I have played wtih 3mm thick abs a lot.
If you heat it too close (means too hot also) it or too much it will start to get pinholes/craters.
Tendency to bend at different directions is still little bystery to me. I manufactured first prototype convex hubcaps and I had same problems. Propably its related to that you are not heating enough wide section of the part and it will get tensions to the part.
If you are building that lower spoiler I would first cut the rough shape with by using cardboard to get correct shape.
then draw the figure to abs sheet
then cut it with knife along the outer line
bend and it will crack along the cutting line. Might need pliers like leatherman etc
Smooth the edges with knife
put the plastic on top of something and where you want to make edges put something under the part so its not laying againt the table etc.
Then heat engouh wide section correct temperature is 120-140 celsius usually. Test it with small piece that will it start to boil or not before you do the actual bending
Lift the part up when it starts to be floppy might need two people for that bumber to get best result
shape it agains the bumber
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