Well that's a hard one. Here is the MSDS for Smoothcast 300: http://www.eagerplastics.com/sc300b.htm
"Polymeric Resin" is a pretty generic term so I can't give you anything specific. If I knew what exact polymer it was I could probably give you better information.
Let's just call it "plastic". Most plastics are vulnerable to Keytone solvents
. I think the most powerful ketone you can get a hold of is Butanone which is usually called MEK or "methyl ethyl ketone". You can buy it from Amazon or your hardware store.
Here is an example from Amazon:http://www.amazon.com/Sunnyside-Corp-84732-Methyl-Ketone/dp/B000BZX49I/ref=sr_1_1?ie=UTF8&qid=1322108882&sr=8-1
From the description "Very active solvent with similar characteristics to acetone, but is slower drying than acetone. Primarily used with specialty epoxies and resins." which is a nice way of saying that it will dissolve the[Censored. Please be nice!] [Censored. Please be nice!] out of things.
I have to warn you though, MEK is pretty toxic so you don't want to screw around with the stuff. Wear two layers of nitrile gloves and don't breath the fumes. If you have a an organic vapor respirator use it and replace the activated charcoal cartridge beforehand.
Some plastics and resins aren't soluble in anything at room temperature. In this case you want to get the MEK boiling but you want to do so safely. The best way to do this is to use an electric hotplate outside so you don't have a toxic fume problem in your house. Use a pot of water and float a borosilicate dish (Pyrex) with the MEK in the boiling water. This will prevent hotspots igniting the MEK because you have a water buffer. Put the metal pipes in the MEK and get the MEK boiling. It should attack the plastic rapidly. I have dissolved a number of things this way.
If MEK doesn't work the other solvent to try would be Xylene.
If Xylene doesn't work, if you can get your hards on "carbon tet" https://en.wikipedia.org/wiki/Carbon_tetrachloride
or Chloroform or Tetrachloroethylene those are almost guaranteed to work. Write you will first before using them though. Chloroform is the only chemical I have even seen dissolve my HD-PET conicals at room temperature.