We were in Bintan last weekend with our friends - We used all the correct potions; but the sand fly bite count came to in excess of 150 between five of us! My usual tactics of plenty of alcohol, garlic, and chilli in the bloodstream failed completely as I personally mustered 43 bites.
The worst time of day is between about 3 - 5 pm at the edge of the beach furthest from the water. The only solution seems to be to apply enough oil to look like an oil slick so the little sods drown in it while trying to bite you.
Next visit for me it will be water sports in the morning and a bar lunch/dinner from 1 pm until falling over time.