This is from another post I've archived and used with great results on old paper with fog.
1) Cut a small piece (about 1 inch square) of paper from the box you are currently testing. Then in full room light place half of it in paper developer (like Dektol 1 + 2 dilution) in order to see how long it takes for maximum black (probably less than one minute). Note the time in seconds.
2) Cut another similar piece of paper. Keep the room light off. Place a small coin in the middle of the paper and firmly hold the coin down with your finger. With the coin held down, turn on the room lights for about 5 seconds. Turn off all lights and develop for the time needed in step one for maximum black.
3) After fixation, note the tone of the coin area: if pure white you have perfect paper in that box. If medium grey it is still usable but will need other treatment to make beautiful prints.
4) If medium grey: you need a two fold approach towards getting that pure white back. Add either potassium bromide or benzotriazole to the developer. (NOTA BENE: To make a stock solution (MY way) of potassium bromide add 12 ml volume of powder (about 18 G) to 100 ml of water. To make a stock solution (MY way) of benzotriazole add 1 G (sorry, volumetric measuring is not practical because this chemical is like feathers) to 100 ml of water). With either of these restraining solutions you add about 20 ml to each liter of developer working solution. Now, the addition of this will slow down the time it takes to achieve maximum black, so test again with the tiny piece of paper like in step two. Add carbonate to the developer if it is TOO slow.
5) Finally, if the coin area still shows some grey you have to make your print a bit darker and then use a reducer after fixation to take off that ugly veil of grey. Reducing solution, like Farmer's Reducer, but slightly different, is this: mix 1 G of potassium ferricyanide (or 1 ml by volume: with potassium ferricyanide the grams are the same as the volume) into 100 ml of water. That is your 'bleach'. Take one part bleach and add to one part 'paper strength' fixer. That is your reducing solution but, beware, that is quite strong. Depending upon how much grey you wish to remove you can dilute this solution up to FIVE TIMES! That means, a solution of 100 ml of reducer (ie, 50 ml of bleach plus 50 ml of paper fixer) can have up to 400 ml of water added to make a total of 500 ml of diluted reducer. Agitate frequently and watch the print like a hawk. Too much reduction means a lost print.
6) The combination of both the restrainer and reducer has meant that many a box of photo paper deemed lost is now found. It does take a bit of experience to 'dance around' with this procedure but, in the end, you will have obtained a king's ransom of paper for pennies.