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21 comments
Associate

Join Date: Jul 2009
Posts: 33

time between sand blast and paint

08/29/2009 3:50 AM

dear ,

can you please help me to know the minimum time (hours) between sand blast and start paintting fro tanks plates and steel structure and i think it is not same in winter and summer

and can you told accoring to wich standard and is there afree copy fom it

regards

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#1

Re: time between sand blast and paint

08/29/2009 6:12 AM

As far as I am aware there is no minimum time, it is the maximum permitted time.

After the surface preparation, the surface is devoid of all protective coatings (even oxides and scales act to protect the surface to some extent).

The primer should be applied within a limited time (depending on the reactivity of the material) of the surface preparation. For steels, it is usually 4 Hrs but it again depends on the ambient. In tropical, rainy seasons, if the humidity and temperature are high, it may be quicker. It is all depending on parameters like Relative Humidity, dew point etc that are measured regularly for proper painting in addition to the job skin temperature.

These all depends on

- Paint manufacturer recommendation.

- The experts on your area (If the painting Engineering deptt exist)

- Process qualification.

For the intermediate and finish paints, some times the gap between the primer and the other coats exist, but it is rarely in hours. More important is the keep the surface clean of all foreing matters.

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Associate

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#3
In reply to #1

Re: time between sand blast and paint

08/29/2009 7:27 AM

thanks for your answer ,

but for the data below cn you tell me the exact time and can you help me to know according to wich standard we can depend for this

1-i have steel structure and i need to paint it with fire protection paint

2-the weather dtda is

Feels Like: 94° Barometer: 29.77 in and falling Humidity: 11% Visibility: 6.21 mi Dewpoint: 36° Wind: NW 10 mph Sunrise: 5:42 am Sunset: 6:48 pm

regards

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Guru

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#5
In reply to #3

Re: time between sand blast and paint

08/29/2009 8:19 AM

As TVP told wait for the dust to settle down and then you can immediately paint.

But are you doing sand blasting or Grit blasting. We have banned sand blasting at our works as well as the vendor works due to the HSE requirements.

The additional advantages you get is the large grits do not need the settling time.

The RH value and the temperature seems good enough and I suppose you may go somewhere to the extremes ie 4 Hrs max (As per our Process Qualification parameters for approx this parameters)

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#6
In reply to #5

Re: time between sand blast and paint

08/29/2009 8:35 AM

BTW: did you check up ISO 8502 ? it has several aspects especially on surface cleanliness parameters (including the checks)

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Guru

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#2

Re: time between sand blast and paint

08/29/2009 7:10 AM

I'd wait till the dust settles. In the summer, that might take a quarter hour, but in the winter it takes as much as 15 minutes.

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Associate

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#4
In reply to #2

Re: time between sand blast and paint

08/29/2009 7:42 AM

the problem not the the settles of the dust only ,

i can change the place of painting but the prblem is the corosion if i leave the sand blasted surfaces for along time there fore i need the minimum time for that and according to wich satndard i can depened

thanks

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Associate

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#7
In reply to #4

Re: time between sand blast and paint

08/29/2009 10:06 AM

thanks

but can you help me to download this standard

regards

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#9
In reply to #7

Re: time between sand blast and paint

08/29/2009 11:49 PM

You have to pay the money just like the rest of us

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Guru

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#8

Re: time between sand blast and paint

08/29/2009 11:59 AM

But, you asked for the minimum time. What you need in terms of corrosion is maximum time.

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Engineering Fields - Piping Design Engineering - New Member Egypt - Member - Member since 02/18/2007

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#10

Re: time between sand blast and paint

08/30/2009 12:06 AM

All posts are good, but I like to add that the surface preparation code is "Steel Structures Painting Council", SSPC. And I think that any painting contractor must have all tools in addition to a original copy of that standard "Pictorial Surface Preparation Standards for Painting Steel Surfaces".

Also, when you apply the primer, make sure that the temp. of steel is higher than the dew point temp. by about 2 oC to avoid formation of invisible film of moisture on the surface of steel under the paint which lead to formation of rust under the paint.

Please see CR4 Thread Painting for Hot Surface 90 Degree Celsius for some additional information.

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#11
In reply to #10

Re: time between sand blast and paint

08/30/2009 2:34 AM

The standard "Pictorial Surface Preparation Standards for Painting Steel Surfaces" issued by "Structural Steel Painting Council" SSPC, can reply your question about the max. time between the sand blast* and applying the primer.

This pictorial standards includes coloured pictures for the different types of steel surface preparation, and to judge of attaining the targeted degree of surface preparation you have to compare between the colour/brightness of picture of the standard and the colour/brightness of actual finished steel surface. And before you apply the primer, you have to attain the colour/brightness of steel surface to be exactly the same as the colour/brightness of picture.

And in some instances, you can let the sand blasted surfaces of steel without applying the primer for some few hours without any change in its colours/brightness when compared with the standard. And in another instances, may be letting the finished steel surfaces for few minutes can affect the colour/brightness of steel surface. So the change in surface colour/brightness depends on the surroundings atmosphere such as humidity, moisture and its corrosivity, temp., dew point temp., .. etc.

Conclusion. The max. time between the sand blasting and applying the primer can be experienced for working few days at the definite region/area, and this time can be changed with changing of region/area.

*Note. Sand blasting is now prohibited because of the cancer due to silica located at sands used, and the grit blast is now recommended by all codes and standards.

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#12
In reply to #11

Re: time between sand blast and paint

08/30/2009 2:56 AM

The following is the cover of SSPC_Guide_to_Visual_Standard.

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Associate

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#13
In reply to #12

Re: time between sand blast and paint

08/30/2009 5:43 AM

thanks for your help mr

my job is quality control ,

and i have stell structure sand blasted before 10 days the company try to paint it by prime coat and i stop them therfore i need the exact time betwenn the sand and prime coat an i do the sand blast according to sa 2 1/2 i have the picture of this type but the time is my prblem and my weather is below

Feels Like: 94° Barometer: 29.77 in and falling Humidity: 11% Visibility: 6.21 mi Dewpoint: 36° Wind: NW 10 mph Sunrise: 5:42 am Sunset: 6:48 pm

ca i paint or i have to do the sand blast again and according to which standard exactly

regards

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#15
In reply to #13

Re: time between sand blast and paint

08/30/2009 3:34 PM

Dear sarmadaoudi@yahoo.com,

The time is estimated and governed by the time just finishing sand blasting until comparing the steel surface with the picture of the standard Sa 2 1/2 and just before happening any change in colour of steel surface.

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#14

Re: time between sand blast and paint

08/30/2009 11:05 AM

I do not think there is any standard for how long to wait between sandblasting and painting. The sooner you do it the better, because due to humidity in the air, also in the compressed air used for sandblasting, the surface tends to rust after the sandblasting and it is better to cover it before this starts. In summer hopefully the humidity is lower the wait can be longer.

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Commentator

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Location: New Delhi, India
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#16
In reply to #14

Re: time between sand blast and paint

08/31/2009 12:50 AM

Hi..

No time limit is specified between blasting & painting. However, there is a unwritten procedural limit. As soon as the blast cleaning of the surface is completed and the quality level is achieved to e.g. Sa21/2, or white metal in IS1477 or whatever according to specified standard the first coat (primer) must be immediately applied.

In addition to the quality standards & quality assurance plans you must have a written sand/shot blasting procedure in place wherein all such conditions are clearly addressed. Moreover, i feel a blast cleaning level of Sa21/2 for structural steelwork is on the higher side and would increase your cost.

Anil Tiwari / New Delhi

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#17

Re: time between sand blast and paint

08/31/2009 7:35 AM

I don't remember ever seeing a maximum time set in any of the SSPC or NACE documents, but I've seen it referenced often in contract specifications for lots of painting projects - usually <24 hours.

Your dew point sounds pretty low per your post, but even if it was as low throughout the 10 days and nights, depending on your location you might also have to worry about surface cleanliness - e.g. is there salt spray or any other source of contamination (soot or other particulates)?

You could freshen the blast in a small area and see how different it looks - if it looks a lot lighter, then you'll have a pretty good idea that some oxidation has taken place.

Also, watch out for the coating manufacturer siding with the contractor - think about where their interests lie - they want the contractor to keep buying their paint, and besides the warantee period, they don't have any long term ties to your equipment. I've heard it a thousand times before where a white metal blast is specified and the inspector rejects a blast and the coating manufacturer comes along to say that near white is OK for the material - they don't have to own it and repaint it when it has failed in 15 years instead of 20.

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#19
In reply to #17

Re: time between sand blast and paint

08/31/2009 10:11 AM

"GA", so far ... and, if I might ... further-to the mention of "surface cleanliness":

The basis of this whole thread is the reason that there exists a profession of "Certified Coating Inspector".

The mentions of "SSPC" have been just slightly outdated (esp. the link to the 1982 version of a document!). The 'old' Steel Structures Painting Council changed it's name years back ... it retains the "SSPC" abbreviation, but the name is now "The Society for Protective Coatings" (<Link).

SSPC and NACE have joined forces and written numerous "Joint Standards".

In addition to the Blast Standards (which, themselves, address visual appearance only), a Coating Specification must also address issues such as the blast PROFILE, surface cleanliness (how many ppm of various contaminants, including oxides that divulge the occurrance of corrosion on the surface even before it can be seen visually) are allowed to be present, and numerous other factors.

The certified coating inspector's job, then, is (much like a certified weld inspector) to insure compliance with (or "enforcement of") the Specification.

The "blast hold" (duration that the 'integrity' of a particular blast finish can be expected to be held under prevailing conditions) is a subject of much controversy.

The various environmental factors that contribute are generally well known for any given region of the planet, and these should be understood and discussed during "pre-job" meetings between the owner, coating manufacturer rep, and contractor.

Are you documenting *everything* that needs to be documented...? Using profile tape (compressible foam tape strips) to do profile measurements every 100 sq ft; testing for the various contaminants (including those that might be present in the blast media and the air supply); relative humidity and surface temperature at commencement and thruout application of the coating...? Air temp should be 5° above surface temp and rising (not falling!) for most coatings.

If you are only addressing a few "hit-or-miss" factors, you will have little to work with (as mentioned before) when the coating fails prematurely ... some years down the road.

Here's hoping that doesn't happen to you. "Cheers" ~

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Associate

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#20
In reply to #19

Re: time between sand blast and paint

09/01/2009 4:09 AM

thanks mr ndt tom very much fro your good informations and the sites but can you help to find the sand blast and paint test report forum if you have any sample,

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#18

Re: time between sand blast and paint

08/31/2009 9:20 AM

ASTM E337 Appendix 1

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Anonymous Poster
#21

Re: time between sand blast and paint

11/10/2010 6:48 AM

Normally we may maintain the time gap upto 4 hours depending upon the climatic condition.if it is hot,yes 4 hrs ok,if the climate is more humidity,then again blast the surface for few minutes & clean with fresh air & go for painting immediately.

S.MURUGESAN

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