can anyone tell me the way to mask off the wiper area so that it looks like its been in use I've forgotten how
to do it

My way is to lay the screen down on a cutting mat - if possible - and put one point of a pair of dividers (compasses with two points) on the pivot point of the washer arm. Open the dividers out to the legth of the arm and lightly scribe an arc in the clear plastic. Do the same for the other arm - if there are two wipers on the vehicle. There's usually a smaller arc at the bottom of the blade too don't forget. Cover inside your area with masking fluid - I use Humbrol's Maskol - Do your weathering and then peel off the maskol - A clean sweep on your screen
Some older vehiles were hand operated so the sweep will be from side to side, electric ones tend to stop just past the upright and sweep back, like modern car ones do.
Was that what you had in mind.. ?

I've swung two arcs (larger radius and the smaller radius) with a cutting compass that I've had for years.... since the prehistoric days when I was doing PC boards in rubylith. I would put a length of scotch tape on a cutting surface and cut there. Transfering the two arcs to the windshieid by hand (and eye). Then I would drybrush on the weathering. I have never had any luck with spraying on the weathering... usually was too much.

I'm just building a GPA out the box and was going to do a very light weathering job on it

Hi!
kkeefe


Don´t hesitate to airbrush the weathering!
Try to dilute the paint very much and the distance of the airgun should be at about 8 inches.
Here an example:

Try all the above ideas and go for it. I have always used simple cellophane (Scotch Tape) for the mask. The weather and/or terrain the vehicle is traveling through (in your imagination anyway) will help determine how you apply the weathering.

Snow tends to stick to the windshield and gob up when it refreezes. You can reproduce that effect with thicker paint, white glue, putty, etc. It tends to build up at each end of the wiper arc of motion.

Rain will run and streak even after the wipers pass. For a heavy rain this can be achieved with a thin wash painted heavily over the masked area. If you want to create a sense of motion stand the vehicle up on it's back end and let the paint dry running off the windshield towards the sides. For a lighter rain thicken up the paint and apply lightly.

A HMMV running through the muck (puddles on a sunny day) will splash mud all over the place. I was trained on these last week by the kind folks at the 731 ORD CO, and made a mental note of the vehicle’s condition afterwards. Mud splatter can be reproduced with an airbrush, or by applying thick paint on a large paintbrush, or toothbrush, and then running your finger nail over the bristles letting the paint fly in small droplets onto the kit.

For really heavy weathering you can do more than one of the techniques on the same kit. If any paint gets under your mask, take a good, hard, look at it before cussing. It may give the appearance of the water that drips and runs down the windscreen after the wipers have passed.

My attempts in no way rival Viking’s, but they might give you another idea for the effect you are going for. (Please ignore the broken, ill-fitting roof seam at the top. The kit didn't quite go back together well after being damaged in a move) Good luck!