Why we recommend the glass test.
The reason glass is a great way to evaluate a product is because it lets you see the dried product isolated from your surfacing. Any differences between the dry product on glass and the dry product on your surfacing will give you clues of what to look for on your surfacing that is giving you an unexpected result.
On glass, you can see:
The only requirements for the glass are that it be flat, clear and clean. We have found "Windex Original Formula" to do a good job of cleaning. The glass can be any size and shape available as scrap from your local glass store or picture frame shop.