Over metal and filler I would definitely want a couple coats of epoxy on them 24hrs before the bed liner is applied. The epoxy just ties it all together nicely. Make sure to get the edges coated well-the edges often get overlooked and usually take the most abuse. The bedliner is a really good product. Some are even reducing and applying it smooth-I think this has opened up doors to a lot of different uses with a little creativity. Consider some of the 20's-40's era cars with running boards, I would bet one could even do a two stage application using a stencil if a raised pattern is needed.
Im coming up part way on the firewall, but not all the way. I suppose the bedliner would wash off good with a pressure washer so maybe my hesitation of too much in the engine compartment is just being overly cautious? I do like your idea of color tinting the bedliner. On the other hand, if you reduce it, its going to be much smoother. You might really be on to something. Hope you post some pictures!
I have not but I am getting ready to spray a bed out reduced a little for flow hopefully by this weekend. I doubt I get to my own car before spring unfortunately. The reduced stuff just be the black pigment that comes with the kit. Will try and get some pics early next week if I get it all done.
Easy as cake. If ya just wanna check it out when your all ready to spray, and I recommend this anyway, just test out on a scrap panel. Start one to two turns out on the nozzle and adjust from there, I can't remember where I set the regulator but I believe it says in the tech manual or somewhere on the forums.