[MPWR]

You can get isopropyl in any decent grocery store, in the first aid section. It is rubbing alcohol, but it will say isopropyl on the bottle, and the concentration. Get 99% if you can, 91% will do. It's really cheap, a bottle will run you $1 USD. While you're in the grocery store, go to the plastic storeage container section and get some of the largeish reusable/recylable storage containers. Glad makes a size that fits car bodies rather well. Isopropyl is highly volitile (evaporative), so it's best to keep it covered and let the body soak. If you get the 99% stuff, you don't have to wory about it geting weaker as it evaporates- as it's straight alcohol, the relative water content doesn't increase.

Yeah, you can use brake fluid instead, and it may not soften your putty work- but that depends alot on the putty itself. Alcohol is safer, and probably cheaper too (and less poisonous, and you don't have to find a safe way of disposing it!).