Hydro blasting is also referred to as high-pressure power washing and high-pressure water jetting. Most people are familiar with the term power washing. A standard power washer used in preparing metal building panels for repaint will operate at 4,000 to 5,000 p.s.i.g. Hydro blasting is performed in the 10,000 to 30,000 p.s.i.g. range.

The current evaluation is on the use of a piece of equipment from JetStream. The equipment is set up such that the optimum operating pressure is in the 15,000 to 20,000 p.s.i.g. range. Buildings with aged, peeling polyester and fluoropolymer have been prepared with this equipment prior to repaint. At these operating pressures, the loose coating is easily removed to the bare metal substrate and what has been considered tightly adhered has also been removed. Observation of this stripping process show that under some tightly adhering topcoats the primers are difficult to remove. Primers have been thermoset epoxy and polyurethanes. These primers can also be removed, but an excessive amount of time is required to remove them completely.

Substrate cleanliness is maximized with this process. Pressure is sufficient to emulsify oils, environmental contaminates and remove all chloride, sulfate and nitrate salts. Jobsite examination of exposed bare metal substrate shows a lusterless gray zinc surface. Pressure not sufficient to remove the galvanize or galvalume surfaces.

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Large panels of aged Plastisol with tight adhesion, new Ceram-A-Star® 950 with urethane primer over HDG and new Trinar® with urethane primer over galvalume were prepared for laboratory examination using the JetStream hydroblaster running at 18,000 p.s.i.g. The tightly adhering Plastisol was quickly stripped from the metal substrate.

The new Ceram-A-Star® 950 and Trinar® required substantially more work to remove completely. The Ceram-A-Star® 950 being the most difficult to remove. See the following pictures of these coatings and substrate after hydroblaster cleaning.

Report continues after photos.

Since surface condition is a prime criterion in any repaint process. The panels were examined using the video/optical microscope in the analytical department. Under magnification, the zinc substrates show some surface texturing caused by this process. See the follow photos for surface profiles observed under magnification.  
The first photo, galvalume under Trinar® shows some of the urethane primer remaining after the process. This material is still very tightly adhering to the substrate.  

The coating in these photos is a competitor’s fluoropolymer. The last photo shows areas of peeling fluoropolymer topcoat. Primer was tightly adhering, expect in area of peeling topcoat.

CONCLUSIONS: as a method for surface preparation, hydroblasting is far superior to the standard power washing equipment. Surfaces are extreme clean, coatings that are consider acceptable for adhesion but are suspect for future adhesion failures would be removed with this method. Metal substrates exposed by this method have a minimal surface profile, which promotes mechanical adhesion. Heavy chalking is not completely removed by standard power washing this method corrects the problem. Method is environmentally acceptable; gives exceptionally clean surfaces, leaves only tightly adhering coatings and leave metal substrate intact.

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