How to prepare for coating to Rust...
2 CommentsMonday, 6 February 2017 | Admin
A Simple Step by Step on applying coatings to Corrosion.
The notes below should be used for true corrosion inhibitors, that are specifically designed to be applied direct to rust. Please ensure all rust is removed for coatings that "contain corrosion inhibitors", request a rust free surface, or do no include specific properties to stabilise existing corrosion. coatings in this category are typically Oxides, ZInc Rich, Phosphate primers, etch primers, and waxes.
So you have Corrosion like this....
But you really want it to look like this
So the first thing you need to do is establish is if its worth saving, should it be replaced, can it be repaired? There's a whole range of means of doing that but for the sake of keeping my ramblings within Essay level lets take for granted its either been replaced, repaired, or is salvageable.
So lets deal with Steel.
1. Establish the current state.
Once you have effected repairs you can be left with a complete mish-mash of substrates. From new steel with Mill-scale on them, contamination from manufacture, right through to new shiny steel, and existing heavily corroded steel that may or may not be contaminated.
Time often dictates we press ahead, give it a quick whip over with a abrasive media like a flap disc and that's enough. The point of this article is to encourage you otherwise.
Once we have our substrates identified, we need to prepare them.
2. Existing rusty steel.
We need to ensure the coating you are using is going to do its job. Now coatings vary massively from sticky waxes, to paint on armour and they all deal with rust differently. Some will ask you to use a "rust killer", some will ask you to remove all traces of rust with a magic wand, and others a happy medium where you remove excessive rust. when using products like CIO, FCP, RCP, and 2K armour we ask you to remove the excessive rust. We want you to prepare back to what we call "firmly adherant rust". This means that you can't pull the rust off, or flake it off. We ask this because paint over rust that soon comes off is simply no good. Equally going hell for leather with a wire wheel on an angle grinder and getting a surface sheen a Chef would be proud of is just as bad, because not much with adhere to it.
So we are after a compromise. This is about perfect.
We can work with the corrosion above for a few reasons.
1. Its not falling off the steel. Anything that adheres to that level of corrosion won't be falling off because the rust is coming away from the steel.
2. The rust isn't so heavy that the coatings are going to trap moisture and air underneath (well, good ones won't)...
3. The rust isn't so pronounced that the peaks of the corrosion (rust looks like a mountain range under a microscope, with peaks and valley's (no not the ones in wales)) that need to be equally protected) will be left exposed to vulnerable.
4. The level of corrosion that remains is rough to the touch. Smooth is bad for adhesion, Cheese grater rough is bad for longevity. Stick to something close to the above, using the products mentioned above and you won't go far wrong.
5. To achieve the above your first step is to remove the contamination. There's no point whipping out the angle grinder only to spread any contamination that was there all over the work piece, giving you a nice looking, heavily contaminated work piece, and preparation media that could go on to ruin further jobs. Just like removing the loose rust contamination takes many forms, and that can be salts, silicone's, or existing chemicals picked up from engines, silicone sprays, or previous coatings (wet waxes are infamous for this)
6. So prior to abrasive media panel wipe the lot down having cleaned it off with a pressure washer/ steam cleaner, TFR/ your weapon of choice.
7. With the boring bit out of the way its time weigh up your options and and choose your method pf prep. Large areas >angle grinder, tight areas > drill, sensitive areas (no i mean around brake pipes and such!), wire brush. Abrasive media on angle grinders vary massively. You have grinding wheels, flap discs, wire wheels, and then you have variants within those categories, so you have crimped or the more aggressive twisted knott, you have resin >zirconium and then ceramic at the top. Your aim is not to polish the steel. Merely to give a good key for the primer.
3. "not rusty steel"
1. You have set new steel in, or its protected with an existing coating. Both should be panel wiped down prior to messing about with them,
2. Once decontaminated a traditional coating is best removed with flap discs and/ or a fleece mop. For new steel mill scale (the darker grey layer often found on unprepared new steel must be removed). Flap disc, angle grinders tend to work best for this. Try not to "blue" the steel by introducing too much heat. We are only aiming to prepare it, not anneal it.
3.for wet waxes and underbody schultz many find the use of an industrial steam cleaner (local lorry service place are usually very helpful), and TFR combined often remove most of the products. A scraper for the softer stuff, and a screwdriver or bolster chisel help for the hard schultz type that cracks. Never had much success with a heat gun and scraper, but apparently also a viable option.
4. As per rusty steel we want it rough ;)...... so a grinding wheel, course flap discs, lower the rpm, higher the grit the better. For exterior bodywork with a high grade finish flap discs and wire wheels are the way to go, and DA sanders between some coatings.