There are different sorts of papers used to cover the cardboard on old jackets, and the same goes for inks. This means that even with experience gained the hard way, trying to clean up old jackets to valuable records is always going to be a dicey proposition. The easiest-to-clean surfaces are almost always those with the most "glossy" texture, with plastic-laminated (think old Impulse Jazz sleeves) of course being the best. The more "papery" and rough the surface, as well as the lighter the color or white, the worse the result likely will be if any liquid is applied, causing staining. Some color inks are also more likely to run or rub off than others, but again this is very hard to determine in advance. Other than urging you to try a small test swab in an inconspicuous area (Ha!), I would just say to stay away from solvents like alcohol and certainly from bleach, which if it's going to work on the mildew is going to do the same for the image. You can forget about doing much cleaning on a lot of back covers, which tend not to be glossy even if the front is (plastic-laminated excepted), and frequently have a lot of white space. Here my safest suggestion is just to moisten with water and wring out a soft white rag or paper towel, and try starting with lightly wiping a small area, moving on to a bigger area and more pressure if nothing untoward occurs. Check the towel to see if anything is coming off and move to new clean spot on it if it has, or stop altogether if nothing is coming off the jacket. If there is grease or oil-based soiling on the back cover, you can try this with a dilute solution of mild soap, again wringing it out until moist before applying. Once this is done, go over the cleaned area with a totally dry towel. But again, with many back covers you are better off doing nothing at all! (All of the foregoing also applies to that small percentage of non-glossy front covers, too.) With an average "gloss" of front cover, you can also try the above method, and oftentimes that will be all you'll need to do. Test a small area if you can for colorfastness, though - some cheaper inks disolve very easily and must be left alone. For a good quality typical front cover with a shiny substrate and better colorfastness that has heavy soiling, mildew, grease pencil, etc., the best cleaning agent I have found is heavy clear mineral oil. Oil disolves and lifts grease and dirt, and "polishes" the cover at the same time. A very little goes a long way, so apply the towel to the bottle mouth and turn it upside-down to wet a small spot of the towel. With many quality covers you can wind up doing a rather vigorous rubbing without causing damage. Keep moving to new clean spots on the towel as you reapply the oil, and watch out for white backgrounds, which can be made yellower and darker. After the surface has been treated, again wipe the remaining oil off with a fresh dry towel. Hope this helps, but be careful, err on the side of caution, and don't come after me if you screw something up even worse than it was!
