Sticky MC

Nah, dust won't short-circuit an air-gap - dust is non-magnetic. If your contactors are hanging up, the common problems are: insufficient air-gap, weak spring (if that kind), incorrect orientation (if that kind), rough contacts, and lack of bleeder.

If you have found dust stuck to the cores of a contactor. Then it is most likely due to iron contaminates in the control panel. Will happen on equipment operated in dusty environment. Have seen similar cases on electric lift trucks in a wire factory. The dust in these cases did not hold the contactor closed. It reduced the maintain pressure on the contacts causing them to arc across until they welded together. Try to seal dust out of the controls. I silicone every entry place i could find. Tighten connectors retaining cables.If they where not sealed sealed them. Replaced cover gaskets.

Another thing to consider is contact rating & load - are you sure you haven't got a surge when you make the contactor which could be welding up the contacts? I've seen this - admittedly with DC systems and capacitative loads - but the same applies to any system.

Dust cannot be the problem. Where I worked had immense of dust. But dust with moisture cause havoc. That I have experience and frustrating days to clean up the mess. Another is insufficient spring tension. But you have not mentioned the contactor details to look into the problem. Poor contacts also very bad . Another fact is frequently in and out contactors get welded if copper get burn due to the arcing.

Naw. That is my opinion too. A decent contactor have to have enough strenght in its spring to open. But if you cause the contacts to weld, most likely by overload, then they will not open. A weak spring also lengthen the time for arcing, same result.

If it built poorly, you can maintain it, but it wont turn into a quality contactor.

How often are the contacts closing?

How much current?

If the current is more than 50% of the rating, try using a contactor with a higher rating.

If the frequency of closings & the current are high you could consider mecury contactors.