I like my own filler. If I think I'll need filler, I chop up some of the sprue in the kit and throw it in some Tenax-7R (or any good liquid, plastic glue). Two to one, glue to sprue, by volume. It can take a couple of days, for all of the sprue, to completely melt. If you need to thin it, add glue. I you need it thicker, add sprue, or let it stand open for a few hours.
It's main advantages are: 1) it matches the original color of the model, 2) it's goes on like thick paint, 3) sands exactly like surrounding plastic, 4) Once sanded, it's invisible (before sanding it's got the same color and finish as the model), 5) with patience, it can be buffed back to the original finish of the model.
There are several disadvantages to this stuff, too: 1) it takes time to fully set up (the thinner the coat, the faster the set-up time), 2) it has to go one in thin layers or it will melt unlaying plastic, 3) if you leave it open for a couple of days, it hardens, 4) it starts to set-up quickly (gets stringy).
If you need to fill a open hole, Put a piece of scotch tape over one side or the hole. Then, paint light layers on, until it is filled.
It can be quirky, so practice with it, on some sprue, till you get the feel of it.
Mike