Think of it as a big walk through a hostile environment. First, you have to go through some bushes, they'll be light, barely there scratches, likely quite small, possibly a few together as many branches and leaves caught you. Although one bush has thorns, so they'll be a little heavier, so use a longer stroke of the paper. And since bushes grow every direction, the scratches wont really have much of a direction to them, unless its on your gauntlets where you've been brushing them aside.
Then you have to squirm under a fence. There are loose stones and gravel there, they'll catch the paint too, heavier than the greenery, and slightly more uniform, but still, rocks shift so they'll never be all in one exact direction, especially where you had to shimmy sideways to free your rifle stock when it got caught. Again, variety here, some tiny scuffs, just press down with one finger on the paper, others use the palm of the hand to hold the paper to get more width.
Seems somebody saw you coming, you ducked most of the blast though and it was just a few tiny bits of shrapnel that bounced off your armour, was still pretty sharp though, so deep scuffing in random directions. And then more fence, had to cut a hole in this one, pretty sharp edges, try using something like a key to get very thin scratches. I find faster works best with this as making a slow scratch will be rather wobbly.
And don't forget the dust storm which blew up as you ran back to your ship, that'd be very fine grained possibly even a slight swirling as the wind bows the grains about.
And then remember as you move and bend the plates will likely catch on each other, so they may get more wear round the edges. Also things like shins, knees, hand plates and sticky out shoulder bells will likely be a lot more scuffed and dirty than chest plates. While the chest plates would likely have more blaster marks than your shins unless your attackers have been very bad shots.
Don't be afraid to use other things either. I usually take several different sizes of keys, the small hand sized pick that's in the house, the end of a run out pen, wiring if you have any heavier cables - I've got a nice copper one with a scratchy end, various sandpapers, scouring pads... I could probably find more things, but that's just what I could list of the top of my head.
(And for anybody who reads this and is using metal armour - I have a few dents from a 12th century style sword - see, totally random things.)
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