It's a great barrier coat to prevent wrinkling but also sets up an even color for your basecoat. For instance, this tool found off the street is in our shop doing some crappy side job for a customer. It was discovered that the red paint on it was soft, which could have been due to bad hardener. So this tool paints the jambs silver and the red, since it's gummy, bled thru the silver metallic, which is semi transparent as all metallics are. So on one hand the tool was fighting coverage because it's not so easy to cover uneven colors with metallic, and on the other hand the paint bled thru just days after. Probably the worst prepping and paint I've ever seen. Anyhow, a sealer could have helped both situations.
I'll try to take some pics of these jambs today.