RM Online - Be in control of the surface

When deciding how to remove surface contaminants or existing coatings with surface preparation equipment, it is important to keep one question in mind: What will the surface condition be afterward? This is often the last thing on the minds of equipment operators. But it should be the first.

Keep control of the scope of the work by relying on proven technology that will reduce the job to a few basic steps. Controlling the removal process can make the job of re-coating more manageable and predictable. It is this degree of control that makes or breaks a job's success.

So how can you take control of the surface?

One way is through a process known as abrasive shot blasting. A centrifugal wheel or drum accelerates steel shot or steel grit abrasive at high speeds and hurls it at the surface to be cleaned. When the abrasive hits the surface, it fractures the coating or rust deposit and causes it to break off in small particles. The contaminants and abrasive are contained and recovered in an enclosed continuous-loop process in which a separation device removes the recycled abrasive particles from the waste.

There are several ways to regulate the effectiveness of this process: 1. using different sizes of abrasives, for various cleaning speeds and surface profiles; 2. changing travel speeds to increase or decrease the amount of removal; 3. changing the size of the motor used to throw the abrasive.

The abrasive used can be steel shot or steel grit or, in some cases, both can be used together. Additional horsepower allows more abrasive to be thrown, for faster travel speeds and higher removal rates. You should match machine sizes to the job requirements and the desired results.

Control of the end result also includes being able to recover the removed contaminants and preserve a dust-free environment. High-efficiency dust collectors are used with filters that remove minute particles at 99.96 percent effectiveness, down to a particle size of .3 microns, and constantly clean the filters. At predetermined intervals, a back blast of compressed air is thrust through a portion of the filters, "back pulsing" off a layer of dust; dust collectors can be used continuously if you pulse only a portion of the filters.

There are only a few coatings that may exist on steel or concrete surfaces that shot blasting will have a tough time removing. If the surface coating is rubbery, the round steel shot will bounce off. Sometimes by mixing steel shot with steel grit, you can remove such coatings. But do take care to monitor the results on the steel or concrete surface below.

Different grades of cleaning and profiling of the underlying surface can be expected. The Society for Protective Coatings (SSPC) and the International Concrete Repair Institute (ICRI) can help you establish a comparative standard for the profiling of steel and concrete. These standards are used to define the predictable end results using repeatable techniques, so there's little discrepancy when trying to match what the customer expects of the coating.

Often the words "shot blasting" conjure up a vision of a machine that will eat itself alive, but that's true only if maintenance is poor. Inside each blast machine is a sacrificial layer of wear-resistant liners that take the brunt of the wear. The liners can be changed for less money than re-placing the outer housing.