I know what you're talking about, and I've done it for a few imports. Whats most important is that the piece being molded in is mounted rigidly and not allowed to flex. what cracks is the mud that smooths the transition. biggest problem is the front bumper. the weight of it hangs out to the front, and the bondo cracks between the cover and the fender. just driving it around will do it. to do the front bumper right, you need to create a lattice-type bracing underneith and make it 100% rigid. not 99%, but 100%. I havent seen it done yet. theres no way to do this without bondo. only need a little bit, but they all crack. rear bumper/sideskirts are ok, just the front sux.
94 V8 on 17" custom paint, R.I.P
97 Mark VIII LSC yeah
08 Ford Edge, white for now