These are stainless steel castings. The rust spots are either from small surface inclusions from casting or where you have touched them with steel. Lifting or resting them on steel surfaces will embed small amounts of Fe in the surface which will later rust easily.
If these are only on the cast surfaces maybe you should have some words with you supplier. The casting should have been annealed, blasted (usually grit or shot) and then passivated (hopefully in Nitric acid). The raw castings should never show rust spots.
1. If you have done any machining make sure that you coolant is well filtered.
2. If you do any 'touch up' with wire brushes, flap wheels, or files make sure that these are only ever used on stainless steel. (and make sure that the brushes are 3xx SS)
3. You can remove Fe easily with acids, the safest is a product called CitriSurf which is citric acid based. (I have bad experience with a quick dip in anything other than nitric acid, but that is more difficult)
4. Any of the corrosion inhibitor companies will sell Rust Preventive (RP) oils some of which are designed for Stainless Steels, ask.
5. Use nylon slings for lifting, rest only on wood, cardboard, or plastic, use flange face covers (also minimizes mechanical damage). No chains, no metal forks, no steel tables or pallets.
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P.E. Metallurgy, consulting work welcomed
