Hi folks
To help modern fly lines shoot through the rings smoothly and with less friction, some manufacturers incorporate lubricants within the fly lines coating and these slowly leach out of the line to maintain a lubricated surface, others apply an additional thin outer coating over the main coating to produce the same result.
If a fly line has a lower specific gravity than water ( ie 1.0 ) it will float, if the fly line has a specific gravity greater than 1.0, as long as it breaks through the water's surface tension it will sink.
Good tips for keeping your fly line tip floating might include sealing the end with a light dab of superglue and attaching your leader with either a nail knot or a needle knot.
Nylon, copoly and fluro etc have a specific gravity greater than water and they can pull the fly line tip down, today's popular braided nylon leader loops do the same of course. It can help to coat your braided leader loop with a silicone paste fly floatant product, but make sure that you don't get these on your fly line, as some fly floatants are harmful to fly lines. Many insect repellants that get transfered from your hands to your line will damage it too.
A dirty fly line will not shoot or float well. Cleaning with mild hand soap is good and so are mild hair shampoos, but detergents and solvents can remove plasticizers and lubricants which will lead to the coating stiffening and cracking. You may find that your automotive lubricants make your line feel smooth and slick, but beware that many of these also have solvents in them which can remove the original fly line lubricants that you are trying to supplement, which can lead to hard lines and cracking again.
Most fly line cracking and hardening etc is caused by the line coming in to contact with other angling products on our hands, especially insect repellants and dry fly floatants. The fly line tip section and the area of line that an angler handles most for false casting are the two common areas for this problem, the false casting one depending on the amount of line that each individual angler carries when casting. Not as some may suggest, anything to do with the length of the fly line head
If you fish a venue that suffers from algae blooms - clean your line more frequently.
Best wishes
Mike