# Shop primer vs. holding primer



## Lavoisier (Jun 5, 2017)

Hello everyone,

I come from a middle sized contractor and we construct storage tanks (crude oil, petroleum products, water etc.) among other things. We are about to start a new project that will include the construction of 4 new tanks. 

As usual, we start our painting works after the completion of mechanical works (assembly, welding etc.)on the tank. Depending on the time needed before we start the blasting/painting works we:

A: Do not paint the prefabricated steel sheets at all (for short storage periods)
B: Ask our steel supplier to blast and paint the steel sheets with a zinc silicate shop primer (for longer storage periods). 

In any of the above cases, we have to grit blast the entire surface, which is rather time consuming.

Now there is an idea to ask our supplier to blast and paint the steel with a epoxy holding primer (75-100 microns, 3-4 mils). Only area to be left out are the edges that need to be welded (2 inches wide). In theory this would save us a huge amount of time. However, there would be also a large amount damage during the mechanical construction phase. The question is whether the damage is small enough to make this kind of approach pay out?

Anyone with an experience with this matter?


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## CApainter (Jun 29, 2007)

Do you typically assemble bare steel materials in the field, then blast and coat?


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## Stretch67 (Oct 7, 2013)

Usually the damage during install winds up being so much that its a total repaint anyway.


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## Lavoisier (Jun 5, 2017)

CApainter said:


> Do you typically assemble bare steel materials in the field, then blast and coat?


Yes, that's how we usually do it


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## Lavoisier (Jun 5, 2017)

Stretch67 said:


> Usually the damage during install winds up being so much that its a total repaint anyway.


I also think that the damage will too large that we'll have to blast the surface anyway


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## CApainter (Jun 29, 2007)

A partner of mine that blasted and coated more tanks than I have, said they would receive sheets of steel where he blasted and primed with a shop primer, except for where it would be welded. Once the tank was assembled and welded, they would white metal blast the welds and whip blast the holding primer before applying a coating system.

The exteriors typically have a different coating system than the interiors.


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## kentdalimp (Aug 15, 2013)

Typically the way I see it done is as follows:

Steel Guys Blast and Shop Prime. They can/will allow Hold backs (2"-6") where welds will go, or they may just prime the whole thing and make the welders grind back the coating. It all depends on who wants what during coordination meetings. The reason for the Shop Prime is to keep the Metal from Rusting too badly in the field. 

Once the entire unit is assembled (If none of the work is taking place in the shop) The Coatings contractor will Spot Blast & Spot Prime. And begin Striping/Coating. Extensive Damage (Depending on how anal you want to be, this could be anything NOT a Hold back) or replacement pieces are Changes (+$$$).

Thing is to make sure you get to specify the Primer. If the Steel guy picks something like Tnemec and you want to use Ameron, well you are screwed. You need to coordinate that early on. Also, make sure the welder is responsible for grinding the welds, not the painter!

Repaints will typically be a Total Blast / Repaint. New Construction tends to be Shop Prime, Field Spot Blast, Field Spot Prime, Field Finish.

Is there a reason you think you would need to Re-blast/Re-prime the whole thing?


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## CApainter (Jun 29, 2007)

I'd like to add that epoxies used as holding primers may be subject to short recoat windows. Thirty days in many cases. These coatings need to be more aggressively prepped before applying the final coating system. Non chemical cure coatings typically have no recoat window. So a light brush blasting is all that is needed to create a mechanical bond with the final coating system.


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## Lavoisier (Jun 5, 2017)

kentdalimp said:


> Is there a reason you think you would need to Re-blast/Re-prime the whole thing?


The thing that there is going to be a lot of damage on that holding primer. There will be supports for lifting, temporally welded to the metal plates, a lot of "hammering" during the welding part and people working around the plates tend to scratch them a lot.


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## Lavoisier (Jun 5, 2017)

CApainter said:


> I'd like to add that epoxies used as holding primers may be subject to short recoat windows. Thirty days in many cases. These coatings need to be more aggressively prepped before applying the final coating system. Non chemical cure coatings typically have no recoat window. So a light brush blasting is all that is needed to create a mechanical bond with the final coating system.


We usually use epoxies manufactured by PPG and the over-coating intervals are around 2-3 months. if we pass that period we abrade them with sand paper witch is quite fast and works really well for the adhesion of the subsequent coat.


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## CApainter (Jun 29, 2007)

Lavoisier said:


> We usually use epoxies manufactured by PPG and the over-coating intervals are around 2-3 months. if we pass that period we abrade them with sand paper witch is quite fast and works really well for the adhesion of the subsequent coat.


Epoxies will have a tendency to resist abrasion better during installation. But, I have seen delamination failures with coating systems applied over epoxies that have exceeded their recoat window.

Abrasive whip blasting will always provide a better anchor than hand, or mechanical sanding. But it does require a lot more logistics to blast, let alone the compliance issues that prevent open blasting in many regions.


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