Coat or not to coat before hydrotesting?

There is a long debate in projects about this.

Should I coat my tank before, or after hydrotesting?

Contractors usually want to coat before hydro, because disassembling and assembling scaffolds takes time.

The short answer is that you should not coat before hydro, because coatings have hole bridging capabilities.

This fact is mentioned in API 652.

However, there are times that you can coat before.


API 650 won´t state if coatings are to be added before or after the hydrotest. No mention at all.

Here you can use the concept of risk to make a decision.

A tank with crude or vegetable oil won´t pose as much of a risk as a tank with naphtha. But that depends, because the same crude oil tank has a higher risk level if it is by the riverside than the inland.

Standards give minimum requirements! You have to use risk driven analysis to make decisions. (BTW: API 580 is the master standard when dealing when an RBI Analysis)

If you think that coating before hydro leaves a high residual risk, don´t coat.


Now, when talking about pipes…

….there is a mysterious excerpt online that talks about how a researcher made some holes in a pipe and tested it at different pressures.

His results indicated that coatings can mask small holes in the pipes.

Several tests were conducted on a 4″ Schedule 40 carbon steel pipe that was internally pressurized with specific hold times at 1100 (30 min), 2000 (10 min) and 2400 psig (5 min). The pressure was increased in 100 psig increments. The various coating systems that were evaluated consisted of;

– vinyl finish over inorganic zinc primer
– vinyl finish over vinyl primer
– epoxy finish or epoxy primer

The pipe section was pre-dilled with twelve, small diameter thru holes (0.0135″ and 0.020″) to simulate weld or fabrication defects.

The results of the pressure tests revealed some interesting results;

– in a test pressure range of 325 psi to 1125 psi, the vinyl coating systems failed. At the onset of failure, the vinyl coating locally disbonded into a blister that ruptured.

– the epoxy system adhered and did not disbond even at 2425 psig pressure.

So, you may want to think twice during fabrication of pressure retaining items to coat after hydrostatic testing.”

What the standards say?

ASME B31.3  PROCESS PIPING says this:

B31.3 allows joints to be painted.

With an increase in risk, comes and increase of testing and precautions. ASME B31.1  POWER PIPING says this:

In pipes under B31.1, joints need to be exposed.

Here again we can see the concept of risk behind the reasoning of the members of the ASME and API committees.

A pipe that carries crude oil has less explosion risk than one with naphtha.


Unless your contract explicitly says so, you will have to decide if you want to paint before or after hydrotest. Both have advantages and disadvantages.

But if your tank is a small water tank, I guess you can paint before hydrotest. If your tank is a large naphtha tank, you better don´t coat.


Depending on the product contained, you may want to coat the higher courses and leave the bottom and lower courses uncoated. That way, you reduce the amount of scaffold to be assembled.



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