Sandpaper is your best friend. Get several grits, and give each piece a through sanding with each progressively finer paper. Some folks use a filler/primer which can help cut down the sanding, but if you want them smooth, there's not really much else you can do.
In terms of weathering, there's many ways to do it depending on what equipment you have. I like to keep it simple so I just get a few different acrylic colours (nice and cheap) like black, burnt umber, and whatever else you like, squirt some into a container, pour a bit of Windex in (helps to stick it to the existing paint) and mix it up. I then use a cheap, beat up brush and jam the mix (the umber and black make a really nice grimey grey colour) into all the cracks and crevices. Then wait 10 seconds or so, and take another rag and do a really bad job at cleaning the piece, leaving some smears and crud in the grooves. You can also use that same rag to wipe down other stuff if you want. Once you're happy with your weathering, let it dry for a day or two and then hit it with a clear coat.
You can also use an airbrush to mist paint, or you can mist spray paint over top of items (don't spray at the item but spray a small cloud over the item and let the paint fall).
There's lots of weathering techniques here and online, especially YouTube. I'm partial to Adam Savage's techniques. It's a good idea to take some scrap and do your paint tests on that so you don't have to worry about making an unwanted error on your final piece. Have fun!
