Wet sandblasting systems

General points
The term "wet sandblasting" is used wrongly to designate every system which projects an air/ abrasive mixture with water added. This wide definition includes two kinds of processes very distinct:
- the traditional processes with a gaseous vector called "humidified" whose aim is essentially to obtain a non-polluting work, without dust.
- the specific processes with a liquid vector called "wet way" processes whose aim is to use very fine abrasives to obtain a soft treatment.
We will talk, for information only, of the first process which is beyond the scope of the so called "wet way" processes, but in the scope of the dry way without dusts processes (see without dust). It is widespread nowadays and it gave rise to a specific soft cleaning technique by impacts called "rubbing-out" or "hydro rubbing-out".

Gaseous vector systems

Sandblasting without dust
The main drawbacks of the outside sandblasting, to restore buildings façades or renovate ancient monuments, are the emission of dust which pollutes the environment, and the diffusion everywhere of the abrasive hardly to recover.
To overcome this drawback, several devices have been devised in order to feed with water the abrasive jet, and reduce the dust emission to more than 90% fitting out a classical pressure pot with:
- either an annular spraying collar fixed around the nozzle,
- or a nozzle carrier which feeds with water the nozzle air/abrasive jet,
- or a special nozzle fed with a water supply in periphery.
Besides the great reduction of dust, the addition of water with a jet avoids the surrounding projections and facilitates the recovery of the abrasive trapped by the liquid. This less aggressive system, used with very fine abrasives, projected at low pressure, is well-adapted to the delicate cleaning works. It allows also minimizing the individual protection of the operator, reduced to a mere eyeshade.

Liquid vestor systems

Suction wet way process
The dry sandblasting, using as a vector the compressed air, is very hard to control when the products to project, non-metallic, are fine or under the form of powder. Concerning the low density products, (quartz, aluminium oxide, glass beads), their projection is very diffuse and limits considerably the visibility and the work control. Moreover, the ventilation-dust removal generates an over consumption of the product carried along by the draught towards the filter.
In the "wet way" process, created by EPPLER in 1937, a liquid vector (water in general), replaces the induced air of the suction system in order to convey the abrasive.

Principle:
A water/ abrasive mixture is put in suspension through stirring in a hopper, and then sucked up to the spray gun by the suction effect. At the nozzle level, it is propelled by compressed air which becomes the "accelerating" agent of the mixture (picture 1). In order to better control projection, it is possible to replace the suction effect by a force-feeding pump which feeds the spray gun. (picture 2)

Applications:
This process makes it possible to do precision works with extremely fine surface finishes. The water film which comes between the projectile and the surface absorbs the impact and makes it uniform, avoiding thus to dig the soft parts and let the hard parts in relief on non homogeneous structures. It also makes it possible to use silica quartz which are forbidden for a dry way process.

Pressure wet way process
As well as for the suction process, the vector is liquid instead of being gaseous, and the compressed air insufflated into the projection hose is used only to atomize the jet and give a kinetic energy to the projected abrasive.

Principle:
An abrasive is loaded with water in a tank which is put under pressure thanks to a water pump. Under the pressure, a valve closes the tank loading top opening. (similar to dry pressure) The tank lower measure is fed at the same time through the water pump and an air compressor which ensures the mixture conveyance to the projection nozzle.
Each parameter is adjustable; the water/ abrasive mixture outflow; the outflow of the water projected by the hose; the compressed air outflow; and all the relative pressures.

Applications:
The effects are the same than these of the suction wet way process and direct its use towards delicate and precise works in any environment, as well in the industrial sites as in the building industry. The main applications are cleaning, polishing, gumming, and stripping.