6061-T6 is a heat treatable alloy. Its mechanical properties are affected by time at temperature.
While removing surface moisture is a fine idea if the plate is being moved from the outside where it is cold to the inside where it is warmer, the better idea would be to allow the plate to warm to ambient and dry naturally. Any preheat should be held to a minimum. This isn't steel, so the need to preheat is not necessary.
You will need much more amperage that expected if you are used to welding steel alloys. Aluminum has a very high coefficient of thermal conductivity which must be overcome by higher amperage. If the weld pool takes a long time to establish, you need more amperage, not preheat. As a matter of fact, with thick material that necessitates using many weld beads, you need to cool the plate between weld beads. Artificial cooling by blowing cool air will be a definite aid if you are interested in maintaining the UTS around 24ksi. You can consider cooling the adjacent base metal with water soaked rags between weld beads. I wouldn't recommend applying the wet rags directly on the hot weld. Do not use compressed air for cooling, most compressors are oil lubricated. Bad, bad, bad.
Good luck - Al
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