I did a little experiment with scrap. I pilot drilled a piece of .025 flush with the edge of a piece of angle. Then I offset the simulated skin .025 and drilled through with a #30. The hole cleaned up pretty good. If I have any holes I don't like I will go up to a #21 and use an AN470AD5 solid rivet...
Any 40's, 50's, 60's, or 70's era Cessna will be unprimed 2024-T3 alclad, except for a minority of airframes. The C140 I was restoring had some corrosion issues. Considering it is 68 years old that is not too bad. I am not sure that an unprimed airplane would be harder to sell than an overweight one...