I don't know what's going on with the filler-primer. I brought it inside at Sofaking's suggestion. It's been indoors almost 24 hours now, and it's still gummy. I think the high build formula is too finicky.
Would it be a viable option to buy the primer that isn't designed to be a filler, but use it that way anyway?
Why don't you try using a car body filler, something like U-Pol or P38 and then thinning it out with some acetone. Thinning it out will allow you to make it more spreadable, it will increase the curing time, the acetone evaporates off and will cure (don't add too much, you will quickly grasp if you have let it down too much). Also you can increase the cure time by adding more catalyst that usually comes the with the product ( you can get it in white, it normally comes in red, but it find white is an easier colour to conceal with paint)