Is it a bad idea to clean and reuse the old gaskets? I like the cost savings even if I have to use a little elbow grease but will it still seal? I need to find a new front and rear soon or go the re-use route.
You can use your old ones with success. But like you mentioned over time the rubber starts to get hard and it can be VERY difficult to install back in the car. The newer rubber will "give" as you work making things easier.
It can be sealed without leaks..... anything can be sealed but how it looks after achieving a perfect seal is factor also. Different methods can be used to achieve the seal so to narrow it down to just one way to seal one is hard to say. Recently we have been using a buthel (sticky rope) between the pinch weld and the gasket instead of the traditional bed a glaze. We did this to mine... so far no leaks... but I'm still looking. My boss is always working on something better and easier to seal windshields and back glass so this one of his new methods. Working with bed and glaze can be a messy situation as everyone knows so this method eliminates the mess altogether.
