Often, life intervenes between foiling and soldering. This frequently results in the foil not accepting the solder very well, because of the mild tarnish that has occurred in the meantime.
It is a good idea to clean the foil of any possible corrosion before soldering when there has been an interval between the two processes. It is enough to clean foil with a mild abrasive such as a foam-backed scrubber from the dish washing, or fine steel wool. I prefer the scrubber as it does not introduce another metal.
Some people prefer a vinegar and salt solution to apply to the tarnished foil. I am concerned about the introduction of an acid into the process causing further problems later. I don't recommend this method of cleaning.
I then coat the exposed foil with a film of paste flux to protect from further tarnish. This acts better than any loose covering of cloth or plastic to protect from oxidation. The purpose of flux is to both provide a "wetting" agent for the foil to accept the solder, and to prevent oxidisation. Liquid flux cannot provide protection, once dry, from the copper tarnishing. I prefer the use of paste flux to reduce boiling of flux and to keep the copper free from corrosion. The paste flux will not indefinitely prevent oxidisation, but will do so for a week or two.