Hey Dave
How is it going!!!!!

First of all you do not need to clear over Pro-mark decals unless you want it to be glossy?
They are fuel proof, what is not fuel proof is the glue that is used to stick them down. All the F100's I have done and the Tam F18 do not have clear over them and with out any problems. I will not be clearing the F4's either as its just not needed and why add in the extra weight
Sometime ago I found an old T-33 tank that I had stuck an old old pro-mark decal over and tried several different chemicals, fuels and such to test their durability and the only way it started to come loose at all was if it was fully saturated in fuel, not likely to happen on our jets. The other reason is for repair, it is so easy to just replace one if you need to after an off road excursion.
Say hi to the Jakester.

Almost forgot, you do want to wipe down the model with ppg acryl-clean so as to remove any oils from the molds or the natural oils/wax that paints give off as they cure.