Back to the Metal

Much of our restoration work is about rust and paint removal. There have been some recent advances in how to achieve this, giving a number of new possibilities. In this article we’ll see some of those options using blasting techniques. A further article will cover more labour-intensive methods.

Due to the hazards encountered in the nature of this type of work, it is essential to wear protective equipment when using these methods and associated machinery!

Sandblasting:

One of the most widely known methods, it removes paint and corrosion quickly and easily. Sand is available in various grain sizes and types, depending on the job in hand, from simple surface cleaning, through to heavy rust removal applications.

Sand’s benefits are it is cheap as a ‘blast media’, it is available widely and many DIY shot blasting guns can use it as well as other granular media if the nozzle size allows.

Grit guns available from outlets like Machine Mart, can often handle a range of media types and sizes, some have renewable tips and a range of tip sizes for different weights of media; for instance, for use on fine sand.

Sand and Grit can be obtained in a variety of milled grain sizes, which also helps determine the ‘ferocity’ and cleaning power of the media. Generally speaking, the bigger the grain size, the more it will remove at around 90 psi.

Avoid sand with stones included as it will need sieving out as the larger particles will block the gun. Some ‘Builder’s type’ sands containing these inclusions are cheaper and not intended for sandblasting (just for mixing into concrete) so they will need sieving before use. The labour costs involved using a non-specific blasting sand might be more than the extra paid for a consistent grain sized sand only media, besides having to unclog the gun when a stray bit of stone frequently gets in the gun feed pipe and stops the work.

Sand can often be reused after filtering through a gauze or sieve, a 90psi air pressure is mostly used to distribute it from the grit gun towards the surface to be cleaned. Don’t linger over one spot too long with the blast as the sand can heat and harden metal sheet, making it brittle and liable to crack. Sandblasting will erode wood away.

The dust and blowback from the surface from sandblasting is a hazard and a filter mask must be worn to avoid Silicosis. Goggles must also be worn to prevent debris entering the eyes. Protective head shields can be purchased to help in avoiding these hazards.

Sand is quite acidic so ferrous (Iron and Steel) surfaces must be treated to prevent re-corrosion occurring, as soon as possible after cleaning. Sand is a hazard to mechanical parts and should not be used on engine blocks etc. Blasted sand can be released from the cast iron skin of engine parts and migrate out into the engine oil and internal parts in time, with obviously disastrous results later on.

Sand grades range from soft sand (sand pit type), silver sand (soft and good for aluminium casting cleaning) to sharp sand (visible quartz granules of a sugar grain’s size). Sharp sand will work effectively and is commonly used as a ‘one size fits all blast media’ by many sand blasters as it does the job of rust removal well.

Another use for sandblasting is for etching on to glass. The glass can be masked and sand can be used to etch the glass surface to make decorative patterns. A small grain sand at a lower pressure, is best for this purpose so that you get a ‘frosted glass look’. A great idea for making decorative panels for household interiors and as found in the old Victorian pubs on interior glass panels between seating booths!

Note: Some Pressure Washers have Sand Blasting feed attachments which sand can be fed into for blast cleaning, remember that these machines often kick out enormous pressure at the nozzle, this can likely damage panels etc not to mention embed sand into the metal surface, so it might be prudent to avoid using these  sorts of devices on your car parts at all costs!

Metal Grit media:

Along the same lines as sand, are granular metal media types. Aluminium Oxide which you may know from abrasive discs and papers, is like granular sand media, available in various grades of grain size. This media can be filtered and reused and is quite robust.

Iron Sinter grit media derived from casting slag is milled to a variety of ‘grain’ sizes and provides a hard media for blast cleaning. The picture shows 20, 36 and 60 grades. Copper Sinter is produced along the same lines.

Metal grit media types are harder than sand, but can be filtered and reused. Mostly, these present the same hazards to the person and to mechanical parts as with sand, so these media types are generally only suitable where non mechanical assemblies are involved.

The hazards from material blow-back and dust from cleaned surfaces is also an issue, as is wearing adequate eye and skin protection. We’ve all probably chanced it in the past, but operator safety isn’t something you skimp.

If you are stripping mechanical parts right down, these media might be possible to use, but I would recommend a non-hazardous media like Soda or alternatively, chemical dipping, to be sure that the refurbished parts don’t cause further expensive issues from unwelcome inclusions!

If using Sinter on mechanical parts, be prepared to fully disassemble afterwards, to remove all traces of abrasive material!

Bead blasting:

Results from use of various media types

(results from left to right: untreated; plastic beads/walnut shells; glass beads; aluminium oxide).

Glass beads are one of the types used in this situation as an alternative to sinter grit on metals, but are not as commonly used nowadays. Plastic Media beads came into use in the 1990s, the advantage is that the media doesn’t damage wood or rubber but will take paint off and are reusable.

The only possible drawback is if plastic bead debris gets into a mechanical assembly and block an oil way for example. The plastic beads can be reused until they disintegrate, Glass tends to shatter depending on the pressure used and the hardness of the material it is used on.

‘Soft’ Media types –

Using a milled Bicarbonate of Soda (Baking powder) grain, larger than used in cooking, the Soda cleans off corrosion.

Soda blasting is a new method of corrosion and paint removal to the market. It doesn’t harm wood (see pic), rubber mouldings or glass and leaves a light powdered coating on bare metal, which should be paint coated within a week to prevent further corrosion.

The soda does need a specific blasting gun which is different from a sand, grit or sinter gun. It is a ‘one use only’ material as it disintegrates on removal of the corrosion or paint layers. It creates quite a bit of dust and an extraction system is preferable.

Media made from Walnut shells is used to polish aluminium parts and is re-usable until it disintegrates.

Larger scale blasting:

If you are going to do any amount of media blasting, a cabinet is worthwhile. It contains the media, the crud and the dust from going over anything in your workshop, including your power tools. It reduces the hazard from flying debris too. A knock-down temporary booth can be made, which can be quickly assembled up for your jobs.

Working on batches of items is a good way to work, you can do a lot at one go. This is worthwhile if you have a lot to do, but safety precautions are a must even for a small job. Remember this stuff removes paint! It can damage skin and eyes too.

Another alternative to blasting is chemical dipping. Ideal for a boxed chassis, the stripped-down part is dunked into a tank and the chemicals dissolve old paint and rust to leave a shiny surface. The parts are then flushed with clean water, air dried and can be zinc dipped or epoxy coating dipped. When dry, they can be repainted.

Chemical processing is expensive, a chassis is about £450 just to strip, but it does make life easy.

3 thoughts on “Back to the Metal

  1. RayWhite says:

    I have had some issues with sand blasting the main one being clogging. I have tried various kinds of media and two different low pressure sandblasters but with not much luck. I have researched the problem and most commentators say they think it could be a moisture problem. I used quality media and tried heating it – and the pressure – vessel to ensure is is dry and have two filters on my compressor but clogging still persists. I have also experimented with different size nozzles and it’s not that. The only improvement came from when – out of desperation – I installed a homemade agitator at the base of the pressure vessel. It is getting the media to be drawn out of the container that really seems to be the problem.

    I would be interested to know where I am going wrong. When I do manage to get a few minutes without clogging I have enjoyed good results. The trouble is that I seem to be spending far more time freeing a blockage than I do actually blasting anything.!

  2. Ed Hosford says:

    It is my understanding that it is illegal to use sand for blasting metals in the UK due to the free silica content, I have been using crushed glass readily available from blasting suppliers for a number of years for open blasting ( chassis and body shells ) but aluminium oxide in the blasting cabinet for smaller parts.
    an answer to Ray White your blockages are definitely caused by moisture either in the media you are using ( not properly dry ) or more likely from your air supply, If you are using a normal garage compressor it will be working overtime to produce the flow required and the heated air contains a lot of water vapour which will condense in the hopper. can be partially over come by making up a condenser coil or better still a compressor with over 50 CFM output @ 90 psi with a very small nozzle.

  3. John Powell says:

    Re Chemical stripping, Yes it does a good job and gets everything off but you need to ensure the chemicals are totally neutralised before repainting. I am seeing corrosion creeping out of some overlapped joints several years after it was done. whether it wasn’t washed off properly by the strippers, not cleaned properly before the Bodyshop sprayed I don’t know but its something I have to get on top of. Saying that I am still planning to have a complete body shell dipped.

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