To echo hcmbanjo's remarks.....
I use it all the time, but first I thin it down with a
little bit of water to about the consistancy of paste or pancake batter. I use it to fill fins and nose cones, fill body tube spirals and to make fillets.
So to answer your questions, here are my observations/experiences:
1. I don't seal mine first, but you could if you wanted. I don't think it is neccessarry though, and my opinion is that it adds extra work. I think the wood filler is accomplishing the same thing that the sealer does.
2. No you don't, although (again) you could if you wanted to, but it isn't needed. I don't do it.
3. It depends on how thick you put it on and how much water you use when you thin it down, but my experience is that I usually let it dry about an hour. If it is still a little wet, it won't sand well.
A couple tips from my experiences:
A. Place a small amount of filler in a small disposable cup or jar for thinning, and add water very sparingly - a little goes a long way.
B. If you use it to seal balsa fins, do both sides of the fin at the same time to prevent warping.
C. Shortly after you apply it, you can dip your finger in water then rub it over the filler to help smooth it out. The smoother it goes on, the less sanding you will need to do later.
For me, I live in an apartment with less-than-optimal ventilation, so sanding sealer always stunk up the place and gave me a headache. That problem completely went away for me when I replaced sanding sealer with wood filler. No fumes and easy soap and water clean up.
Here are some fillets on a Big Daddy that I used watered-down wood filler for:
