I've modified a few Coco gauges with deeper funnel and started using a spray called Hydrobead inside funnel area. This stuff is so slick it's amazing, you virtually get no beading in funnel area.
I started looking for a solution after comparing rainfall between the standard (non modified) and extended (modified) funnel and was finding .01 less measured rainfall with the modified funnel because surface area was collecting more water droplets.
Haven't actually had a rain yet since the hydrobead just came in but my testing showed virtually no beading at all so every drop of water will get measured.
Reason for the extended Coco top is to prevent splash out which I've experienced many times when comparing a fully open coco gauge against fully assembled Coco gauge on heavy downpours and hail collection.
A few pics...I also sprayed inside the standard coco gauge along with both tipping buckets. Don't use a spray like this with the standard NWS 8" diameter gauge because you could contaminate the dip stick.
This Hydrobead spray may last up to a full year outside (have doubts on that claim) and also repels snow and ice, just a warning as you can see it puts a white glaze on surface.