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Blast profile
2

Blast profile

Blast profile

(OP)
I have a specification that calls for a surface preparation of 1.5-2.0 mils.
It was blasted and measured with Press-o-film testex and measures 4.0 mils.
Is more mils better for adhesion? Is this acceptable? What should I know?

RE: Blast profile

first... check the limitations on your testex film. if you are using "course" film that reading of 4 could actually be more but is not registering because you run out of the white stuff to inject into the crevices.

The product rep for the coating is the best for this question. 3 to 4 mils is pretty typical result from blasting for recoat jobs on bridges and water tanks using epoxies and zinc-primers. Apart from getting better adhesion, this high profile makes it harder for an evenly applied coat to cover all the peaks.

RE: Blast profile

(OP)
We were using x-coarse press-o-film so I believe the values are accurate. Up to 4.5. We got an average of 4.1 mils and we took 18 measurements.
It will be coated with Sherwin Williams Zinc Clad II primer.

RE: Blast profile

2
On recoat jobs where someone else has previously blasted, you are pretty well stuck with getting the same (or deeper) profile. It's almost impossible to reduce a profile once you have it.

On the flip side, some contractors just blast the hell out of the surface and achieve too deep of a profile all on their own.

To determine where the deeper profile is coming from, you would need to clean a small area in a way that will not deepen the profile. Three methods come to mind:

1) Blast with a very fine abrasive (maybe 200 mesh or less) and determine DFT on that area.

2) UHP water blast determine DFT.

3) Chemically strip and determine DFT.

Deeper profiles (and sharper profiles) will generally give better adhesion. HOWEVER (and it's a big however) - you have a much higher risk of pinpoint rusting. Blasting almost always results in scattered "rogue peaks" which are much taller (think 2x taller, maybe more) than the typical measurement with Testex tape. Typical solution is to require a thicker primer, with a letter of approval from the owner/specifier and from the paint manufacturer.

Epoxy zinc coatings are generally pretty forgiving of extra thickness. IOZ are decidedly NOT forgiving of over thickness. Zinc Clad II is an IOZ. Over thickness in IOZ generally leads to curing issues and mud cracking.

The data sheet (linked below) has a recommended dry film thickness (DFT) of 2-4 mils (50-100 microns), and a specific warning not to exceed 6.0 mils (150 microns).

http://www.paintdocs.com/docs/webPDF.jsp?SITEID=ST...

RE: Blast profile

I have a Sherwin Williams letter from one of their NACE3 product reps that should be similar for your condition.... i copy/pasted the text below and removed the reps name and project since this is the internet after all.

"Per our conversation I understand the surface profile on the XXXXXXX County Bridge has averaged
approximately 4.1 mils with a high of 4.3.

Sherwin Williams states a blast profile recommendation on the data sheet at 2 mils. While this is optimal, I
am comfortable with the application of the Zinc Clad II Plus Inorganic Zinc over the higher profile. The
key to the application will be to limit the total dry film thickness of the coating to 6 mils dry. Our primary
concern if applied over this film thickness is possible mud cracking and/or shearing of the inorganic zinc.

Please be advised that cure schedules will also be affected slightly with higher film thicknesses."

RE: Blast profile

The deeper profile can affect the surface of the coating and the little peaks near the surface can form a 'stress point' for the coating surface. Best to minimise rogue peaks. Is this a primer? and do you have a top or intermediate coat over?

Dik

RE: Blast profile

Contractors love that solution "we'll make it up on the next coat!"

Nevermind that the next coat is cheaper per mil, easier to apply and won't offer the same corrosion protection that a proper primer will.

The primer is your primary corrosion protection.

RE: Blast profile

Tom... funny, I've heard the same line up here... must be standard coursework for Contractor 101 classes...

Dik

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