Steel Fabrication & Industrial Facilities – Choosing the Correct Surface Preparation Before Painting

There are two primary ways for steel fabricators and industrial facilities to perform surface preparation before painting. These two most common ways are SP2/SP3 (hand & power tool clean) and SP6 (Commercial Blast or Sand Blast). I will go through each one in regards to their relation to the coating that will be applied.

Hand & power tool cleaning (SP2/SP3) is usually fine for single component products. These products would include metal shop primers, universal metal primers, alkyd/oil topcoats, acrylic topcoats, and silicone-alkyds. However, the performance of these coatings over time is always dependent on the level of surface preparation. Even though this method of surface preparation might be sufficient, the coatings that are applied will almost always perform better if a commercial/sand blast (SP6) was performed.

For epoxies, urethanes, zincs, or other multi-component products, a commercial/sand blast (SP6) should be performed. Yes, I know what you are saying already. I lot of data sheets from different paint suppliers say that SP2/SP3 is the minimum surface preparation for many of these products. I can only tell you from my experience, I have seen disasters from this level of surface preparation when high performance coatings have been applied. The most specific problem I’ve seen is when an epoxy with a good adhesion rating has literally pulled off the existing mil scale on the steel like it was a magnet. The trick here is that even though some data sheets on high performance coatings say you can perform a hand & power tool clean surface preparation, it will also state the existing mil scale on the steel needs to be removed. How long does this take with a hand & power tool clean surface preparation (if it is even possible) and are your employees going to work that hard to get it done?

Another item you need to keep in mind is the level of profile that is exists after your commercial/sand blast (SP6) has been completed. Having a profile on the surface of the steel is important because it gives the coating a tooth to grab onto. However, too much of a surface profile can be a problem depending on your paint system because there might not be enough coating being applied to cover up the depth of the profile. For each project, make sure you contact your paint supplier to find out exactly how much profile you need.