Finishing your DIY parts, overview
When I started writing this article the idea was to tell about one specific tech that I have developed for coating steel parts with higher-end requirements (like ones in this series about Spot Welder) in my workshop. But the introduction quickly grew into a whole separate post, so now you have it here.
When it comes to finishing the parts of your making, the options are numerous. Let’s discuss some of the most popular ones from a hobbyist perspective and with a pinch of historical and chemical background.
Painting with “drying” paints
That would be the first thing on a DIYer’s mind because of availability and simplicity. The “drying” here means that the solidification mechanism of the coating is either related to evaporation of the solvent or to oxidation (usually of unsaturated compounds) by air’s oxygen, but often to both of those. A wide range of coating properties may be achieved, but the shortcomings usually are lack of hardness/wear resistance and a relatively mild heat/chemical tolerance.
The somewhat outdated but still very popular (because it’s dirt cheap) option are alkyd-based enamels. The alkyds are polymer chains based on two kinds of interleaved units. The most-known alkyd PET (the one in plastic bottles) is comprised of units of terephthalic acid and ethylene glycol that make linear chains because each of the units has two reactive ends.
When glycerol with its 3 OH-tails is used instead of ethylene glycol, the result would be a more rigid mesh-like structure used in glyphtalyc enamels.
Even more rigid structures of the most popular pentaphtalics are obtained with 4-tailed pentaerythritol. The glyphtalics therefore are somewhere in-between on the scale of elasticity, which explains their historically wide usage for impregnation of transformer windings.
Practically it is easy to dip- or brush-paint with alkyd enamels, but then it usually takes several months for the resulting thick layer to be properly dried, due to slow solvent diffusion. It is possible to do a better job laying thin layers with a spray-gun while using high air flow, so that most of the solvent evaporates right on the fly. Yet, even when done right, the final drying can take a week.
Another well-known class of coating substances is drying oils. Those are unsaturated fats and “unsaturated” means that their structure includes carbon-carbon double-bonds that are easily oxidised on air and participate in linking of individual fat molecules into polymer meshes.
The oils do not produce as strong and thick outer film on the surface as enamels do, but they have other advantage: they are applied in a raw state, when individual molecules (monomers) are relatively small and therefore easily sip into the narrowest cracks and pores. Then they polymerise inside those pores, creating protective impregnation inside the part’s volume. Obviously, this makes them unsuitable for finishing shiny metal surfaces (no pores there), but they work great for wood, where this hydrophobic impregnation makes a good protection against water. Their slow polymerisation speed may be seen as a positive property, as it gives the oil time to seep deeper.
There are nowadays complex formulations of oils and enamels with improved properties, that dry faster and produce better-looking finish even after simple brush-painting, and probably do not emit as much nasty solvent vapours. You have to pay a premium for those fancier ones, but for smaller projects it may be a sensible way to go.
Powder coatings
Powders are probably the next big thing. The process consists in spraying thermoplastic or thermoset polymer powder on the part and then curing it in an oven at ~200°C. Generally produces greater-looking and more robust finish, does not require additional time to dry (as there is no solvent involved) and is fast and cheap when done at scale. The powder-painted parts are ready to use right after the curing. Whenever you’re looking at a mass-produced painted steel part, most probably it is powder-coated. But if your part doesn’t like to be heated in the process or if your local factory with a powder facility does not take your small order, then you’re out of luck.
If a thermoplastic is used, the coating’s heat tolerance will not be great, as it softens above ~100°C. The thermoset polymers, on other hand, undergo cross-linking while baking and after that might be resistant even up to ~300°C. In general, powders are superior to drying paints and there is a wide range of special visual effects and physical properties that are achieved by various powder formulations, but as a DIYer you’re always stuck with the specific option that the factory deals with. Although there is nothing to stop you from building your own (smaller) powder facility in the workshop, but that would be a whole project all by itself.
Binary paints
“Binary” here means two components that are mixed together before application (think of epoxy). The binaries take care of a more “luxury” slice of use-cases, producing (arguably) better results than powders and not requiring the baking stage. Are mostly used for bigger automotive parts and are relatively expensive. But it’s not a problem to find a place that will take your smaller order, for a price, of course. In principle, you don’t even need professional service and you can mix and apply them to smaller parts in a workshop.
Electroplating
In cases when what you need is the metal surface being, well, metal surface, electroplating is the way to go. The idea is to cover your steel part in layer(s) of other metals to increase corrosion resistance, or just for shiny looks. While there are different ways of placing one metal on top of another one, only a few are really viable for a hobby processing. The basic idea is simple: when a metal atom, like Cu (copper) loses a pair of electrons, it becomes an ion, Cu++, that, under conditions, is water soluble. If then the pair of electrons is given back to the ion, it becomes an insoluble metal atom that falls out of the solution, trying to stick to the nearest surface of neutral metal atoms alike. Therefore we can dissolve one metal electrode (anode) and plate its metal on another electrode (cathode) by putting them in a proper solution and passing electric current through them. While it sounds simple, there are practical tricks to it: the solution has to be of right composition and right temperature, the surface for plating has to be in perfect condition (polished, degreased, stripped from oxides), the base metal of the part has to be compatible with the solution and the metal that is plated with. For example, plating steel with copper is not as straightforward, because in the absence of external current source, there is an immediate reaction between iron and copper sulphate solution. Overall the process is much more involved then any kind of painting (if you want to get actual results and not just a TikTok demo), but with determination it is doable and there is enough information online to get started. Except for the classical copper/sulphate plating I would recommend looking at nickel/acetate process. From my experience, the later one is even easier to establish and produces decent results on steel straightaway.
Conversion coatings
This is about covering metal with layers of metal oxides, salts or other compounds (typically of ionic nature), while the metal in the resulting compound is the metal of the part’s surface, converted to this compound by a reaction with the bath solution or atmosphere. Whence the “conversion”. One prominent example of conversion coating processes is application of “rust converter” compositions to rusty (or not) steel parts. Those compositions are based on phosphoric acid that converts iron and iron oxides to iron phosphates. A typical result of this process on a steel part would look like this:
Another group of conversion processes is known under the umbrella of bluing. The idea is that when a clean steel surface is heated to ~200°C and above, a uniform film of a dense magnetite (Fe3O4 iron oxide) is formed. On a highly polished surface it goes through colours from yellow to blue, depending on temperature. Watchmakers have excelled at this process for preparing highly attractive finishes on screws:
Such treatment produces a very thin, and therefore not very robust film. The colours produced are actually resulting from interference of light in thin films.
On other hand, if the surface has higher roughness to begin with and if the heating is done in multiple cycles, it is possible to get a thicker and slightly porous finish of black colour. This porous layer is then usually treated with linseed oil that polymerises inside the pores and makes the surface hydrophobic (phosphated and other conversion layers may often benefit from this oiling step as well). This second version of the process is easily reproducible and always gives decent results without much effort (if only you can afford bringing your part up to ~400°C):
Industrially black finishes are more often produced by boiling in a caustic bath (mixture of sodium hydroxide and potassium nitrate, the latter acting as oxidising agent) at ~150°C. This does not mess with part’s tolerance and heat treatment as much as the previous approach, but the boiling hot caustic mixture may be not something you’d want to deal with in a smaller workshop.
Finally, there is a number of pre-mixed “cold blue” solutions (usually with proprietary formulations, but mostly based on selenium oxide) that may be applied to the part at room temperature. They do not produce as robust film and are somewhat expensive, but again, not heating the part at all has its benefits.
Conversion coatings are mostly electrically insulative and being non-organic have higher heat tolerance than all kinds of paints. The best thing is that they are converted from the surface layer of the metal and therefore do not suffer from adhesion problems. In fact, they are often used as an adhesion layer for further painting and some paints (like the ones marked “works on rust”) contain a phosphating agent in their formulations.
Conclusion
For sure the discussed finishing options do not cover it all, but that’s what I found to be a good “short-list” to select from in my DIY experience. Here is a decision flowchart that works for me when there are no special requirements:
Sorry If I have bored you with technicalities without showing many fancy pictures, but this context would be a useful introduction to the next article about phosphate coatings. Meanwhile, get subscribed not to miss the next part of it.
