I am completely ignorant to blasting and paint profiles, by my choice I might add.  Now, however, it has become part of my job to see that our coatings are correct.
I need information on the following:
If I read a blast profile of 3.7, does that not mean that we would need DFT paint mils of around 4.0 for sufficient protection?

The only specs I have are a SP-6 prep with 2-3 mils of Carbozinc.  I understand that your blast profile cannot exceed your primer and/or cannot exceed 1/3 of your total DFT.  Is that correct?

Thanks!

Firstly, study the manufacturer's datasheet for the recommended surface profile range. Secondly, you may wish to ascertain the measurement basis of the nominal dry film thickness of '2 - 3 mils.'  Generally, it is understood to be the film thickness factored for the profile, but this has to be clearly stated in the DFT measurement specification.  ISO 19840 gives guidance on the subject.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04
 

Thanks SJ.  I will try to get my hands on that ISO.

The surface cleanliness grades are summarised bellow:
Grade of Surface Cleanliness ISO 8501-1 SSPC
White metal Sa 3 SP-5
Near-white metal Sa 2½ SP-10
Sweep blast cleaning - SP-7
Solvent cleaning - SP-1
Power tool cleaning St 3 SP-3
Power tool cleaning to bare metal - SP-11
Water jetting (ISO 8501-4) Wa 2½ SP-12
Wet abrasive blasting - VIS 5

Dry film thickness is measured starting at the peaks of the profile. The instrument is zeroed out on the blasted surface. Paint in the profile itself is not considered part of the DFT.

However - the deeper the profile, the higher "rogue peaks" you will have. These are scattered "end of the bell curve" peaks which can lead to pinpoint rusting, even with 2-3 mils of coating over top of the nominal peak height.

I don't like to have a blast profile deeper than my DFT due to the rogue peak concern.

The blast profile can be changed by changing the blast media or the amount of pressure at the blast nozzle. Also, the blast nozzle orifice can be a slight factor, as well. An excessively deep profile can lower DFT's.

If the profile is too deep and the inspector/engineer wants to prevent painting, a good compromise is usually that if the profile is 1.0 mils - 1.5 mils deeper than specified or that is called for in the primer's product data sheets, the contractor shall add an extra 1.0 mils - 1.5 mils of primer.

Is 3.7 mils the average of a few profiles? Are you taking anchor profiles with testex tape and a micrometer?

3.7 mils will work fine with Carbozinc 859. I do not represent Carboline, but I'm sure your rep will send you a letter to that effect if requested.

What type of DFT Gauge are you using? Type 1 or Type 2?

NACE Certified Level 3 Coating Inspector
8 Years of Painting Inspection Experience