1. Blasting

There are basically two different methods of blasting. Wet blasting and dry blasting. Shot-blasting, sand-blasting and grit-blasting employ different types of blasting material, but are essentially the same. In this manual we will use the word blasting.For small applications we can recommend our Wencon Blaster (dry blasting). See chapter I in this Manual. 

1.1. Dry blasting

Dry blasting is the most common kind of blasting. There are a large number of companies who use dry blasting and the way we become involved is usually to specify. Blasting is not only used for cleaning metal surfaces, but also for cleaning houses before painting them. Blasting is also very much used before painting steel constructions, etc. Blasting is without doubt the best way of preparing the surface for Wencon applications. It offers the best physical adherence, and is by far the most efficient way of cleaning the part. Accordingly,  whenever possible we advise our customers to use blasting prior to an application. If blasting is impossible, other ways of surface preparation  must be adopted. Machine parts that have been in use, for instance in sea water, will have quite an amount of water and salt penetration into the metal structure. This penetration should be removed before application, or, said in another way, sufficient salt and water should be removed to ensure  salt and water do not come to the surface of the substrate before the Wencon  repair material has cured. If the surface of a machine part shortly after blasting turns black, or perhaps very dark, it is a sign of salts sweating from the substrate. A way of accelerating the sweating out of the salts is by heating up the part or leaving it in a warm place for at least 12 hours. The most efficient way of removing water from the substrate is by using dehumidification units. (See Wet Blasting) In many instances it may be a good idea to use written specifications. This may sound rather formal, but does not need to cover more than indicated below. 

Specification for dry blasting

1.  Blast  the  machine  part  to SA 2 1/2 using  sharp-edged  blasting media, to a roughness of min. 75 microns.

2.  Leave the part for sweating out salts in a warm place for at least 12 hours or heat it up to 30 - 40 °C (86-104 °F) using gas torches.

3.  Blast again to SA 2 1/2 immediately prior to the application.

4.  For parts containing lots of water and salt it may be necessary to repeat 2. and 3. until the surface remains light grey for at least 2 hours after blasting. Most companies will understand a specification like this, but it may be necessary to check the blasting prior to application. If there are any signs of salts sweating out, the parts must be blasted again.  

1.2. Wet Blasting               

Some applications do not allow dry blasting due to dust coming from the process. For instance when the application should take place on site and you have lots of motors, pumps, valves etc. in close proximity, your customer may not be too happy about  dry blasting. The reason being that dust will enter roller bearings, etc. even inside the motors. Under such conditions the answer may very well be wet blasting.  Wet blasting is carried out in almost the same manner as dry, but using water in union with air and grit. We use the same standards as for the dry blasting (SA 2.5, SA 3 etc.). There are two problems that will often follow dry blasting. The first being that the technique leaves the substrate wet, the other being the fact that a wet surface will oxidize before getting dry.Dehumidification. Prior to application on a surface that has been wet blasted, a dehumidification must take place.  Dehumidification units can be hired from many firms who rent out machines. Be sure to get the firm to help you to choose the correct type and size for the application. After dehumidification a light dry blasting may be necessary. It may be done using equipment that retrieves the blasting material and dust by means of vacuum suction. 

2. Perago

Using a Perago disk is a little like blasting, but without sand.Perago is a rubber disk with hard steel spikes mounted on the perifericke. Perago can be mounted in a normal drilling machine, and gives a surface close to a blasted surface - clean and rough with sharp edges. Perago dishes can be ordered at Wencon and all Wencon distributors.

3. Grinding

For small repair jobs we can often meet too many objections to blasting. Wheel grinding is an acceptable way  in cases, where it is possible to grind the entire surface. Most often when repairing mechanical damages. Corroded areas are often in a state where a wheel grinder will only be able to cover part of the surface, and is therefore not advisable here. When grinding use a coarse stone. Use the Wencon Cleaner before and after grinding. Grinding with sandpaper or emery cloth is only advisable when, for example, carrying out shaft-repair on a lathe. 

4.1. Needle Gunning

Needle gunning is a method that has almost been forgotten in recent years. Or should we say is mostly used for very rough cleaning or removal of rust. It is possible to do a very nice job using a needle gun, but it takes time and should be closely supervised. It is essential that the marks from the sharp needles cover the whole surface so that none of the original surface remains. It is recommendable to steam clean the surface before needle gunning. (see steam cleaning) 

4.2. Turning

When carrying out mechanical repair jobs, turning is often the best and easiest way of preparing the surface. When, for instance repairing a worn shaft damaged by a loose roller bearing, turning to a rough standard should be done prior to degreasing and application. 

4.3. Steam cleaning

An effective method of removing salt and oil remaining in the substrate is to use steam cleaning. Steam cleaners are available from most plant hire firms. Use steam temperature of approx. 95 °C and repeat the process three times, leaving the substrate to dry for some 15 minutes between each cleaning. Steam cleaning is not sufficient in itself. It must be followed by blasting or one of the other surface preparation methods.

5.  Dehumidification

A very simple and effective method of removing moisture, whether it be moisture in the metal or from wet shot-blasting, is dehumidifying. Even parts which appear to be completely dry can contain large quantities of moisture. When coating tanks, or other totally or partly enclosed parts - large pumps, pipes, etc. - dehumidification is recommended, as humidity emanating from the persons doing the work can condense on the part and reduce its physical adherence. Dehumidification units (DU) can be hired in most towns, and plant hire firms are very helpful in advising the type and capacity of the unit. Position the DU close to the substrate to be treated and insert the tube carrying dry air into the tank or part. If the parts to be treated are numerous and small, make a tent of plastic sheets and feed the tent. Usually a few hours of dehumidification is enough to obtain a dry and good surface ready for the application.A quality requirement of dehumidification is that during the coating process there must be a temperature of minimum 3°C above the point of condensation. The point of condensation temperature is the highest the part must have, when moisture from the surroundings falls on the part and condenses. The point of condensation is thus dependent upon the temperature of the part, the ambient temperature, and the relative humidity of the air. Instruments exist for measuring these values, and where the repair of large parts is concerned, conditions should be laid down regarding this, if the application is to be done by subcontractor or customer. 

6. Steel brushing

Cleaning by using a steel brush (by hand or on a machine) we can not recommend. A steel brush leaves a polished surface which will reduce the adhesion of Wencon or other products - paint etc.