DIY Sand Blasting
Sandblasting has gained a reputation for bending body panels or even blowing holes in thin metal. That's because most professionals like to get the job done quickly so have massive air supplies that have enough force to bend panels, or with enough grit to heat the panels up causing distortion.
None of this is a problem for the DIYer. Sandblasting is possible with low cfm, and while it's a slow business it is possible to blast the thinnest panel without distortion.
The photo shows me completing the sand blasting on a floorpan chassis. The job probably took about 10 hours, but I'd already removed the underseal with a heat gun and scraper.
I had attempted sandblasting many times in the past and always didn't have enough air flow (cfm) for the job. That's because I was using the wrong equipment. More experienced forum members have pointed me in the right direction, and this page covers what they told me to make it easier for anyone else to sandblast at home.
Matching the sandblaster to the compressor
The amount of air a sandblaster uses is proportional to the nozzle size:
cfm for continuous running
at 80psi at Nozzle
|20 gallon eBay sandblaster||
|Machine Mart sandblasting gun||
|Random syphon gun for blast cabinet||
Given that the biggest compressor that can be reasonably run from 240V single phase is 3.5hp and 14cfm only the little ceramic nozzle has half a chance in a DIY installation. The two syphon guns in the photo above do have smaller nozzles inside - they are positioned inside the larger nozzle to create the suction to pick up grit, but this means that much more than half of the air supply is wasted in picking up the grit.
Pressurising the grit container means that grit is forced into the air supply with no wastage of air. The photo shows the air supply entering through a water trap, then a tapping to the grit container pressurises the grit supply. The orange flexible tube carries air down to a mixing valve at the bottom of the container where grit is forced into the air supply and into the outlet hose.
Expensive versions have been available for some time, but recently someone has started advertising them on eBay for around £70 (2008 prices). A couple of sizes are available - sold as 10 gallon and 20 gallon sandblasters (a UK gallon is about 4.5 litres). I went for the 20 gallon version as it's only a little more expensive. They are brilliant, but if you do buy one bear in mind the nozzles are ceramic and don't last very long at all. I'm already on my second nozzle, and will replace it with a tungsten nozzle soon. The ball valves have a short life too.
I used J-Blast Supafine from Hodge Clemco which ought to have a grain size between 0.2mm and 0.7mm, but there are a few rouge bits in there that will block up a small nozzle. The link also shows alternative blasting media and describes the applications.
For DIY use the media needs to be sieved before use to prevent occasional nozzle blocking. It needs to be sieved again before re-use to get rid of the big bits of paint, underseal, sealant, nails and old nuts and bolts that really would block the nozzle. I used a sieve from Tesco with approximately 1mm mesh.
Sandblasting is a messy business. I did the work outside on a tarpaulin nailed up to surrounding buildings at the edges to encourage grit to flow back in ( I couldn't find a big tarpaulin so taped two together to get something like 6m by 10m in size). My grit recovery rate was about 80 percent, so starting with six 25kg bags I ended up with three bags after shotblasting the chassis.
I live on a farm where the nearest neighbours are about 500m away. A car parked just 20m from where I was sandblasting was coated in grit and dust after a day of blasting. The grit and dust really does travel, so for a residential area blasting in a big tent or cleaned out garage would be more sensible.
A tarpaulin would still be useful wherever you blast - it allows the grit shot to be gathered in a pile for recovery without any need for sweeping. Here the edges of the tarpaulin have been lifted up to move the grit to the middle for collection. The tarpaulin also ensures you don't sweep in concrete dust (which again is bad for you).
I used around 200 litres of grit blasting the chassis. (That's about 350kg of grit, though it was reused a few times along the way). The grit becomes finer and more dusty with each use. I'd not want to use it after about 5 or 10 uses). The finer grit is less effective than new grit, so it takes longer to blast, but it does produce a finer finish which could be handy for delicate work.
I had always understood that sandblasting would be bad for thin metalwork. It turns out that's only because people who sandblast for a living like to get the job done quickly so they use huge compressors with massive nozzles and have enough force to bend little panels (or they can go fast enough to heat thin panels enough to distort them). These things aren't an issue with DIY sandblasting.
If you blast through a 3mm diameter nozzle at 80psi air pressure that's the equivalent of placing a half pound weight on the panel (250 gramme). So he force isn't sufficient to bend even the thinnest panels. And the work is slow so heat doesn't build up enough to distort a panel. The trade off is time - it took about 15 hours to blast the paint from this body shell.
With DIY sandblasting it's even possible to blast thin flat panels without distortion. The photo shows an outer rear wing pressed from 0.8mm sheet. I blasted the inside as I needed the key for the high zinc content paint I planned to use.
Though it's touch and go - One of the panels I blasted was an old stock panel with those rust worms under the paint. That took a lot of time to clean up, and the panel did make a pop noise at one point. No lasting damage to the panel, but from there I was more careful.
For the other panels I turned the pressure down to about 70psi and would blast a 100mm square in one part of the panel, then move to a different part of the panel and blast another 100mm square and so on until the squares joined up. That was to avoid localised heat build up which could have distorted the panel.
I haven't quite got this one completely figured out yet. Covering every part of the body is a must as the grit rebounds from the panel at much the same speed as it is applied. A thickish long sleeved shirt and welding gloves work fine. The hood I'm using in the photo isn't up to the job - grit does get in there and can make it into your eyes.
Various better hoods are available from £20 (search for sandblasting hood). The ones to chose are heavier, have much more material, and drape down to waist level. The pros will use air fed helmets which must really help keep them cool. The visors can can be replaced. I got through about 5 of them when blasting the body and chassis.
A respirator (proper fine dust mask in the style of a gas mask) is essential. Any blasting media will break down into fine particles which the lungs can't get rid of. I use a respirator bought from a paint shop which is designed to stop solvents. They are cheap enough from your local car paint shop, or eBay, but will likely clog fairly quickly with the dust.
How to strip what?
DIY sandblasting isn't very good for removing rubberised paint or sealant. That needs to be stripped using a hot air gun and scraping knife. I found it fairly effective for the old underseal sprayed fairly thinly on this chassis, but thicker underseal would be a problem.
While the DIY blasting will remove paint it's much quicker to do large areas with a strip and clean disc and paint stripper.
Don't use sand!
Sand is terrible for use in sand blasting. Most of all because it breaks down into very fine particles which stick in your lungs causing silicosis. That's another of these cumulative things that you don't want to start with. Also drying grit costs a lot of money, so builders sand isn't supplied dry - it would clog in the blasting pot.