While waiting for a welder, I cleaned the rust off the front section of the frame.
Then today the welder came. Very professional. Of course there are limits to what he could do, but he fixed all the welds and added some reinforcement. It’s now far stronger than when it was new.
And I was able to get two coats of the Alumathane on. It’s curing now. 
Then I need a couple dry hours to get this floor section in and the next one out.