Mr. Surfacer is a product designed to help smooth out imperfections and tool marks in plastic as well as act as one heck of a primer. I picked up a jar for 5.75 and thought I would show what it does for anyone interested. I am using the 1000 grade Mr. Surface. It comes in 500, 1000, and 1200. I think the 500 would have filled much more quickly, while the 1200 is a primer only type.
I have been working in green stuff so I started with a few marks in the Gs. Thumb Print, deep tool marks, and a rolled texture you might find on a fig.

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continuing to apply

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As you can see after four coats even the deep marks are almost gone. I hope you find this helpful. I think this is a pretty useful product.

EDIT I forgot to mention that is makes a very hard surface on which to paint.

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Here is what I want to know about it: 1) if you used it to prime a knee or shoulder joint, would it stand up to the friction of the joint? and 2) would the same be true after painting over the top of it? Finally, 3) is it possible to mix any color in with it, or would that kill the effect?

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What I want to know is, where do you find it?

This would have been a godsend countless times when I marred/knicked bare skin areas (particularly forearms) for smoothing out those little spots. And since I'm far from done customizing, I know I'll definitely be needing something like it.

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sorry drbindy, I just got the stuff. No idea on the answer to your questions.
Doc Rob your answer is much easier, hobby town USA. They had 3 or 4 jars of each grade.

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Wait! This stuff is available in the States, again!? I've really been missing it, since my jar 500 thickened up on me.

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Dang, no Hobby Town USAs in this area (we really could use one, craft shops are nice but...lacking.) Thanks for the heads-up, I'll check online.

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Apparently it has been for a while now, store clerk said it was a heavy seller, thats why they keep so much in stock. I guess model aircraft folks grab it up like mad.

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Some months back, U.S. Customs decided they weren't going to let it into the States, anymore. Next time I get into Hobbyland, I'll have to see if they've got it again.

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did a little digging. Here's a helpful review: http://www.swannysmodels.com/Surfacer.html

and here's a place to order online:
http://www.greatmodels.com/~smartcart/cgi/search.cgi

edit with proper address for ordering

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this is by mr. hobby who also make awesome primer spray & clear coat!
the surfacer should hold up on knee joints, it's pretty tough stuff! as far as paint holding up, you can put enamels over this & they should hold up better than acrylics although you would have to prime the rest of the figure
dont think you can mix color with it..........

which grade of surfacer are you having the most luck with? Sounds like you are using the spray rather than brush on, but I think that came in different levels right? I'd probably go with brush on still, only since I don't have a good place to use sprays

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i'll check when i get to the studio, i just make sure i put plenty of news paper down, you want 2 or 3 coats rather than a heavy coat, so if your careful & you open a window you could do it inside.
didnt have much luck with brush on primer, this stuff is way better!