That is the downside of bondo. You can't expect it to hold up if it's used thick or you fill a seam because it will shrink and that seam will show in about a year.
That's simply not true. 2-part polyester fillers cure through chemical reaction, not through solvent evaporation and therefore maintain dimensional stability.
Things like tube spot putty shrinks and can take months to do so unless applied very thin.
