I never did like bondo. I use fiberglass for any kind of body work. Both are a pain in butt lol.
X3, X4, X5 etc on the above....Clean is the key. Get all the old paint removed down to bare metal if possible or at least ruff up the paint with 60 or 80 grit sandpaper. Prepping the metal is the key to getting bondo to stay on there. Start off with 80 grit to knock down any imperfections after putting it on. Use a plastic or very flexible putty knife. The smoother you put it on, the less you will have to sand and maybe not even need to start off with 80 grit. Just remember this stuff is for very small dents and working out any imperfect body problems. Try to work your sheetmetal more first than just trying to cover with bondo. If you put bondo on to thick and it will and can pop off later on down the road.
The key here is to work slowly with it and try not to sand it all down at once. I go at it with 80 grit to get it down, unless I put it on really smooth then I will start with 100 grit and work way up to 220 then to 400 dry/wet and finally 600/800 wet sand. You want to get that as slick as possible when finished. Feather all the edges back down to the metal when sanding it down, because you don't want it looking like bubble gum under the paint job or to see the edges of the bondo showing through the paint. Try to make your body knife larger than what you are trying fill, so when you spread the bondo it will fill the impression and feather the edges blending it back to the sheetmetal.
Block sand it or use a good electric or air sander if you have a compressor. Using just your hand and fingers with sandpaper can give or leave finger grooves or a wavy feel to it when done sanding. You can pick up a cheap block sander at Walmart. I have used a small piece of 2x4, but the block sander is made of rubber, works a lot better and is flexible with most rolls or curves in the metal.
If you have to, see if you can pick up a body tool set at any local fleamarket. The hand dollies in the kits sure have some nice different curves and angles. I bought a cheap seven piece kit for about $15. It came with two small body hammers and 5 hand held steel body shapes. It was made in china, but hey it works. I have also used those to sand with. Just wrap the sandpaper around the shape/contour I needed.
Wished you live a lot closer, would be glad to show you everything. Another trick we use is "spot putty filler". It is used to fill any little minor sanding scratches or air bubbles that may pop during the sanding process on the bondo. It comes in a tube that you can just squeeze out, put it on the bondo and its already pre-mixed. I normally just smear it on with my fingers working it into the scratches or popped air bubbles. I wet sand with 600 or 800 on the spot putty and bondo then. Liquid Steel works if you can't find any spot putty, it comes in a tube also and can be found at most hardware stores. Should be able to find the spot putty at any auto part store. Sometimes you can find it at Walmart.
Just remember take your time, use patience and keep running your hands over to feel anything not smooth. It should feel as smooth as glass when completed and you shouldn't be able to feel any edges between the metal and the bondo. If you do, wet sand some more and use the spot putty on the edges to blend the bondo to the metal. When you think you have it all done, shoot a light coat of primer over it. I use just a cheap can of .98 cent gray or red primer from walmart for that. Any imperfections will show through the primer.
My Step-Dad use to own his own body shop and taught me quite a few body tricks of the trade. He just couldn't understand or figure out how i got the fiberglass to look better than bondo lol. A plus for me on fiberglass... its waterproof for years to come an go and seems to me to bond better to metal than bondo. My El Camino is proof of that... 20 years since I have done that body work and the fiberglass still hasn't popped, flaked or chipped off anywhere.
I have some real old pictures of my El Camino, before and after on my "Found 1964 Barracuda" Post.
http://www.forabodiesonly.com/mopar/showthread.php?t=70558
