Why do fly lines need cleaning?
Although they get rinsed every time you cast them, fly lines can build up a coating of dirt, especially if you're fishing in muddy areas. This layer of dirt makes the line much less slick and will begin to impact your casting if you don't clean your line periodically.
How much difference does it make?
A clean fly line will have a slick, shiny surface just like when the line was new and you'll be able to shoot line better and cast a lot further than you could when it was dirty. It should make a massive difference to your casting.
How often should I clean my fly line?
I'd recommend cleaning your fly line after every trip, irrespective of whether it looks or feels dirty or not. I also clean my lines mid-session, often several times if I'm fishing somewhere muddy or if I feel it is taking more effort to shoot line due to the build of dirt on the fly line's surface.
How should I clean my fly line after a fishing trip?
Fill a bowl with warm water and add some Fairy liquid or similar washing up liquid. Give the reel itself a clean using one of those foam sponges with a scourer on the other side and make sure any signs of mud and dirt are rinsed off. Then pull the fly line off the reel and put it in the bowl of soapy water.
Leave it to soak for a minute then put a small amount of washing up liquid on the sponge itself and pull the fly line through it under pressure and place the cleaned fly line onto a towel. Once you've cleaned it by pulling it through the soap impregnated sponge give it a quick dry off in the towel then wind it back onto the fly reel.
Do I need to use a special fly line dressing?
In most cases just washing the line in washing up liquid after your trip should suffice, but you can also get special fly line dressings, such as Rio's Agent X and Orvis' Zip Juice which do seem to improve "shootability" a bit.
Some people do apply all manner of polishes and other compounds to their fly lines to help make them slicker, but doing so is a bit risky and could damage the plastics the fly line is made from, so I'd stick to the proper fly line dressing if I were you.
How can I clean my line during a fly fishing session?
I take a few unscented wet wipes or baby wipes with me in a resealable bag and add a small squirt of Orvis Zip Juice and then pull my dirty fly line through the wipe to clean debris and help make it slick again. You should find the line casts a lot better after doing so, although it will be slightly slippery for a little while.
What about silicone based products?
Yes, silicone based products such as Silicone Mucilin are also great for making fly lines slick again. Just give the line a wipe in an unscented baby wipe first, then work some of the silicone paste into the clean pad and run the line through it.
The silicone paste will add a water repellent finish to the line and make it smooth and easier to cast. It also makes the line float better, so it's great for reviving a floating fly line with a section that's started to sink. Just don't use it on an intermediate or sinking line, or it too may start to float!