When spray chroming items that are subjected to environmental thermal shock and harsh UV radiation special procedures are to be followed.
If you wanted to spray chrome for example a 1955 Buick front bumper bar, here is a typical list of procedures to follow:
1 >First start with a bumper bar that is fully primed – sanded and sprayed with 2 X coats of high gloss 2K urethane, then wait until fully cured.
Clean off the chemical residue that forms on the surface of the 2K urethane basecoat with alcohol – acetone or similar.
Or better still you can use a high performance 1 part polyurethane that can be brushed – rolled on or ideally sprayed, for example POR – 15 or Rust Bullet brands.
Sensitizes can also be added to the base coat pre-application.
2 > A hot solution of acid or alkaline based roughing solution is evenly poured over the bumper.
The roughing solution should be concentrated enough to etch the urethane surface.
The etching solution should be suitably formulated to open up the pores on the surface of the urethane, and most importantly increasing the surface area.
3 > Next a blue flame is methodically passed over the entire bumper bar energizing the surface to be silvered, and making it more chemically receptive to the metal ions that are part of the sensitizing solutions.
The tiny particles of the following sensitizing and super sensitizing solutions will have more surface area to attract the silver crystals.
4 > A neutralizing PH correct wetting agent is also sprayed on under pressure to the surface before the sensitizing and super sensitizing procedure.
A thorough rinsing with de ionized or distilled water is most important after each procedure.
5 >After the super sensitizing procedure has been completed and fully rinsed, a light coating of silver is applied by spraying with a two component spray gun.
6 > Next a 2 x part complex copper sulphate solution using a 2 x component spray gun is applied on top of the lightly silvered substrate.
Copper is useful as a sacrificial metal
7 > A coating of nickel sulphate combined with other chemicals using a similar 2 x component procedure as the copper, is now sprayed and rinsed with the ionized or distilled water.
8 >> A final coating of silver is sprayed on to the nickel coating.
An experienced silverer will know how much silver is to be sprayed on, if you try to apply too much silver at one time, the reducing agents will stop reacting and the caustic and ammonia components of the silver solution will start to burn the outer silver layer.
White clouds will appear when too much silver is applied, also if too much silver is sprayed on to one area of substrate roundish brown stains will appear.
A good dousing of hot de ionized water can usually resolve the problem.
Alternatively not applying enough silver will result in unsightly blackish clouds appearing on the substrate.
These may only be noticed later under certain light conditions.
9 >Next a powerful silver passivator solution containing special light altering metallic ions is sprayed on to the freshly silvered substrate.
When you apply a thin coat of silver to a clear urethane – acrylic or enamel etc the cured top coat will give off a yellow haze which of course is unwelcome when trying to replicate the look of hard chrome.
To counter the yellowing effect, careful chemical additions must be made to the sensitiser – silver and reducer solutions the passivator and urethane top coat.
FORMULATIONS FOR THE ABOVE WILL BE DISCUSSED AT A LATER DATE.
For the moment, a violet dye is used by most spray on chrome chemical suppliers, with sometimes less that excellent results.
10 >The bumper bar is now air blown and flame dried by propane fire or similar, before applying an automotive urethane grade top coat.
At this stage, a violet dye can be mixed with the urethane top coat.
Other paint protection products can be applied on the urethane coating, for example silicon dioxide and titanium dioxide.
The roughing of the urethane prior to sensitizing and silvering give the small silver particles a chance to fill up the cavities and give the silver layer a mechanical platform to gain a better adherence to the urethane base coat.
The problems associated with spray chroming could fill a telephone book.
It would probably be true that 90% of people who rush into spray chroming too quickly without doing a lot of research will fail.
If you spend $200 on a small spray chroming kit, for example from ebay, and discover corrosion pits appearing on your bike helmet a week later, not much sleep is lost.
However if you throw in your day job at the plastics factory – spend $25k on an expensive chemical drinking mythological chroming machine, at best you will probably get that insect caught in a venus fly trap experience, or at worst, you will be wandering around in a daze looking for a rope and stool.
DUAL NOZZLE SILVERING SPRAY GUNS
There are many brands of 2 x component silvering spray hand guns on the market, prices vary from about $85.00 to $1500.00, an all plastic spray on chrome gun is now available on the market, but I have yet to test it.
Perhaps a user of this gun can email Guy Molloy sales@decomirrors.com and let us know of their experiences.
Another advantage, mainly to the chemical suppliers of the 3 nozzle system, is the spray chrome chemical formulas will only be suitable for their expensive unnecessary spray on chrome machine.
SHORT LIST OF PROBLEMS WITH 2 X OR 3 X HEAD SILVERING GUNS
1 > No controls for fluid balancing, useless for silvering! or has balancing controls, but needs an alum key for adjustment.
2> Cheap brightly coloured aluminium nozzle covers combined with brass needle outlet holes, will corrupt the silver mix giving it a cloudy appearance.
MORE TO FOLLOW .....
|
