Selected Answers

From Karen Fischer of Amstar of WNY on February 8, 2012:

     Adding to Ishita's com ...read more

     Adding to Ishita's comments... assuming we are discussing a steel substrate being prepared with a recycled abrasive....it is very important to insure that the substrate (steel) surface be free of potential contaminants BEFORE abrasive blast surface preparation begins. Oil, grease, bird droppings and various salts (chloride, sulfide etc) should be removed from the surface by a variety and/or combination of methods (power washing, steam cleaning, solvent wipe etc.) prior to abrasive blasting to reduce the levels of contaminants that will be taken up by the abrasive.

     Preventing the initial contamination of the blast media is just as important as routine checks throughout the surface preparation process. Removal of these contaminants by recycling equipment is not very efficient in most cases. Preparing specifications to address the removal of contaminants prior to abrasive blast cleaning will help to address this problem and will reduce the incidence of the contaminants being forced into the substrate (reducing the probability of coating failure) as well as reducing (or slowing down) the contamination of the recyled blast media (saving the contractor money by reducing the frequency of adding and/or replacing the grit due to contamination.)

From Phil Zammit of Brooklyn Iron Works Inc on February 6, 2012:
Depending on what you're loking for, there's dirt, ...read more Depending on what you're loking for, there's dirt, salts and various other abrasive tests. But the most adverse contaminant in an abrasive is oil. You cannot beat the solvent wash test and then pour the virtually evaporated solvent on a mirror. Look for oil streaks when dry and smeared.

From ishita Bhattacharya of Berger Paints India Ltd on February 7, 2012:

A proper surface preparation is very important ...read more

A proper surface preparation is very important for the coating system to perform. But along with surface preparation, abrasive cleanliness must also given proper attention so that the unclean abrasives do not recontaminate the surface, which will lead to the failure in the coating system. 

The most dangerous contaminants of abrasive can be listed as water, oil, grease and chloride- and sulphate-containing salts. Any one of these contaminants, if transferred to the steel surface, can prove to be detrimental to the coating system.

Thus, proper checking is important. An oil and grease check is very easy. Take a glass jar and place some abrasives into it, add water, put the lid on and agitate the glass jar well. If the oil and water separate, then it implies that the abrasive is contaminated. A dust check can also be done by this method. If the water appears cloudy or hazy, then it can easily be concluded that the abrasive is contaminated and should not be recommended for future use. Oil and grease check is very important for each blasting-recycling step because  it is quite possible that the abrasive can pick up oil from the surface of the steel during the blasting process, which can later recontaminate the steel surface.

The abrasive should be dry, since moist abrasive can cause pin-point rusting on steel. Dry abrasive check is determined visually. A salt contamination test of the abrasive is generally done in the lab. Chemical contamination is checked by litmus paper. Abrasives are generally neutral, 6-8 pH , but if the litmus test comes positive, then it can be summarized to some extent that it might be chemically contaminated. ASTM-D 4940 is the standard test for determination of salt contamination.

Another point that should be kept in mind is that abrasives constantly break down on successive use. The finer abrasives which are too small to be useful for cleaning must be rejected in order to obtain better surface profile.  If such small points about abrasive cleanliness can be kept in mind, a much better surface can be obtained for paint application and, along with it, we get a better coating life.