thePetroStreet

We have a case at our site. We are repainting entire structure of our Urea bulk stores as the deposits have increased on the structure & are a threat to its integrity.
Is there any standard which governs the DFT of applied paint to be maintained on structures in corrosive dusty environment. The type of paint we are using is Epoxy based.

A viable approach for a corrosive environment would be:

Surface Preparation:

1. Structure cleaning: SSPC-SP10 - Removal of all oil and grease using solvent and then drying it out with compressed dry air
Abrasive blasting as per ISO 8501-1 SA 2.5

2. Roughness 50-60 µm as per ISO 8503-01 or as per ASTM D4417 (segment “1” grade “Fine G”);

3. Abrasive shall be as per ISO 11124 parts 1-4 - usually use of silica-based sand is discouraged but if the silica content is crystalline, it is ok

Painting:

The three coats would usually be

1. Primer: Solvent-based inorganic zinc (or Exoxy zinc if the inorganic one is not available) - usually ranging from ~50µm depending upon your requirements/recommendations

2. Mid-coat: Epoxy-polyamide coat 100 µm - this is should at least be 100µm or more

3. Top-coat: Aliphatic Polyurethane coat ~ 50µm. If you go about doing this in the field using a roller or a brush, make sure that you double your coat thickness.

This should give you a total DFT of 200µm. However, here you must ensure that the maximum DFT must not exceed manufacturer's recommendation.

Why I gave you these details (based on good work practices) is because as far as I know, there is no specific standard which specifies the DFT of a particular coat of paint. It is usually based on a middle-ground between the Paint Manufacturer's instructions and the designer.

Further to my earlier post, I just checked my sources once more and would advise you to stick to the paint vendor's technical data in case of any ambiguity.

Some guidelines might help:

In a corrosive environment, heavier film thickness may help as it would prevent moisture from penetrating the coating and ultimately prevent it from reaching the metal surface.

However, also note that thick coatings (values higher than the upper DFT limit stated by the manufacturer) can be damaging if your coating is solvent based (which many epoxy coatings are). When you use an solvent-based epoxy you bank upon a thickness which allows the solvent to evaporate with time. A thicker coat would not let that happen which would lead to bubbles/blisters and ultimately peel off of the paint.

Solid-based coatings don't pose that risk.

Thanx Kamran for such a detailed reply & helping me out.
Regards
NAQ