¶ Why painting the steel tubes ?
Painting your PrintNC is a good thing, for two reasons :
- Cosmetic : A well painted CNC will look better
- Prevention : Paint will prevent rust
Rust is a result of the steel oxidation with oxygen, when put in presence of water. Normal air humidity will bring enough water to allow steel to rust. That's why protecting your steel tubes is important.
You can powder coat the steel frame pieces, but it may cost more money. Many use spray paint from a can. Part of the experience is to paint it really cool colors.
¶ Painting before or after assembly
There's advantages and disadvantages to do the painting before or after the assembly.
However, painting before the assembly should be avoided as much as possible, because drilling / tapping operations will generate metal shards that will scratch and destroy all the paint applied.
¶ Before the assembly
- No need to cover tapped holes
- Steel will be easier to handle, as it'll be dry and clean
- You don't have to disassemble your build after the initial assembly
¶ After the assembly
- No risk to scratch the paint when manipulating / drilling / tapping during the assembly
¶ Preparing your steel tubes for painting
Before painting. You will need to prepare your steel tubes.
¶ Removing rust
There's several ways to remove rust / mill scale
¶ Sanding
You can use a classic orbital sander with a 120 grit then a 180 grit in order to prepare correctly your steel tubes.
You can also buy a Scotch-Brite Clean and Strip XT Pro Disc - Rust and Paint Stripping Disc - 4.5” diam. x 5/8-11 Quick Change Thread - Extra Coarse Silicon Carbide for an angle grinder to remove the mill scale.
¶ Steel pickling
Another method to clean your steel is to “pickle” them. This method is purely optional.
Please note that this method involves manipulation of dangerous materials. Please be mindful when manipulating them! Also, plan for where the liquid will drain into. It may leave a yellow stain on concrete.
¶ Procedure
- Make a box big enough to hold the largest piece of steel + some extra height/length/width. I used two wooden boards I had leftover from my workbench build, and some smaller scraps, and built a 4 sided box (no bottom) because I didn’t have enough. A bottom doesn’t really make a difference if you use thick plastic sheeting. Another box alternative is to take some 2"x6" 6' boards and screw them together to make a box and 2'x8' for the plastic. In retrospect, I wish I would have made the box bigger.
- Pick up your ingredients
- HCL (aka Muriatic Acid) in some form (I got 30% at hardware store)
- Baking Soda (to neutralize the acid)
- Scotchbrite pads (or another scrubber to clean the mill scale)
- A couple of buckets and/or a hose for washing / rinsing
- Plastic sheeting (I went with thick since my box had no bottom)
- Some lightweight oil
- Recommended to wear disposable gloves when working with this
- Put the plastic sheeting into the box and tack down the edges so that it can’t fill inside.
- Fill a “neutralize” bucket with water.
- Add baking soda to the bucket – this is your neutralize bucket, also useful in case you spill or splash or otherwise need it. Probably want 10:1 water to baking soda ratio.
- Fill the box for the steel up with water. Pay attention to whether there is any slope to the ground. May consider adding shims to “level” the box.
- Add the acid to the water in the box (not water to acid) until you reach your desired concentration. When using HCL, even low concentrations will work (for example : 1.5L of 30% HCL with 80L of water or about 2%; for Americans that would be about 1 cup per 5 gallons).
- Put a test piece in, and run it for 20 minutes or so and see how it is.
- To remove the mill scale, you can use water and scotchbrite pad to remove most of it really easily. It will slough off. I used the baking soda water instead of plain water here since I wanted neutral runoff – although with the low concentration and a liberal amount of water its likely fine without it.
- Use the baking soda water and wipe everything on the steel liberally again to neutralize any remaining acid.
- Wash the steel with water one last time.
- Now dry the steel with some paper towels, regular towel, or whatever you have on hand. Then you want to apply the oil to the steel (it will begin rusting in about 30 minutes or less otherwise)
- When done with the liquid in the box, add the “neutralize” bucket into it. Also add baking soda until the soda no longer bubbles. Be sure to wash off the area really well, perhaps even power wash. If the water does not run to a close drain, you may have to power wash your neighbor's sidewalk, too.
- When you’re ready to prep for painting, remove the oil with your cleaner of choice, sand with some 60 grit to rough the surface a little more, and paint!
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¶ Lemons
| If you cut a lemon into 3 pieces and use the lemon as a sponge, rub it on the steel. Wait about an hour. Be sure to remove the acidic residue before painting. Also, once you clean the steel with water, it will begin to rust very quickly. So, be prepared to prime the metal. (Thanks to user “salvo” for the tip.) |
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¶ Vinegar
An alternative to pickling with muriatic acid is to use white vinegar. This method will eat away all the rust and mill scale leaving a clean smooth surface to prime/paint on. See photo with and without mill scale (dark mill scale tube in the middle).
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Make a waterproof container similar to the method used for muriatic acid. I instead used some building construction vapour barrier because I had some on hand and instead of building a box, I stacked up some 2x4 wood to make a makeshift bathtub. See photo.
Put some paper towel over top of the steel to keep the tops wet and let soak for 2-3 days.
Use a hose to remove the majority of gunk left on the steel from the vinegar bath. Hosing off all the vinegar is important to neutralize the acid, don't forget the inside of the tube. After hosing off, use a wire wheel on an angle grinder or drill to easily remove the remainder. Be prepared to paint the steel as they no longer have the protective coating of mill scale and will start to rust quicker. Keep them dry away from moisture till painted. If you do find you need to store them before cleaning for a short period if time, look out for any rust, flash rusting etc. If you do find any hit it again with the wire wheel before painting. Any rust at this point should be minimal and come off quickly.
¶ Cleaning
Acetone can be used to clean your steel tubes and also de-grease them thoroughly. Use rags and wipe all the faces until all surfaces are clean.
¶ Covering tapped holes
If you paint on tapped holes, you may have trouble to screw bolts in them afterwards. In order to avoid any issues, apply some painter's scotch on all tapped holes.
¶ Painting
Now that your steel tube have been prepared, they are ready to be painted.
¶ Preparation for painting
Before starting the actual painting job, run some lumber through each steel tubes to allow easy painting of all faces. Be sure to put cardboard or something under/near the paint area for any overspray.
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¶ Primer coat
Apply one coat of primer. (some spray paints act as a primer and paint)
¶ Finishing coat
Apply one to two coats of paint for a nice finish.
If you have drips or over sprays, wait until the paint is dry. Then with a scotch brite pad or fine steel wool, rub in the same direction. Then lightly spray paint that area a few times.