Sweep blasting before galvanizing?

A. Someone has given you poor information.
1. You don't need to shotblast if you intend to galvanize the steel unless it has previously been painted. If it's new steel, it needs no blasting to galvanize it.
2. Galvanizing is a corrosion protection. If protection is what you need, then you don't need to paint it. If you need a special colour, or a highly aesthetic finish then you could paint.
3. There is no such thing as "sweet" blasting. There is "sweep" blasting. (could be a typo?). This means a very light blast by "sweeping" the nozzle over the work. It means a very light blast. Its a poor pretreatment method for preparing galvanized steel for painting. It's expensive, messy and ineffective.
4. Painting. You can paint steel that has been blasted but not galvanized. In this case, shotblast, prime, paint. If the steel has been galvanized, then you need a pretreatment or the paint will not adhere properly. There are a number of proprietary brane chemical pretreatment systems available. You apply (spray or brush apply) the pretreatment, then after drying you paint.

Good luck, hope you get a "sweet" job.

A. Hi Mahesh. I am not seeing a specification for "sweep blasting" by any standards body, so it remains mere slang rather than detailed instruction. To hot dip galvanizers it is a pre-paint process and involves lightly blasting with a soft media, at an angle, and at low pressure --

The Galvanizers Association of Australia says: "Blast pressure 40 p.s.i maximum, Abrasive Grade 0.2 - 0.5mm (clean ilmenite), Angle of blasting to surface 45 angle, Distance from surface 300-400 mm, Nozzle type min 10mm of venturi type. These controls will ensure that the severity of blasting does not damage the galvanized surface and should remove only 10 µm of surface zinc." (see www.azobuild.com/article.aspx?ArticleID=7982).

The American Galvanizers Association says: "Some materials that have been successfully used to sweep blast galvanized steel include aluminum/magnesium silicate, soft mineral sands with a Mohs hardness rating of five or less, organic media such as corn cobs or walnut shell media [on eBay or Amazon] , or stone materials such as corundum and limestone. Particle size for abrasives should be between 200 and 500 micrometers, Blast angle should be between 30 and 60 degrees relative to the steel surface, Blast pressure should be 40 psi or less. Finally, the substrate should be maintained at least three degrees centigrade above the dew point." (see www.galvanizeit.org/education-and-resources/resources/technical-faq-dr-galv/sweep-blasting-galvanized-steel).

In your case the substrate is stainless steel, and we don't know why you are blasting it (preparation for painting?) -- but it seems reasonable to interpret "sweep blast" pretty much the same way the galvanizing associations do. Good luck.

Regards,

A. Hi Abbas. This thread explains sweep blasting in pretty good detail. But what are you blasting and why?

Regards,

A. Sweep blasting by means of sand or al-OX/ garnet is limited to Air-pressure blasting and generating huge amount of dust !

Nowadays Stainless steel shot is being applied :
SS Chrome steel with hardness 280 - 400 Vickers and or mixed with Stainless steel grit (one Size-grade finer) as Sweep or brush blasting media.
Major international HDG'ers have approved such media since meeting spec for Brush / Sweep blasting/

Such media can be applied in Air Pressure as well as in Centrifugal wheel shot blasting equipment.

New ways are right now also in practice tests: the use of Zinc/Al-Alloy shot or respective Zn/Al-Alloy cut wire:
Hardness of such media is > 70 HV as like usual zinc cut wire but less than 120 HV in use.

Since hardness of these media is significant higher than hardness of HDG layer, the brush / sweep blasting can be realized to way lower cost and by means of more efficient centrifugal wheel shot blasting machines !