I have read many replies about shot blasting. No one has mentioned what size shot to use. I am looking to rent an 8 inch wide blaster to do 1500 square feet (yes, I know it will take awhile, but it is a basement floor, not a garage, so access is limited.) I have read that if not used properly, the shot blaster leaves "corn rows". I would think this would be a function of the shot size. But if I use too small of a shot size, I will be at it awhile plus some. Any opinions on the proper size to use would be greatly appreciated. Then I can get down to the business of epoxy. Thanks in advance.
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Shot blasting - shot size
- Thread starter superdel
- Start date
Edger
Well-known member
I can't remember the actual size of the shot, but I had an 8" Blastrac for several years. It did not work better with heavy or fine shot so I used to mix the shot. The heavy will leave small dimples while the fine shot leaves a nicer clean surface. The fine shot does not bounce back up as readily so if you have holes or joints it can dump the shot. If the shot does not bounce up it goes back onto the floor and the next shot will hit that and not bounce either so it ends up just dumping it on the floor. The heavy bounces much better, but does not leave such an evenly cleaned surface so the mix of the two maybe 50:50 was better.
The only time it will leave "corn rows" is if the floor is very soft because an 8" is not very powerful. That will also contaminate the shot with sand prematurely too. I used to blast at 20-25 sq.m/hr so that would be ten times for feet - 200-250 sq.ft./hr so it should take you a day or a long day.
Some of the shot inevitably ends up in the vacuum system so there are losses although not that great unless the vacuum is too strong. You will need a magnetic broom and a piece of heavy plastic film or material to keep recycling. As the shot is left on the ground in small amounts you run over it with the magnet and pick it up. Then you dump that on the ground and flatten it out to release the sand and dust. Then go over it again to pick it up and dump it on the plastic or material. After that just funnel it back into the container of shot.
When you get too much sand in the shot it fails to bounce properly so you dump it and move the blaster forward so the shot lays on the floor. Pick it up with the magnet and leave the dirt behind. Refill the shotblaster and keep going.
The heavy shot was about two grades higher than recommended for the machine.
Hope that helps a little.
The only time it will leave "corn rows" is if the floor is very soft because an 8" is not very powerful. That will also contaminate the shot with sand prematurely too. I used to blast at 20-25 sq.m/hr so that would be ten times for feet - 200-250 sq.ft./hr so it should take you a day or a long day.
Some of the shot inevitably ends up in the vacuum system so there are losses although not that great unless the vacuum is too strong. You will need a magnetic broom and a piece of heavy plastic film or material to keep recycling. As the shot is left on the ground in small amounts you run over it with the magnet and pick it up. Then you dump that on the ground and flatten it out to release the sand and dust. Then go over it again to pick it up and dump it on the plastic or material. After that just funnel it back into the container of shot.
When you get too much sand in the shot it fails to bounce properly so you dump it and move the blaster forward so the shot lays on the floor. Pick it up with the magnet and leave the dirt behind. Refill the shotblaster and keep going.
The heavy shot was about two grades higher than recommended for the machine.
Hope that helps a little.
dcs Inc
Well-known member
Rental company will know what size to sell you. Use a steel shot. (looks like little bb's.)
Edger, thanks for the info. I went to the rental place and they told me almost exactly what you did. The rental of the 8 inch machine, with vacuum is $377 per day and a bag of shot (50 pounds) is $70. The only shot size they carry is 280. This is obviously 28 mils in diameter and is toward the smaller end in available sizes. The counter man says this will take off the paint and about 1/16th of concrete, leaving it ready for epoxy. Also included in the rental is the magnetic broom. (Again, thanks for the tip on using the broom). The only question that the guy could not answer, was how much shot do I actually need for 1500 square feet. We both think one bag should be enough, but I will take the second one and return it if I don't use it. I will attempt this in about 2 weeks. In short, it should cost somewhere around $500 for 1500 square feet for the prep. My preliminary figure on the epoxy cost is around $900. So I am looking at getting away with $1/sq ft. We'll see.
Edger
Well-known member
Superdel, I guess one bag should be enough, but you never know, I think grinding will take longer so it sounds like a reasonable deal. Good luck, would be interesting to know how your got on.