Hydroquinone is a component of almost all paper developers.  Are
you using hQ as the sole agent in combination with sulfite, bromide,
etc., or are you using hQ as the only chemical in the developer?

You don't give enough information to determine the cause of the
staining.

Selenium toner is often the cause of permanent staining.
 

If this is the case:

Developer stains are, in general, impossible to remove.

The most expedient solution may be to throw out the stained
prints and make replacements.

If don't want to do this then I would begin by making up a stack
of hQ stained prints to sacrifice in the name of experimentation.
In finding a way to consistently make stained prints you will find
the cause of the staining.

From a position of ignorance, mild reducing agent may work:
s. sulfite, vitamin C ...  A dash of carbonate may help to unstick
the oxidized hQ.  A re-fix in fresh fix is a good idea.

Potassium permanganate and nitric acid make up a universal stain
remover.  However, permanganate can leave permanent brown stains
on anything organic [paper and gelatin] and nitric acid if not
very dilute will eat away the gelatin and turn the paper into
highly flammable and unstable nitro-cellulose.  OTOH, it might be
worth a try ...

Don't use chlorine bleach, it will remove the emulsion.

   What is the formula for the developer?  Hydroquinone as
a sole developing agent is a very high contrast developer
when used at high pH, usually with hydroxide as the
accelerator, but very warm tone developers can be made using
carbonates.
   If there is not adequate sulfite in the developer stains
can be produced as the result of the reaction products of
Hydroquinone.
An example of a warm tone developer using Hydroquinone as
the sole developer is Ansco/Agfa 110

Agfa 110 Brown Black Paper Developer Stock Solution

Water (at 125F or 52C)                        750.0 ml
Hydroquinone                                   22.5 grams
Sodium Sulfite, dessicated                     57.0 grams
Sodium Carbonate, monohydrated                 75.0 grams
Potassium Bromide                               2.8 grams
Water to make                                   1.0 liter

For use dilute 1 part stock with 5 parts water. Expose
prints 3 to 4 times normal and develop 5 to 7 minutes at 68F
(20C).

  Although the above formula shows the Hydroquinone being
dissolved first its probably better to dissolve the sulfite
first. Hydroquinone will dissolve in Sulfite solutions,
unlike Metol.

  I am copying this formula as an example. It may be
practical but I've not tried it.
Agfa also had a warm tone developer using Hydroquinone and
Glycin (No.115).   This does not require the overexposure
and long development of the above. From the description it
is not as warm as 110, which is supposed to produce brown
images on warm tone paper, at least the warm tone paper of
the time. No.115 dates from the late 1930's some time.

Thank both of you for your answers.

Actually I forgot to put some more detail. The developer is self made,
the formula as follow:

NaOH: 7gr
Ascorbic acid: 1 gr
Hydroquinone: 2 gr
Tap water: 1 liter

it's woth to remember to any reader how is dangerous the NaOH - pay
attention to protect yourself and use cold water.

I added the ascorbic acid just to avoid a fast oxidation. I tried with
some sodium sulfite (5 gr per liter) but there is no more the lith
effect. It keeps for 1 hours more or less, I know too short, but
enough to develope some paper. With Agfa MCC it works very fine, with
a smooth infectious development. With Agfa MCP instead it's very fast,
very high contrast and difficult to control, not recomended. The
overall tone is just slightly warm.

The staining I had I think it's due to exausted developer. As I
understand, no way to remove it... Ok, not a big issue.

Regards

Elia Freddi
················································
"Sii tecnico spietato con il mezzo e poeta con la mente" - il ratto
"In ogni fotografia c'è sempre qualcosa di troppo, tranne quand'è riuscita" - Edouard Boubat

MypagE at http://efreddi.altervista.org/