The plastic coated ones, do lose a bit of strength, but they still penetrate well. On the foot it would be no problem. The plastic covered ones are much thicker so depending on where you place it you may not be able to walk on it safely. You'd have to tape one on, in that location. The plastic ones are regular neodymium ones with plastic on the outside...this prevents breakage and corrosion too.

When I did my feet I used non-plastic smaller ones, that were like this:
http://www.amazon.com/Disc-Package-E...dymium+magnets

You'll have to figure the polarity yourself. Basically the wrong pole will stimulate nerves, and the correct pole will block pain impulses. I put tape between my skin and the magnet, to prevent corrosion of the nickel coating. And also some people react to nickel with irritation anyway. I use surgical tape.

If you buy small magnets be aware of the danger to children and pets. Death can occur if swallowed.
Also you cannot use magnets if you have an implanted medical device.(cardiac pacemakers, SCS spinal cord stimulators, and
shunts for the brain.)

I love the magnets and still use them. I made a pad of 3 sewn in a row, for my back to lie on in bed. I use the one inchers with no plastic cover for that.

Without the plastic the really high gauss ones can penetrate 6 inches, which is enough for the back, but perhaps not quite enough for buttocks, depending.

You can leave the little ones on, if you want.
I stop at 20 minutes now, because I get relief usually by then, so why keep it on longer?