General Sandblasting Information for Code Rubber Lining and Painting
Before beginning any kind of blasting you must verify that the humidity and dew point are at acceptable levels. Not respecting the proper dry bulb wet bulb parameters will only lead to flash rust. This is generally not accepted in the coatings industries and can cause failures between the sub straight and the adhesives. There are charts available to understand this phenomenon and will help you find the acceptable ranges.
The second evaluation most codes require you to do, is to perform a blotter test. A blotter test is usually with a white cotton or white cloth testing that your air is free from oil, contamination or water. Given you have a modern, proper blasting or painting system with an in line air water separator and filter system you can be assured that this test is to reassure your customer that your air is good.
Adding oils and water to a blasted surface will negatively affect the adhesives and the overall performance of the liner.
There are other blast tests required for some specification, such as the soluble salts test. Although this test comes up on occasion, for most rubber lining it is not required. We will cover this subject in another post.
For the lining of steel pipe, tanks and other steel sub straights, you must achieve a blast profile of at least SSPC-SP5 classified as a white metal blast.
There are visual charts such as the example below to visually compare. These also have detailed descriptions to evaluate the blast achieved.
In addition to getting the surface clean you must also provide a profile for the adhesive. You can verify the profile of blast using a Testex strip shown below. This is a common way of evaluating the sandblast profile and proving to your customer the work was performed to specification. This is by far the most important part of the blasting process. Sure, cleaning scale, mill varnish is important, but in order for the adhesive to properly hold the liner in place, the tooth or profile will give the adhesive a larger surface area to hold onto. Very important.
Blast profiles need to be a profile of 3-5 mils for adhesive as it is required for good surface adhesion. For a paint anywhere from 1.5-4.5 mils is acceptable depending on the specification provided.
If you glass bead a surface you will get a nice finish, looking like SSPC SP-5 but your adhesion will be terrible due to the profile being flat. You can effectively double your adhesion surface by blasting a proper profile. Achieving the correct profile has everything to do with your blasting media and once you find the correct blast media and pressure you can achieve this result 100%of the time. Many option of blast media will create this profile.
For painting the exterior of pipe and tanks most specification call for SSPC SP-6 which is classified as a commercial blast.
The profile for paint is important but not as critical as for adhesives.
Once the proper blast profile has been achieved, adhesive needs to be applied before flash rusting can occur. A good rule of thumb is within 20 min, environmental conditions affect this time one way or another dramatically. On a dry sunny hot day with no humidity the piece can stay without adhesive longer with no risk of flash rusting.
If you have followed all of this you will have an excellent base for which the adhesive can be applied.