On my M38A1....no doubt....body work...ESPECIALLY sanding....and sanding...and more sanding.....
On the half track...…
Probably the entire bogie/suspension system.
Just rusty/stuck/stubborn hard a$$ work.
Of course the clean/sand/prime/paint routine for a bazillion small parts
is pretty "fun" too.

But all this aggravation is great therapy for me.
I absolutely love the sense of satisfaction you get from
working on these mechanical history books and
slowly bringing them back to life!!
Keith