While working on a Japanese WWII tank, I noticed that there were going to be a lot of spots to fill. My wife had purchased a tube of Bondic earlier and suggested that I try using it. Bondic is a liquid acrylic that hardens using an ultraviolet light (provided). I used it on a gap on a coupe hatches and it worked great. It hardened and can be sanded and painted. It is probably best to use it on the backside to eliminate extra sanding. I will post a couple of pictures later.  (There are a couple of companies seling the same product on TV. It is a liquid acrylic that hardens with a UV light.)