I do not think that thinner will affect auto paint so much. Rather, I will worry about whether you can color the part evenly and without bubbles. This is a common problem for beginners. If you have an unheated garage, then you should pick up the part in the house for uniform drying without temperature differences. Anyway, jeep jl bumpers are not very expensive, check it out. You can always replace this if you do not like the result of your efforts.