Evaporust does have a gel, but I haven't had good success when I tried it. It has to stay moist to work and then needs to be rinsed off.
Like bartamos said, cut out the rotted stuff that has to be replaced, then deal with the rest. My solution was to get the cab sandblasted. Even that doesn't get to all the nooks and crannies. You do the best you can with whatever you have available.
