I used "regular" POR15 on my chassis and not the paint. I found that if you pour out to much and then start putting it on the chassis it will start to skim. When it does this it did not flow and level out as well as when it was fresh out of the can. I might add that the brush itself may start to getting a bit stiff and you need to change brushes?? Thats why I use the cheap throw aways. Sometimes a bristle may come out and stick on the chassis but hey, keep on trucking. Who's going to see that.
Yes, you can spray over it to smooth it up some. After I did my chassis I made a mess or two doing some things and touched it up with rustoleum spray paint. Looks good.
Sometime ago a bolter recommended drilling a hole in the por15 and only pouring out SMALL amounts at one time. Well, I just opened up a can of por15 and I drilled a 1/4" hole in the center of the lid. Poured out some, wiped it off, put a piece of duct tape over it and it works like a champ. I've "opened" it and resealed it several times over the paat 3/4 weeks and its still good inside can. The good part is I pour out very little at a time. Then replenish.
I'm sure other bolters with experience with the chassis paint will chip in here.