I painted external woodwork on other structures on the property.