My guide to body filler and when to use it: body filler has a legitimate place in panel repair when used correctly and a destructive role when used as a substitute for metalwork.

Correct use: filling surface imperfections after metal repair. If a panel has been straightened and the metal is back to original contour within 1/8 inch, filler is appropriate for the remaining surface work. It should not be more than 1/4 inch thick anywhere and should not bridge over a repair — it should be applied to solid, correctly-contoured metal.

Incorrect use: filling holes, building up heavily rusted areas, or replacing metal repair entirely. Filler over a hole will crack. Filler over severe rust will bubble when the rust continues to work. Filler over incorrect panel contour will never look right under paint and lighting.