TCBloomer,
When I receivde my Mr Funnel, the instructions said to fill the funnel so that 1/3 of the screen (media) is covered with water. It should not pass or leak water.
Thus I assume the filter media is small enough not to let water pass through it.
from their web site:
Since there is no way to know if a unit was damaged during shipping or since you last used the funnel, some testing is a must. You must test the funnel as soon as you get it. Add enough water to cover 1/3 of the screen. If it does not leak it is doing what it was designed to do. A leak after shipping is rare but it can happen so if you discover a leak contact your dealer at once to get a new replacement at no charge. If you fill it full, it will leak since water is so much heavier than fuel but a water and fuel mix will not be a problem. From then on check it periodically just to make sure it was not damaged since your last use.