Is the water shield coating worth the cost?

I'm doubtful, but I've never heard about it. Grass will keep growing and you will be cutting off the ends.

Just wash it with vinegar and water and you will save yourself a whole lot of money. Not sure if this is the correct answer to your question as it is a little vague.

You can buy a gallon of it at Sears for 43.00. All it does is keep water from staying on surface long enough to stain. A quick wipe down with a towel does same thing it it's you shower door causing the problem. Vinegar can also clean off buildup.

clean the glass with with vinegar using a dryer sheet then apply Rain X