One of the keys aesthetic features of 40K is over-sized rivets, that and of course skulls! When you are doing a conversion or scratch-build, you often need to add rivets to help you model fit into that 40K vibe. Now for Orks, you are fine using a punch and make lots of round, flat rivets, but those really don't cut it for Imperial tech. For those I've discovered Tichy Trains' rivets. These are available in several sizes, and can either be fitted by drilling a hole, and dropping the rivet's shaft in, or but cutting off the heads and simply gluing them on. The first I assume is the correct way, but the second has advantages when you can drill through the model.
Here's how they come on the sprue, top to bottom, 0.050", 0.040", 0.030", the ones I've used most on vehicles are the .040", but the smaller ones would look better on powered armour.
Here are the packs so you can spot them at the hobby store, and you can see what I paid for them in Canadian dollars, but you can always get them from Tichy's website for less:
Here's some of the .040" on my WIP Howitzer:
Here's Tichy's website: