P3Steel Build Log (#02) – Preparing and Painting the Frame

The steel needs to be protected from the elements before it starts to rust – even if kept indoors the moisture from the air will begin to rust it. Because of this you need to either apply a protective layer of paint, use stainless steel which is less susceptible to rusting, or use galvanised steel. The cheapest option for me is to simply paint the frame, so that is what I did… read on!

Preparing The Steel Frame

The steel frame comes raw from the sheet metal workers, which is usually coated in a thin film of oil from the rolling process which makes the sheet steel, and some dust from the laser cutting. You cannot simply paint on top of the frame with all these elements on it, as the paint will fail to ‘stick’ to the metal successfully and will eventually flake and peel off. To get a good adhesion of paint to the frame, you need to give it a good ‘key’ which involves scuffing up the surface with fine sandpaper to give a microscopic rough surface to help give the paint something to hang on to. As you can see, I used 120 grid sand paper on a sanding block. I believe the RepRap Wiki page suggests 150 grit paper, but this seemed too difficult for me to find, so 120-180 will be fine, just don’t go lower than 120 – you’re looking to create small lines in the steel, not great big scratches.

You need to make small circular movements all across all parts, front and back, and also the sides. Don’t be scared with marking the steel, your paint layer(s) will smooth it all out nicely!

It was a little difficult to picture the difference, but I tried below. The keyed steel looks more like a stainless steel work top after you have finished. IT also helps to remove any rust and muck which has already started to creep onto the surface of the steel.

After all the parts are sanded you need to clean them – literally in the sink with dish soap and warm water! This will clean off any oil residue and other contaminants which are on the surface, leaving a clean and shiny steel frame ready to paint. Make sure you leave your frame time to dry, several hours, but don’t leave it days, as your frame will begin to rust, even indoors.

Painting The Frame

Hmmm… I live in a flat (apartment) and don’t have garages or sheds to paint in, and I don’t fancy painting out in the street! The only real solution I had to paint my frame (using spray paint for a clean and even coverage) was to use my bathroom, taped and masked up like some kind of make-shift meth lab in plastic tarpaulin sheet!

It did the job though, and the parts were easy to paint up. I did them in batches which fit across my make shift table on top of my bath(!). All parts got two coats of matt black acrylic spray paint, with a couple of hours between coats. Don’t forget to spray at an angle to get into all the sides and edges of the parts, as these are often missed.

Below is the result of my labour, a freshly painted steel frame, left to dry over night. Over all I am very happy with the results and the matt black really takes to the steel well and produces a clean flat finish.

There were however issues in the way I painted the parts…

Quality Control

With the parts laid out flat on my make-shift painting table they were easy to spray, but after two coats had dried and turning them over the paint on the plastic tarpaulin cover stuck to the finished side, causing the clean flat finish to be speckled and spoiled.

Most of this could be cleaned off as the excess paint hadn’t actually bonded to the surface, but some remained. Careful part choice will be needed to help hide some sides! If I were to paint the frame again, I would make more room and have the parts stood up more, so less contact to the sheet would be possible.


2 comments on “P3Steel Build Log (#02) – Preparing and Painting the Frame

  1. hey 🙂

    Regarding the problem you encoutered, maybe I can help a bit. Indeed, i’ve already paint some parts for my car, and learned this:
    – As you said, less contact is better, so you don’t have excess of paint
    – I always put one ore two layers of primer (it must be so thin that the part is not covered well the first layer), than up to 3 layers of black mat. Again the layers are sprayed from a good distance, very thin
    – I had the exact same problem with you when I painted some wood, and it was because of the plastic film. Someone adviced me to use aluminium film, which seems like a very good idea, but I haven’t tested it yet :p
    – You can try to use a thin sand paper (something like 200 or more) between the layers. Haven’t try it yet on metal (because for my car it was some parts under the hood, so i didn’t need a perfect finish), just on wood, but i’ll definitely try it when i’ll build my frame 🙂


    • Some good tips there Matthieu, much appreciated! I like the Aluminium film idea, I may try that one! If I paint another frame, I think I will use a primer or have three coats, or two and a matt varnish – i’m already starting to scuff the black finish when I man-handle the printer too rough!


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