[TheForge] Re: Permalac steps

[Mike Spencer]

Cindy and James <jallcorn at suddenlink.net> wrote:

> I tried painting a simple little hand railing last year... with a
> brush using a Sherwin Williams Industrial enamel.  (I too have
> painted w/ a brush for 28 years).  It looked like crap and would not
> dry.  So after a week's lost time, I stripped it, applied phosphoric
> acid, neutralized the acid, rinsed and then used a hand held pot
> sprayer.  Wonderful smooth finish.

Some points that invite clarification here.

   + Treatment/pickle with phosphoric acid doesn't produce "rust". (I
     know you didn't explicitly say that it did.) That is, it doesn't
     produce a coating of some form of hydrated iron oxide.  It
     produces a coating of closely adhering iron phosphate.

     Rust is hydrated -- contains closely bonded water even when
     apparently very dry -- and, even under a watertight finish,
     propagates further rusting.  A phosphate finish, successfully
     acheived, does not.

   + Phosphoric acid doesn't need to be neutralized on iron.  You can
     just rinse it off with water.  If there are no crevices to trap
     acid (where it can evaporate to leave pockets of unsightly white
     gunge), you can just wipe it down.  Any traces of phosphoric acid
     remaining on the surface do not promote rust but promote
     phosphating.  The closely adhering phosphate "patina" helps to
     resist rusting.

Googling "parkerizing" or "phosphatizing" should turn up a lot of
info, mostly about proprietary "treatments".

    Phophatizing (phosphating or phosphate conversion coating):

    A number of proprietary treatments such as 'Parkerizing' or
    'Bonderizing' are available for use on steel. They are applied by
    brushing, spraying or prolonged immersion in an acid
    orthophosphate solution containing iron, zinc or manganese. For
    example a solution might contain Zn(H2PO4)2.2H2O with added
    H3PO4. The coatings consist of a thick porous layer of fine
    phosphate crystals, tightly bonded to the steel. When forming
    steel sheet, the parts are often phosphatized in order to improve
    the surface properties of the sheet. The acid recirculation stream
    from the phosphatizing bath must be cleaned after contact with the
    metal.

    The coatings do not provide significant corrosion resistance when
    used alone, but they provide an excellent base for oils, waxes or
    paints, and they help to prevent the spreading of rust under
    layers of paint.  Phosphating should not be applied to nitrided or
    finish?machined steel, and steel parts containing aluminum,
    magnesium or zinc are subject to pitting in the bath. Some
    restrictions apply also to heat treated stainless and
    high-strength steels.

But you can get good results fiddling around with straight industrial
chemicals instead of branded, black-box products.


FWIW,
- Mike

-- 
Michael Spencer                  Nova Scotia, Canada       .~. 
                                                           /V\ 
mspencer at tallships.ca                                     /( )\
http://home.tallships.ca/mspencer/                        ^^-^^