Additional suggestions for weather proofing rig.

The newer rigs have weather proofing end-caps and hole plugs in the header.
This does not SEAL the rig but greatly reduces the amount of rain that gets inside header.
These plugs can be retrofitted to existing rigs. $10, incl shipping

Although rig was not intended to be set up long term out side.
There are things you can do to make it a little more weather proof.

Like any steel product left outside, rust can develop. But like cars, bridges etc. that are quality painted can withstand weather for very long time. Powder-coat, as I use, is very good at protecting the outside.
Issue is more with the INSIDE of the tubes where there is NO paint.
( I have actually had a old style rig with just cheap spray paint set up outside for 2 years through Colorado winters and covered with snow. There was just a little surface rust discoloration. and this was without any special winterization process as described below)

The eyebolts at top are galvanized for weather protection and the nuts on the eyebolts are plated.
The set screws that hold the legs together are Stainless Steel as are the nuts that are welded to the legs that the set screws go into.

One issue is that water from rain and sprinkler can get INSIDE the FEET through the adjustment and tie-off cleat holes.
On the newer rigs I have put an extra hole in the bottom plate of the feet so that water can drain out. If your rig does not have the drain holes in the middle of the feet bottom plate then just simply drill a 1/4" holes and then spray the drill hole with some clear paint.
also, if you know the rig will be outside for long time you could even tri putting old pieces of bicycle inner-tube over the feet adjustment holes to seal the holes. 

Additional things that you can do if you know that the rig will be up long term.

Leg section attachments
As the legs are added and screw is tightened, you could put a thin coat of silicone sealer in the seam of the join there the leg sections come together.
Also put a thin coat of silicone sealer on the out side edge of the set screw that holds the legs on but don't put it in the hole that the wrench goes in so that you can easily put the wrench back in for disassembly.. ( PLEASE make sure that the screws are properly tightened)

Some of the newer rigs also have a snap button to hold the leg in place during assemble, you can put silicone around that button also.

Another possible idea is to get some old bicycle inner tubes of the right size, cut it into aprox 4 or so inch long sections.
Put a tube over the leg section without the set screw. After tightening the screw, (MAKE SURE IT IS TIGHT)
THEN WORK THE INNER TUBE SECTION UP OVER THE JOINT BETWEEN LEG SECTIONS AND
 OVER THE SET SCREWS YOU JUST TIGHTENED.
thIS IS A RATHER EXTREEM MEASURE AND IS PROBABLY MORE THAN ACTUALLY NEEDS TO BE DONE, BUT IT IS AN IDEA.