Painting of Heavy Duty Castors Fallshaw HOME
NOTE: These sheets are summaries of complex matters originally prepared as background material for the internal use of Fallshaw staff. In allowing others to access them without charge we assume no legal liability. If it is important to you, you should check the information for yourself.
Background
Corrosion can be prevented in three ways:

1. Barrier coatings
Most paints interpose a corrosion resistant barrier over the steel that prevents corrosion, so long as the barrier is not broken. Different paint types have different resistances to chemical or physical attack, and the paint type should be chosen to suit the actual conditions of use. Barrier paints (chosen for their ability to resist the outside environment) are often applied on top of a 'primer' whose main function is to adhere to the steel substrate. The efficacy of the barrier depends on the type of binder, the thickness of the coating and its adhesion.
2. Sacrificial coatings
These provide an electrochemically active metal (usually zinc) over the steel that corrodes in preference to the steel. They are positioned either by dipping in molten zinc (galvanising) or held in place with an organic or inorganic binder. These are Zinc Rich Paints (ZRP). Pure zinc (as in galvanising) is a well proven coating against the weather, but it is attacked by weak acids or strong alkalis (outside pH of 6 to 10). ZRP have both the sacrificial benefit of zinc dust (loaded at 90% by weight in order to ensure electrical continuity, and thus cathodic protection) and the barrier resistance of a suitable chosen binder.
3. Inhibiting coatings
These contain pigments that inhibit and passivate the steel surface against corrosion.

Painting of castors
Heavy duty castors are used outside, sometimes being left in the weather, and sometimes used in more aggressive industrial environments.
Standard Fallshaw practice is to paint the X and V series castors with a matt black etch primer of a modified epoxy type, specially formulated to give excellent adhesion to the steel substrate. This has a dry film thickness of 15 to 20 microns. This primer is sufficient for mild exposure conditions, both indoors and occasional outdoor use. The A,B and U series are bead blasted and polyurethane coated to 15 microns.
Good practice, if the exposure is to be severe, is to coat over the primer with a paint chosen according to the conditions of use.
Summary of properties of chemically curing paints:
  Family
Resistance to: Alkyd Epoxy Polyurethane Zinc Silicate Silicone
water fair good/excellent good good/excellent good
weather good good good/excellent good/excellent good
mechanical wear good good/excellent good/excellent good/excellent good
heat good
(80°C)		 good/excellent
(120°C)		 good/excellent
(120°C)		 excellent
(400°C)		 excellent
(250-550°C)
chemicals (acids & alkalis) poor good/excellent good poor (pH6-9 only) good
solvents fair good good/excellent excellent good
animal & vegetable oils fair excellent excellent good good

All these families will coat over the primer, except zinc silicate which needs to bond directly to steel (no intercoat).
The choice of the coating and its application is usually done by the customer using their knowledge gained in specifying the trolley coating. Customers recoat the castors at the same time as they are coating the trolley,or they follow this advice.

Upgrade Program (delivery from the Sunshine plant)
If the customer wants Fallshaw to 'upgrade' the coating, we offer two levels for factory delivered product in the X, V, A, B and U series.
Remember, the standard castor comes with a matt black etch primer coating 15 to 20 microns thick of modified epoxy coating, suitable for mild exposure conditions.
• Second primer upgrade applies a second coat of etch primer over the standard coat. To ensure full coverage, this second coat is coloured dark grey. It is again a modified epoxy with a thickness of 15 to 20 microns. Thus the total coating thickness is 30 to 40 microns, and is suitable for average exposure out-of-doors, or for medium corrosive environments. Customers may still choose to paint over the primer to achieve a colour match, but it is not necessary from a medium protection viewpoint.
Minimum quantity: 8 castors
Price addition: $10 ea list
• Cold galvanising upgrade is to respray the forks and wheel sides with a light grey (silver), zinc-rich primer of the organic epoxy ester type, containing 90% zinc by weight in the dried film. This adds 50 microns to the coating, and gives extremely good protection against atmospheric corrosion, water, alkalis and salts. It has fair acid resistance and good mechanical strength.
Minimum quantity: 12 castors
Price addition: $35 ea list
• Note on overcoating: Galvanising and zinc-rich paints do not accept top coats easily. This cold galvanising upgrade is compounded with the features of a primer, so it can be top-coated to allow colour matching if necessary. The coating should not be too thick, and epoxies or polyurethanes are preferred to alkyds for better protection. The top coating should occur immediately the castors are delivered from the factory. If this is not possible, zinc salts may form on the surface, and the castors would need to be prepared by mechanical cleaning before applying the top coat.
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