well we just did a few(a bunch) of pieces for a m3 we have in the shop and that is exactly what we did. used the mdf for the molds and bondo'ed for contour and used filler primer(sem brand, in a spray can) sanded until it was to the installer liking then too it to the paint shop... got it back today and the pieces look great. i'll try to get some pics up tomorrow.... and check out the shops website....
www.audiodesignsofatlanta.com