It won’t help with the smoothing part, but for gluing, I’ve seen several people saying friction stir welding works great for this. Basically, stick a length of PLA in a Dremel or other rotary tool and let the tip of the filament spin against the joint you want to weld. The friction creates enough heat to melt the tip of the PLA rod as well as a bit of the printed parts, which then rejoin into a solid piece of PLA as it cools.
There’s a YouTube video of someone trying it her: http://www.youtube.com/watch?v=4xrKbo8f_Rg
I’ve seen this technique work excellently for metal and it makes sense that it could work for PLA as well. I haven’t tried it myself, but it’s something I have on my list of things to try eventually.