I would always say that there is no point preparing and painting a tractor
if you need to go back at a later date and perform some mechanical repairs;
basically because all the good paintwork is often scratched and damaged when doing the repair work.

When a job such as an engine rebuild is been undertaken this is a good
opportunity to prepare and paint the engine as you go along.  When all the
engine ancillaries are removed from the block it makes it much easier to
clean and paint in all those hard to reach areas.

Also whilst a major component of the tractor is dismantled it makes sense to replace all the gaskets (usually contained in the parts kit) and hence stop
all the oil leaks that are part and parcel of an old tractor.  Oil leaks that soon show up on a new paint job.  If you do nothing else mechanically to a tractor before painting it, then stopping the oil leaks is a must – there’s just little point painting the tractor if oil is going to leak all over it.

We are not experts at painting here at Vintage Tractor Engineer, but I think the advice above is sensible.  Customers usually prepare and paint their tractors at the same time as we are doing the mechanical repairs.  We often just do a few hours work on an engine and then wait for the owner to catch up with the paint work.