Sandblasting advice

Hello all:
I am working on a series of metal, welded sculptures that I would like
to gently sandblast or clean to bare metal without texturing the metal.
Mostly mild steel, hot rolled, cold rolled, etc. These include bars,
pieces of sheet, pipe cutoffs, tubes and bar stock. Some have mill
scale some do not. I usually finish it to bare metal, remove cut lines,
grind off any imperfections. My point is that they are not super smooth
even in their final form.
In the past, I have typically finished the surface to almost final form
and then done my welding and assembly. Then, all is required is some
touch up to the final form. I put a lot of work into this pre-finishing
and discovered it can greatly impede my creativity. Sometimes I abandon
a part that I spent a great deal of time working on. More often, I feel
compelled to use it because of the effort it took to prep it. I'd
like to either speed up the pre-finish by softly sandblasting (without
the tiny pits) the pieces before assembly and/or hit them with
something after assembly. I've heard of walnut shells, or plastic
sandblast media. Does anyone have experience with these? TIA
St. Louis, MO
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IF you want to be able to remove the scale and other contaminants you mave need to start with sand or aluminum oxide and then work your way down to something finer. Glass bead or even baking soda. Walnut shell is amazing for cast parts or for when you want to remove contaminants but not any metal. Walnut shell will get rid of the grease etc. We would use it to remove grease, wax and clean heat exchangers etc that were not going to be painted afterwards. Worth a try.
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You are not going to get white metal with softer media. Experiment with lower pressures/abrasive flows to get the results you want, using a more aggressive media like coal slag. I'd stay away from aluminum oxide due to the cost and the tendency of the particles to embed in the work. The only time you get major pitting is when the original steel is heavily rusted, which is really from the rust, not the blasting. Very light guage sheet metal would show marks but any substantial steel should look pretty smooth unless you leave the gun blasting in one spot for an extended period.
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from experience beadblasting an entire aircraft.
use a rounded blast media . glass beads of a type called ballottini come in many my aircraft work I used two buckets full of them about eleven thousand times. I just swept them up and passed them through a flyscreen filter to remove the detritus. I used AH grade to good effect. water washed dry sand which is quite rounded also works really well.
the way to prevent peening of the surface is to either reduce the pressure in the airline or to direct the gun at an angle to the surface rather than perpendicular to the surface. my setup used one of those taiwanese units sold in discount auto shops. they work quite well. at 120psi the beads will remove rust and peen the surface with a finish that accepts paint really well. at about 80psi the action is gentler and at about 60 psi you can have the gun perpendicular for quite some time with just a polishing action.
btw you dont need to do all the surface at once. just bead blast the welding areas first to clean them and then after the welding is all complete go back over and finish the job.
btw the use of a single compressor is futile ( I have months of experience behind the comment) go and hire a trailer mounted compressor. these will give continuous air pressure for the duration of the job. with a home workshop compressor you will be waiting for most of the time for pumpup to occur. I thought with a 15cuft/min unit I'd be ok but to me the hand nozzles all seem to need about 60cuft/min of air. ...and the job takes forever.
the bead blasting booth bears comment as well. you only need the thinest of plastic sheets to contain the beads. I use a frame suspended from the ceiling which is draped in light weight plastic sheet paint drop cloths. when pegged up at the corners it contains the beads really well.
use air filters for your breathing air and a face mask and gloves. my face mask is an old welding helmet with a clear plastic piece in the lens. I tape a piece of transparent plastic sheet over the lens area and this takes all the abrasion. it gets replaced when it becomes frosted. Stealth Pilot
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Stealth Pilot

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