Hi Mustafa,
Dilute your Dektol about 1 + 5. Then:
To deal with VERY age-fogged paper, remember that the paper does not see any difference between age-fog and emergent shadow detail. They are one and the same thing. Thus, it becomes mandatory to expose such paper well above the lower portion of the characteristic curve so that any image detail will be ABOVE such fog threshhold. With such badly fogged papers you still have a chance, but it does take work and preseverance.
Two things must be done: 1) during development I use BOTH restrainer (BZ or PB) plus baking soda (sodium bicarbonate, try 10g per liter, more or less if necessary) in order to greatly slow the development process down. The need for this is to HOLD BACK that damn fog. 2) Then, still there will be some fog, so after fixation, you will immerse the print into a dilute Farmer's Reducer.
One of the essential things to remember if you are dealing with VERY fogged paper is this: your enlarger exposure can get to even five stops more than with fresh paper. You have to 'burn in' that image and, as result, might have problems with highlight separation (after all, how much of that characteristic curve can you 'steal'!?!?!?) It is frustrating but amazing results can be obtained with patience, perseverance, and fortitude (and, time!) Coin tests, throughout, will determine the real fog level. And, forget about using a high contrast filter with fogged VC paper. You will not achieve anything better, as the paper has lost that ability. With fogged paper, severly fogged, you are after every ounce of contrast you can get ahold of. And that is where the dilute Farmer's comes in handy: it slightly increases contrast in paper (but NOT in film). - David Lyga