A-Mac is right-on. I use both Alumilite and Featherlite. Over the past year my preference has turned toward the Featherlite. With the Featherlite you mix only two compounds, then pour into your molds. With the Alumilite you have to measure out part A and part B, then add microballoons to each, mix well, mix the two parts together, then pour. To get consistent baits your measurements must be exact. Yes, the Featherlite must be weighted. It is very easy to sand and/or carve right out of the mold. It does become more durable the longer it sits. I usually demold, wait about an hour, clean up any imperfections (mold lines, etc.) and then put the piece aside for about week to ten days to let it reach full hardness. At this time it is ready for ballast, hook hangers, line tie, any further sanding (at this point it is still easier to work with than a hardwood) or carving. When ready to paint I wipe it down with denatured alcohol and paint. The clear coat I use is Dick Nites and 4 coats makes each bait almost as durable as steel.
Let me add that if you are looking for a bait that will sink quickly the Alumilite without microballoons will give you what you are looking for. No need to add any ballast. The bait is brittle until you clear coat it for protection.
Both products are very good, and each has it's place in my arsenal.
David