It is a sad fact of life that where there is moisture in the air then there is a real possibility that valuable machine tools will succumb to rusting. In my workshop I am fortunate as I have not had any problems with rusting over the last 30 years and I thought it may be of interest to see why. In my opinion the key things are environment, constant temperature, few windows, low moisture, cover machinery and use of an oily rag on machine surfaces.
The workshop environment is key and my workshop is a double car-sized garage which is part of the house with the main living room above. The garage has a single large window on one side which is away from any machinery and a wide north facing up-and-over door. The door is a commercially made unit which is fully internally insulated and has rubber seals. This door is remarkable in that it is just over 1" thick yet heat loss is minimal. As it faces north there is no sun to warm it up so by and large the workshop keeps to within a 10-16deg. C range year round.
Maintaining a constant temperature is essential and changes should occur very slowly. If temperature changes happen quickly by using external heating then moisture will condense onto colder metal surfaces and rust will soon appear.
Maintaining a low level of moisture inside the workshop is ideal and the use of a dehumidifier will help however I do not use a dehumidifier and also park the car inside the garage even when it is covered with rain drops. So why do my machines not rust when there is evidently a lot of moisture in the air? The constant temperature is again a major reason but I also cover my machines with these 'blue' polyethylene woven tarpaulin water resistant sheets which are readily available and very low cost. These sheets are arranged so that they prevent any moisture droplets getting onto a machine surface but crucially allow a free passage of air. Never use cotton or any covering that absorbs water!
If there is a need to raise the temperature of the workshop then this must be done very gradually to allow the cold metal to also heat up. Never use propane or similar heaters as they produce a lot of water.
The final step is to keep an oily rag available and give the machine surfaces a quick wipe if the workshop is not going to be used for a week or so.
See my website for more information: http://www.gwhengineering.co.uk