After each mowing (about 2 hours for what I do) I blow of the tractor with a leaf blower. The next day, when things are cool, I open the hood and blow out the radiators with an air gun attachment, pressure set at 60 PSI. I have a small piece of 3/8" pex pipe (like you see on toilets) I used to get between the radiators. No 90 degree, it just blows air by them. By working the radiators from both sides (hence the need for a cool engine) everything stays clean.
On the cab, clean the filter after each use. Then I take a small brass bristle brush and I have to comb the "smutz" off of the evaporator coil where it is exposed. I'm thinking of finding a "water pik" (do they still make them?) to clean that area out better.
No screen on '09 4520 cab unit.
Sure seems like this cleaning has to happen on each mow otherwise things could build up and you'd have a real mess on your hands. For 2 hours of mowing, you have about 12 to 15 minutes of maintainace to do. Worth it for me

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I also extended the condensate drains so that it was easier to blow of the rear of the tractor (no water everywhere to hold the dirt and grass debris). 3/8" line, simple barbed brass splice device. When I tried it to see if it would fit, it would not come apart. I have since changed out the nylon tie-wrap (UV eats them up) with a black one (UV east them up less).
Pete