Media blast cleaning metal, fiberglass or aluminum surfaces is often used to remove paint, rust removal or corrosion and to prepare the surfaces for a new protective coating.
Media Blasting can be virtually any type of pneumatically accelerated granular or powder material. Silica sands are rarely used anymore because they are not predictable in granular shape, hardness and can be expensive to remove salts that may be present. Additionally because of high silica that may cause respiratory hazards to unprotected workers or workers downwind if it is an open environment, this type of abrasive is not very popular anymore. Each type of media has benefits; sometimes it is a low price point per pound, sometimes it is the cleanliness of the raw virgin materials. No one blast media works for all applications, and many times the wrong media can create more problems than intended for a novice or untrained operator.
Many types of softer metals are blast prepared including, zinc, aluminum, copper, brass, bronze. Soft metals are often stripped using soft abrasive media such as plastic, walnut shell, soda blast. Soft abrasives do not however remove stubborn coatings or rust and corrosion without significant additional effort if at all.
Hard abrasive media may include manufactured materials such as aluminum oxide, silicone oxide, minded garnet, crushed glass, coal slag, silica sands, steel grit, stainless steel grit and many others. Hard abrasives are most often selected for rapid cleaning and cleaning to produce a specific profile and used on steel surfaces. Profile is used to insure protective coatings, thermal sprayed metals, epoxies or powder coated finishes adhere according to manufacturer’s specifications. Typically the higher the profile the better the adhesion.
Associations such as NACE or SSPC have identified ranges of grit size and weight to achieve predictable profiles and levels of cleanliness and guideline production rates. These guidelines use clean mild steel as the baseline for surface preparation.
VA Welding Expert at Asco American Stripping in Manassas Park, VA
While the media selected for a particular project is important so are other factors. Air pressure, volume of air and clean dry air to propel media. Clean media is important to be sure that it has not been contaminated with oils, chlorides or other contaminants. The type of blast nozzle will determined stand- off distance required, production rates, air volume required. Blast nozzle and pressure may be changed several times on each project based on complex shapes and coatings to be removed.
Safety is important when using certain media such as walnut shells that can produce large amounts of static electricity or CO2 dry ice can displace oxygen in unventilated spaces and create unsafe work conditions.
A byproduct of any blasting process, including soft media blasting is peening. Peening is the compression of a surface and can be measured using Almen strips. Low pressures and soft media peen less than heavy hard spherical media accelerated at high velocities. Peening can be a desirable and beneficial result of a blasting process or it can create warping of light and heavy gauge metals.