"This thread is useless without pics."
So, here we go!
I must say, I am amazed with how wonderful Bondo is. To the same degree of my happiness with the Bondo, I am severely disappointed with how badly the MDF reacts with standard primer. I emptied most of a can on the marquee in several light coats, and the MDF just seems to suck it in, and then, causes the surfaces it's coated with to turn to a sandpaper consistency. I realize this is how it's supposed to be for the first layer of primer, but it's really, really disheartening to feel your work turned rough and looking ugly.
I know what my future plan is, mind you, and that is to find a local supplier for putty primer, such as the same product as Ond is using on his cab.
Everything looked really nice before the primer was applied, but the one positive I am taking away from this is that it really helps to show the defects in the preparation... ...in the seams that I tried to fix up with the Bondo, and other sections that I had thought were complete that suddenly jumped out at me as very, very incomplete. I walked away very disappointed and disheartened, but I must be more positive, and look at this as a starting point.