I recently completed some Necromunda guys with some great metal deck bases, perfect for the Underhive or urban landscapes of the 41st millennium. They were cheap, fast, and easy to do, so I thought I would share.
First thing take a blank base (easier if they aren't slotta bases, but I had to use that for my necro) and put a glob of Green Stuff on it.
Next just smooth it out with your fingers and get full base coverage.
If using slots, just dig out the green stuff and open up the slot with a sculpting tool
Now find something REALLY smooth. I had the bottom of this resin mini, but anything flat will do, however you can put a ziploc or other plastic down as well to make sure you get a very slick, smooth surface. This also really helps keeping the green stuff from sticking to your object.
Now that it is smushed out, pry you slot back open if you made one/need one.
AND WET THE BASE. LIKE SUPER WET. Sorry this is blurry. BUT you get the idea. ALOT of water on it. You want the impression you are making to happen but nothing to stick.
Now I had this cheap brass grating from secret weapon miniatures, but any similar pattern/producer will work.
Place it down, put your flat/smooth surface down once more (including the ziploc if you used that) and press down evenly. Enough to make a surface impression everywhere, but not to cut deep and completely smush out the base. It may take a few tries. if you make a mistake, just keep it wet, smooth it out and try again.
Remove your flat object, remove grating, carefully pry open the slot if required. Let dry and cure (I put mine under a fan and bright light to speed the process, but you can leave them over night too). If interested, the mini is a toad I am going to use as a Beast of Nurgle for Blood Bowl from Toad King Castings.
Prime. Anything cheap and black will do.
Drybrush in your metallic colour of choice.
Add rust effects as desired.
Wash (nuln oil or argax easthade work well) and drybrush again.
Once competely dried and cured, if doing slots you may need to use an exacto knife to cut away any excess to get the slot to fit. I don't do this before it is dry so that I ensure a snug fit in the end. Finally you have CITY BASES! YAY!