Well you are not alone on problems like that. I have a half way build Caterpillar that the same thing happened,,,paint just would not lay flat. And when I tried to use purple bath to start over the paint and primer wrinkled and just refuses to come off.
That is the last time I use Tamiya paint, I a lot of modelers use it but it just never does good for me. I am sure its not the paint but somethings I do but Tamiya went out the door and going to use Vallejo from now on. Had used Vallejo before with good results but I tried Tamiya for only
one reason, only hobby paint I could buy in city where I live. So waiting on order of Vallejo.
Sure do like the looks of that truck you are building,,