I don't think any special prep is required Keith, other than making sure the surface is clean and dry, however, it would be best to contact Dinitrol for their advice. What I can say is all these rust proofing treatments work best applied over a surface that has not yet started to corrode, the ideal time to apply therefore is when the car is new.
Ziebart is another good treatment, they've been in the game since the 60's, I remember seeing a 1968 Volvo Amazon in the early 90's that was Zeibart treated from new. A full 25 years later the condition was amazing, the Zeibart treatment had kept the chassis and panels completely rust free, further proof the 'treatment from new' approach is best.
I believe Morgan themselves used Dinitrol 4941 as their final pre-delivery underbody treatment, sprayed over the new inner surfaces of painted aluminum panels and a new galvanized chassis has to be a very good recipe, add in a stainless steel bulkhead and these later Trads should be very long lived cars indeed.
