They only crack if there not done right I've moulded a good few now and no cracks at all most lads put a shit load of filler and don't bond them right that's why they crack. If you bond arch wit fiber glass To the body of the car and then grind a mm or two off the edges of the arches and use p40 fiberglas to mould it then just a really light skim of filler to get any low spots. Gaurentee won't crack if you do it like that. Takes alot of time to do it right but looks really well if it's done right if u look through my threads should be a few pics there