Hi Joe,Well, I'm not CNKS but I will throw out an opinion. If you are talking about rust pits, etc. you probably want some two part polyester "finishing putty". It really is nothing more than a smooth but more fluid bondo. Evercoat makes several versions of this as does 3M. It is good to fill areas that are not deep enough for "bondo type" plastic fillers but are too deep for fixing with a surfacer such as the MP182 CNKS mentioned in a prior post. Rust pits usually fall into this category. I have used the 3M product and it works well but I always seem to wind up with a few minor imperfections remaining, even after 2 or 3 coats of surfacer. I usually take care of that with some regular lacquer type spot putty. Many will not recommend using this "outdated" product but I have had excellent luck with it. The key to it's effective use is to put on very thin (no more than 5mils per coat), give it plenty of time to fully dry (overnight even) and seal it with epoxy primer or a surfacer before the final topcoat. I recommend this type of work sequence for rust pits: 1. Sandblast. If you can't sandblast, wire brush and apply a rust treatment such as Permatex (Loctite) Extend.
2. Prime with a two part epoxy primer such as PPG OMNI MP-170.
3. Apply the polyester finishing putty where needed.
4. Sand smooth with 80 grit.
5. Re-prime with epoxy.
6. Apply a two part surfacer such as PPG OMNI MP-182 - 2 or 3 coats overall (caution health hazard here because of isocyanates). A one part surfacer can be substituted if you lack the proper safety equipment but the same application issues that go with the lacquer spot putty also go with it.
7. Sand with 220 grit.
8. If ANY noticeable imperfections remain, apply the spot putty where needed.
9. Spot sand with 220 grit.
10. Apply a final coat or 2 of surfacer and sand with 400 grit.
11. Be absolutely sure you have no imperfections showing at this point. The surface should be as smooth as glass to obtain the best gloss in the topcoat. If it is not, repeat any of the above as needed.
12. Topcoat as desired. It sounds like a lot of work and it is. But you only need to go to that effort on the sheet metal and only if you want the best possible result. Forget it on the cast. There are also variations on such a sequence. Different people go about it in different ways with different but similar materials and get equally good results. There is a sprayable polyester filler on the market nowadays but I have never tried it. In fact I doubt I have the larger gun fluid tips and needles likely needed to use it. I simply have had good success with the above.  Rod
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