On another BB, we were discussing why the tips of fly lines begin to sink.
I did an experiment. I got an old 4 wt fly line, actually my original 4 wt fly line from the 1980's. It had multiple cracks in the first foot so water could enter the braided core in multiple places. I fished it for an hour and the first foot sank under the water.
Then I coated that part of the line with a thick silicone fly paste and line treatment. The treatment coated the line and filled in the cracks. The line then floated. Silicone fly floatant is hydrophobic and when to coated the line, it allowed the surface tension to float the line.
Modern fly lines are supposed to renew their plasticizers and hydrophobic coating by slowly releasing it from the line coating and keeping the line slick and water repellant (hydrophobic). When these are used up, the fly lines coatings become brittle and crack.
The reason even new fly line sink is that they get dirty and the coating is no longer next to the water. The dirt on the coating is. So they sink. Clean the line and it will float again or treat the line with line treatment.
"Believe it or not, a fly line can get dirty from water as well. Dirt, algae and dust that are dissolved in water will adhere to the outer skin of a fly line. This is why we make most of our fly lines hydrophobic - they repel water. After wiping the line clean, apply a light coating of a fly line dressing. This will help to re-lubricate the line and gives the fly line a slick and clean finish."
"Within the balance of the coating are a number of chemicals that essentially act as moisturizers. These chemicals keep the plastic coating supple and strong and prevent it from drying out, or de-plasticizing. The course of time and ultra-violet rays has an aging process that will eventually make the fly lines dry out and crack."
Cleaning Fly Lines - Fishing Tips | RIO
If the fly line won't float and you are on the river, use a Scientific Anglers micro abrasive cleaning cloth to remove the dirt and treat the line.
Cleaning and Dressing Fly Lines | Scientific Anglers
Fly lines float for 3 reasons. Glass beads between the core and coating lower the specific gravity, hydrophobic line coatings repel water, and the surface tension of water support the line.
Hydrophobic coating that repel water and surface tension result from the fact that a water is a polar molecule and are attracted to each other but repel non polar molecules.
Even when the line is cracked, if you fill the cracks with a silicone line paste and coat the line, it will float,