Things that have worked for me
I have used clear packing tape, Saran Wrap and the clingy static plastic used to wrap furniture when you move.
The packing tape works great but usually leaves a sticky residue you have to clean off later.
On aluminum, if the blue protective coating is still on, I will just lay up directly on that and put packing tape over any rivet lines or holes where it was removed. Works great.
A heavy coating of wax will also work, although I prefer the former methods.
RV6, RV8 (2), RV12 & RV3
A&P, Tech Counselor & Flight Advisor