Quoted:
Quoted: Harcoat anodizing is the coloration process, it's not like they anodize it then put color on top.
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That's not quite right. Anodizing itself has nothing to do with the color. It's a process that converts the surface of the alloy metal into a hard coat with microscopic, honey cone shaped patterns.
the colorization is only done afterwards using special dye, which is held by the honey cones, and then sealed.
Factory anodizing is fairly thick and it isn't something you can easily remove by blasting. Otherwise why would refinishers need to "chemically" strip anodizing, when all they need is blast it?
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Actually you are both right in some respects. Type-III anodizing will color the aluminum, anything from Gray to Black, depending on the thickness of the coating and the chemicals added to the anodizing tank, however it is expensive, time consuming, difficult to control and many of the chemicals used are very hazardous and poisonous. Type-III is difficult to dye due to the small pore size of the coating, so in most cases it is not even attempted. The problem is, that in most cases, the term Hard-coat no longer means that it is Type-III anodizing. The specifications for Type-III are very strict and the only method of confirming that the anodizing is in fact Type-III is either by using an electron-microscope, or by destructively testing a completed piece by chemically removing the anodic coating, titrating it, drying it, and weighing the results to determine the number of cells per square inch. Rather expensive and usually only done in scientific and military applications.
Type-II anodizing is dyed and the dye is absorbed into the cells of the coating. Basically the only requirement is the thickness, and that there be enough cells created to hold the color dye and give a uniform appearance.
In either case, if it is blasted and the color is removed, so is the anodic layer. Think about it. In Type-III, since the color is from the thickness or chemical composition of the cells themselves, removing the color removes the cells. In Type-II, if the cells hold the color, and the color is removed, then so are the cell walls.
Why chemical removal is used, is it is easier, more precise, and if done properly is less destructive to the base aluminum. Other reasons: (1) You can dunk 100 pieces into a tank and have them all stripped in 10 minutes. Much easier and cost effective than having someone blast each piece individually. (2) It is also easier to control the amount of material removed, since the stripping action proceeds at a specified rate. Controlling the amount of material removal, when dealing with thicknesses of 0.001" is almost impossible when blasting, increasing the potential of base metal removal. (3) Blasting also tends to excessively change the hole dimensions more than chemical stripping does. (4) Getting into small nooks & crannies is difficult with blasting. With chemical removal its not an issue as long as the remover can reach it. There are even stripping chemicals available that only remove the aluminum oxide coating and don't harm the raw aluminum underneath.
As for thickness, its all relative. Thickness spec's for Type-II are a coating LESS THAN 1mil (0.001"), while Type-III is anything OVER 1mil.
Although most AR-15 manufacturers state that they are hard-coated, they are just a thick Type-II coating, usually 1.1-1.2mils thick. The term hard-coat has come to mean different things to different people. Hard-coat, at one time, was just another term for Type-III. (Type-III specs require a specific process, with a coating thickness in excess of 1mil, that was also of a specific density of cells per sq. in). It no longer means this. Most manufacturers use this term to mean that it is a Type-II coating slightly thicker than 1mil. The cell density requirements are ignored.
How to tell the difference? You can't, and the manufacturers know it. Unless they say specifically that it is a Type-III coating, it all means nothing.
Now, on to NYPatriot's question. If you are going to Duracoat it yourself, do nothing but clean and degrease. The anodized coating makes a good base and requires no additional prep work. The anodized coating, microscopically, is very rough, so the bond should be good if the Duracoat is applied with multiple, VERY thin coats. I've used Norrell's Moly Resin and an airbrush and have gotten superb results with no adhesion problems. Based on others who have used it, I assume that Duracoat will perform the same. Actually it is the best of all worlds to Duracoat or Moly OVER an existing anodized coating. The anodizing adds the surface hardness and corrosion resistance to the raw aluminum, while the Duracoat adds an additional layer of corrosion protection above this (as well as providing an easy to apply appearance). Moly Resin also adds lubricity to the coating, which aids in reducing wear. I'm not sure if Duracoat does this but from what I've heard it is an excellent product.