I've been fiddling with this too long. I tried a number of brands, even got chems from Beyer Chem where I burned $500 worth of repairs into a North 20 - long story... After all this, the product: 303 Fabric Guard seems to do what I want.
I rinse and dry after a session. The line is a simple thing and the only thing needed to buy are those inexpensive Chinese-made, plactic clamps you can buy from Harbor Freight Tools or other discount hardware sources.
I expect this to keep this black kite from having those brown mud streaks we get at my local launch. It also makes it easier to rinse off the sand and then provides another layer of UV protection - which will be of more value for the person I sell it to than me..
303 comes in 473ml (16oz) bottles and I used a little over two on this kite. I think I could have done it better with a paint gun rather than the sqeeze sprayer attached to the bottle, and I was a little wasteful because I buy it by the case. Expect a retail price of about $12-15.
It's not needed ..but beyond food, water and sex - none of this is needed, eh?!