Advice please
annual anti foul stretched to 18 months
International Ultra
some cracking and peeling around waterline,but otherwise sound
i wonder whether to prime these ares after scraping and feathering.
which primer or just slap on antifoul
ABSOLUTELY, you have no option and the antifoul will stick on much better because of it. 
prime bare surface with PRIMOCON unless you plan to use a vinyl based antifooling like VC offshore (www.yachtpaint.com/MPYACMDatasheets/Primocon__+eng-usa+A4+Y+20100608.pdf).
2 pack products (interprotect) should only be used if your hull is dry
Select to expand quotecazou34 said..
prime bare surface with PRIMOCON unless you plan to use a vinyl based antifooling like VC offshore (www.yachtpaint.com/MPYACMDatasheets/Primocon__+eng-usa+A4+Y+20100608.pdf).
2 pack products (interprotect) should only be used if your hull is dry
Any of the special purpose grey primers. Check on the tin of antifoul for the directions and use the same brands if it is convenient. I would remove all the loose paint and fair the surface with coarse wet and dry. The top photo shows too thick paint and it has started to crack. I would give the surface a hard water blast to shift all the flakey stuff. That looks like a timber hull. If it is just be cautious of the seams with the pressure washer. Cover the bare patches with several coats of grey primer.
use epoxy primer before anti ........ it should never have cracked off like that .
And if you apply with a roller you will need a minimum of three coats.
Check the product manual! 
