Abrasive Blast Cleaning

A coating is only as good as the preparation of the underlying surface

Our customers recognise that no matter how good SELEMIX® paint is or how well they apply it, failure to properly prepare their surfaces before the painting process starts will most likely result in sub-optimal performance.

A coating is only as good as the preparation of the underlying surface.

Indeed poor initial surface preparation has been estimated to account for up to 80% of subsequent paint problems.

One of the most common methods used to prepare surfaces for painting is Abrasive Blast Cleaning. This method involves propelling abrasives at high speed towards the surface to be cleaned.

Blast cleaning readily removes oxides formed at high temperatures (mill scale) and those formed at room temperature by corrosion (rust). It can also remove soldering, welding or foundry residues and is an excellent way of providing an anchor pattern on the substrate to support better coating adhesion.

It is an extremely effective way of preparing a surface, but we’ve also seen it cause problems if done incorrectly.

With the benefit of many years experience in supporting our Selemix customers, we thought it may be helpful to provide some tips to get the best out of Abrasive Blast Cleaning:

• Choose an experienced, professional operator and never let untrained operators blast your work. Ask to see examples of their previous work or any certifications/approvals they have gained to satisfy yourself that they will be able to successfully complete your project.                                                                                                                                                                                                                                                                                               
• Never let an operator choose their own abrasive without your technical input. You should always specify the quality of the blast and the surface profile that you require. Choosing the right abrasive is key to achieving a coating application that lasts. With the right abrasive, profits will be maximised by getting the job done more efficiently whilst also utilising the minimum volume of abrasive.                                                                                                                                                                                                                                   
• When preparing metal surfaces for Selemix is only as good as the preparation of the underlying surfacepay close attention to the anchor pattern you require and the DFT of the primer. Coatings adhere poorly to flat, hard surfaces and so the blast operator will be required to develop a pattern of indentations that the coating can easily adhere to - commonly known as the “anchor pattern”. When an abrasive particle strikes steel, it deforms some areas of the surface into valleys and pushes up others into peaks. The distance between the top of the peak and the bottom of the valley is the depth profile. For the best coating adhesion, the Selemix coating should completely fill the valleys and cover the peaks. If the profile is too deep, peaks can extend beyond the coating surface and cause pinpoint rust and subsequent coating failure. Typically, the correct depth profile will be 25%-30% of the DFT of the total Selemix coating system. If the primed surface is to be left outside in a holding situation (before a top coat is applied) it is always advisable to specify a higher DFT to provide additional protection.                                                                                                                                                                                                                                   
• Always specify the upper limit of humidity for blasting. High atmospheric humidity will increase the likelihood of moisture condensation on the surface resulting in flash rust, blistering or delamination of the coating. When coating a surface prepared by blasting, the steel temperature should always be at least 3 °C (5 °F) higher than the dew point. This safety margin is generally considered to be sufficient for all types of coatings. Never allow blasting to occur whilst it is raining.                                                                                                                  
• Never allow a blasted surface to stand uncoated for longer than 2.5 hours. The longer a blasted surface remains uncoated, the more chance that dust or humidity could compromise the eventual coating process.                                                                                                                                   

• Always specify a high quality anti-corrosive primer. The Selemix product range contains a number of high quality primers that are specifically designed to work in conjunction with our top coats to deliver an optimum coating solution. Never choose to prime with one of the cheaper red oxide primers that may contain sub-standard ingredients and are likely to compromise the finished coating and lead to corrosion and subsequently a reduced coating life.                                                                                                                                           
• Do not recycle blasting abrasives. Abrasive that has already been blasted may contain contaminates that have been removed from the surface. Collecting and then reusing such abrasives risks adding these contaminants back onto the surface, thus compromising the final preparation. Always blast using high quality, “one use only” abrasives.                                                                                                              

The key to getting the perfect Selemix ​finish is to ensure that the surface to be painted is thoroughly prepared. This is never more true than when using abrasive blast cleaning. We hope the above tips will help you ensure you get the best out of our products.