If you choose alcohol, don't use any that has a percentage on it at all. Isopropyl with a percentage on the label has water in it. Any water is too much. Rubbing alcohols are whatever percentage of alcohol that is listed on the bottle. The rest is water. If it's 70% isopropyl, or rubbing alcohol, it's 30% water.

You can buy denatured alcohol at hardwares and home improvements and it's not very expensive. It's 100% alcohol.

Bondo is a thixotropic. They are often blended with a large filler content. The filler won't thin.

Let us know how the experiment works out.