As with all protective treatments of steelwork, it is of great importance that preparation of the galvanized surface is carried out in a thorough and considered manner. In particular, failure to degrease the galvanized steel surface properly is the most common source of failure of duplex coatings.
As with many other substrates, organic coatings cannot usually be applied directly onto galvanized steel. However, simple direct application paint systems, designed specially to adhere to non-ferrous metals such as zinc, are becoming more available in a full range of colours. The reasons for the need for effective surface preparation in most cases are quite straight forward. When the steel is withdrawn from the galvanizing bath it has a clean, bright, shiny surface. With time this changes to a dull grey patina as the surface zinc reacts with oxygen, water and carbon dioxide in the atmosphere to form a complex but tough, stable, protective layer which is tightly adherent to the zinc. The patina takes time to develop and the exact time depends upon the climate around the galvanizing. Typically, the time can vary from six months to two years or more. During this transition of the zinc outer layer into its final state simple oxides and carbonates form, which do not adhere strongly to the surface. As most duplex coatings are applied whilst the galvanizing is in this condition the surface layer must be modified by chemical or mechanical means. Coatings may be applied directly to the initial pure zinc surface or to the weathered surface but the results are not always consistent and ‘taking a chance’ is not recommended.
Where the aesthetic requirements for a duplex system are particularly high, a degree of surface finishing or fettling may be required on galvanized coatings as small surface projections may become more obvious after the application of an organic coating. This is particularly the case for powder coating systems. Care must be taken when smoothing a galvanized coating because the zinc coating may be damaged by heavy or excessive grinding.
Kama ilivyo na matibabu yote ya kinga ya chuma, ni muhimu sana kwamba maandalizi ya uso galvanized ni kufanyika katika njia ya kina na kufikiriwa. Hasa, kushindwa degreasing galvanized chuma uso vizuri ni chanzo cha kawaida ya kushindwa ya mipako duplex. Kama ilivyo na substrates nyingine nyingi, mipako ya kikaboni kwa kawaida haiwezi kutumika moja kwa moja juu ya chuma galvanized. Hata hivyo, mifumo rahisi ya rangi ya matumizi ya moja kwa moja, iliyoundwa hasa kushikamana na metali zisizo za rangi kama vile zinki, inakuwa inapatikana zaidi katika aina kamili ya rangi. Sababu za uhitaji wa maandalizi ya ufanisi ya uso katika kesi nyingi ni moja kwa moja mbele. Wakati chuma ni kuondolewa kutoka kuoga galvanizing ina safi, mkali, glossy uso. Baada ya muda, rangi ya zinc inabadilika kuwa rangi ya kijivu, na zinki ya uso huingiliana na oksijeni, maji na kaboni dioksidi katika anga na kuunda safu ngumu lakini ngumu, thabiti, ya kinga ambayo huambatana sana na zinki. Patina inachukua muda wa kuendeleza na wakati sahihi inategemea hali ya hewa karibu na galvanizing. Kwa kawaida, muda huo waweza kutofautiana kutoka miezi sita hadi miaka miwili au zaidi. Wakati wa mabadiliko haya ya tabaka la nje la zinki katika hali yake ya mwisho oksidi rahisi na kaboni hufanyizwa, ambazo hazishikamani sana na uso. Kama wengi duplex coatings ni kutumika wakati galvanizing ni katika hali hii safu ya uso lazima kubadilishwa kwa njia ya kemikali au mitambo. Coatings inaweza kutumika moja kwa moja kwa uso wa awali safi zinki au kwa uso weathered lakini matokeo si daima thabiti na "kuchukua nafasi" si ilipendekezwa. Mahali ambapo mahitaji ya aesthetic kwa mfumo duplex ni hasa juu, kiwango cha uso kumaliza au fettling inaweza kuhitajika juu ya vitambaa galvanized kama protrusions ndogo uso inaweza kuwa wazi zaidi baada ya matumizi ya vitambaa kikaboni. Hii ni hasa kesi ya mifumo ya powder coating. Uwe mwangalifu wakati wa kusafisha mipako ya galvanized kwa sababu mipako ya zinki inaweza kuharibiwa na kusaga nzito au kupita kiasi.