I don’t think we ever even though of filling the grooves in body tubes when we were kids but nowadays it’s standard operating procedure for a good finish. The main body four-inch phenolic tube has spiral grooves in it you could drive a truck through.
Filling them with a 4:1 mix of Elmer’s Carpenter Wood Filler and water works well. VERY tedious to fill these with a stick, then sanding them down lightly, repeating the whole process two to three times. Lots of dust, too.
Now it awaits some primer to really show off the flaws.