Honestly I feel that there is no true waterproof blanket. Over time the blanket's properties end up wearing out or they never actually end up doing what they say they should. Typically I find that the more you spend, the better quality you get and a lot of the manufacturers that will guarantee their quality, so if it fails you can talk to them that it's the fabric itself that isn't lasting. If it rips due to normal wear and tear usually that is something they cannot guarantee. Sometimes they'll be able to help you out with replacement options if you talk to the right person and you are amicable.
One solution though is with the blankets whose properties last for a good long while is that every few months after you wash it or get it cleaned, spray a light coating of waterproof spray on it. I think this helps prolong the life of better blankets. You just have to be careful to not get this on parts of the blankets that will be in direct contact with the horses skin as this can sometimes create an allergic reaction.
What is the name of the water proofing agent you are using to topdress your blankets? I'd be willing to give it a try on my older sheets.
I'd love some of the Rambo's but really could build a small barn for what they would cost.
We are currently trying to figure out the most cost effective way to mkae a 4 stall shelter. It will only be used in inclement weather so it doesn't need all the bells & whistles (especially since money is tight)
I've got 4 of these and if you hit a good day you can get them for around 70 bucks. They are realy well made and water PROOF! My horses are out 24/7 I've had them on 3 days straight without wetting through.