Sandblasting Tips

[kyleb]

OK, for fun this weekend, I'm gonna sandblast the tailfeathers for my project. I've used a bead blast cabinet before, but have never used anything more abrasive than glass beads. This time, I plan on using my pressure type sandblaster loaded with 40/80 black diamond blast media. I figure I'll put down a few tarps to catch the media, and of course, I have the correct PPE - long sleeves, gloves, hood, dust mask/filter, etc.

But what I don't have is experience. I figure I'll start on some random steel scrap I have laying around, just to make sure I get a feel for pressure, how close to hold the nozzle, etc., then I'll move on to the tailfeathers.

But it sure would be awesome if those of you who have been there/done that could offer suggestions before I start...

How close? How much pressure? Work the entire part (to avoid localized heating), or focus on an area until it is clean?

Any and all thoughts are appreciated.

Kyle Boatright

[David Johnson]

Kyle, you will find the black diamond pretty aggressive. Tater likes it but we don't. Went away from sand a while back. Using "first choice" media from tptools.com. Tried all different stuff before settling on that. Leaves a smooth clean surface. David J.

[Jerry Jackson]

Kyle

I just finished cleaning a fuselage--boy, what a mess it makes! Here are some of my experiences.

(1) I used 100 grit aluminum oxide in a pressure blaster with 3/32 and 1/8 inch nozzles, operating at 80 psi. My 15 cfm compressor could keep up but was working hard.

(2) Be prepared to change nozzles often, maybe once an hour. They wear fast and then the air consumption skyrockets. I found that the pink ceramic nozzles from Northern Tools wore unbelievably fast. White nozzles from Marco lasted 2-3 times longer ($6 each).

(3) You must filter the media before reusing it. Northern Tools has a nice filter for about $15.

(4) If you use the valve in the gun to stop or throttle the media then the valve will be destroyed in about 15 minutes. I fashioned a latch to hold the valve open and controlled the media flow with the media valve on the bottom of the pressure tank (you must have that valve). To start a session, have the media valve off, latch the gun on, apply the supply air, and then open the media valve. To stop a session, close the media valve and then remove the supply air. I found that simply removing the supply air to stop a session (leaving the gun on) cause trouble. The gun air stopped flowing before the tank pressure dropped and that caused the tank to continue pushing media into the hose after the gun air stopped flowing---the hose got packed completly full of media. Sometimes I could clear the clog by starting gun air but other times I had to disassemble the nozzle and shake the media out of the hose.

(5) You must adjust the media valve for an appropriate media flow, it should not be wide open. I found that I could make the valve adjustment based on listening to the gun's sound. With air through the gun but without media flow, the sound of the escaping air contains a high-frequency hiss. That hiss tends to diminish when media begins to flow. Slowly open the media valve until you hear the sound change character. You should be able to see media exiting the gun but is should be quite faint.

(6) Use paint remover to get most of the paint and glue off before blasting. The media has a hard time cutting anything that is a little soft or pliable. If you want to mask an area just use duct or electricians tape.

(7) Wear rubber gloves to handle cleaned parts. If you don't the parts will begin rusting in an hour.

(8) The cleaned parts will have a lot of dust and media adhering to the surface that must be removed before priming. I used red Scotchbrite, mineral spirits, and cloth towels.

(9) I bought 100 pounds ($1.15/pound) of media and I recycled it not less than 5 times while cleaning a fuselage and landing gear struts. It still cuts good but I have lost about one half of it and it is getting dirty (a lot of dust).

(10) Parts can be destroyed by overly agressive media blasting but it is easy to avoid damage. Use fairly fine media, use fairly low pressure, keep the gun 3-4 inches from the surface, aim the gun at about a 45 degree angle to the surface, and don't continue blasting a spot after it is already clean. Adjust pressure and distance so that the leading edge of cleaned part progresses at a rate of about 1-2 inches per second.

BTW, if damage occurs it is not caused by heat. Most of the media will have an elastic rebound from a steel part and will not cause a perceptable temperature rise. (But the inelastic collision with soft material like tape and rubber gloves will cause pronounced heating.) Instead, the damage occurs due to shot-peening which streaches the impacted surface, leaving a lot of compressive residual stress which tries to relieve itself by bulging and twisting. Any blasting will leave compressive residual stresses but the trick is to adjust the blasting operation to keep the thickness of the affected zone to be a small fraction of the part thickness. To minimize the damage use (a) fine media (b) low pressure, (c) don't use blasting to remove spots of soft materials like paint and glue, and (d) don't dwell.

Finally, media blasting is a dirty business. It certainly can be done but it certainly is not fun. That stuff gets everywhere and it is hard to clean up. I recommend trying to hire it done if you can find someone you trust to do a good job on somewhat delicate parts. I didn't find anyone so I did it myself. Now I wished I had looked harder!

Good luck

[kyleb]

Well, today's test run was a disappointment. The nozzle clogged repeatedly. This was a 1/8" ceramic nozzle on a pressure blast rig with 40/80 media. Tomorrow morning I'm gonna try a slightly larger nozzle (5/32) that I scrounged around and found. If that doesn't work, I'll try a syphon blaster I have.

Right now, I'm a bit PO'd at Harbor Freight. If you read the fine print on the instructions inside the packaging for the pressure blaster, it mentions that the 1/8" orifice nozzles they supply are recommended for 120 grit and finer media. They don't mention this on the display unit in the store or in the advertising for the unit. So you buy the unit without the knowledge that it won't handle anything but fine media. To complicate things, HF doesn't sell fine enough media, nor do they offer nozzles with a larger passage that fit this pressure blaster. A Catch-22 all the way around.

The plan this weekend was to do a practice run today on some scrap then blast and prime the "real" parts on Sunday and Monday AM before it gets warm...

We'll see.

[kyleb]

Well, today's test run was a disappointment. The nozzle clogged repeatedly. This was a 1/8" ceramic nozzle on a pressure blast rig with 40/80 media. Tomorrow morning I'm gonna try a slightly larger nozzle (5/32) that I scrounged around and found. If that doesn't work, I'll try a syphon blaster I have.

Right now, I'm a bit PO'd at Harbor Freight. If you read the fine print on the instructions inside the packaging for the pressure blaster, it mentions that the 1/8" orifice nozzles they supply are recommended for 120 grit and finer media. They don't mention this on the display unit in the store or in the advertising for the unit. So you buy the unit without the knowledge that it won't handle anything but fine media. To complicate things, HF doesn't sell fine enough media, nor do they offer nozzles with a larger passage that fit this pressure blaster. A Catch-22 all the way around.

The plan this weekend was to do a practice run today on some scrap then blast and prime the "real" parts on Sunday and Monday AM before it gets warm...

We'll see.

Today's effort with the 5/32 nozzle suffered the same fate as with the 1/8 nozzle - the nozzle clogged almost instantly. And the syphon blaster wasn't a big winner either.

I guess the good news is that I won't be spending Labor Day sweating and gritty under a sandblast hood.

[Jerry Jackson]

Kyle

If the nozzle is clogging then the media flow is too high. The media tank MUST have a valve for controlling media flow. I have used a NT blasters that did not have that valve and the in that case the nozzle clogged constantly. That valve should be used to shut off media flow before starting and stopping a blasting session.

Buy your media from a commercial media blasting supply house. Use the yellow pages to find them.

I think the 40/80 mix is too coarse for the parts I have been cleaning.

Good Luck

[kyleb]

Kyle

If the nozzle is clogging then the media flow is too high. The media tank MUST have a valve for controlling media flow. I have used a NT blasters that did not have that valve and the in that case the nozzle clogged constantly. That valve should be used to shut off media flow before starting and stopping a blasting session.

Buy your media from a commercial media blasting supply house. Use the yellow pages to find them.

I think the 40/80 mix is too coarse for the parts I have been cleaning.

Good Luck

Jerry, per your suggestion earlier in the thread, I did try throttling the media with the media flow valve on the bottom of the tank. For whatever reason, no joy. Maybe I'll try again later in the day. I'm beginning to think that the 40/80 is too coarse as well.

[Captgrumps]

We use bagged sand from equipmental rental places, including Home Depot and Lowes--no problem. Sometimes those exotic grits have its own mind.

Doug

[Jimmy Payne]

Kyle,

We just sand blasted a Super Cub last week and used a 1/8 nozzle and sand bought from a paint supply store. The Home Depot sand is not fine enough. The best advise I can give is make sure the air is dry. When you think it is dry enough, dry it some more. We plan to use two inline dryers for the next job.

[Jerry Jackson]

Jimmy is right, the air must be dry. One way to dry the air is to install a good drier but that is expensive. I wondered if I was going to have to bite that bullet but it turned out that the long piping run for my air allowed enough cooling that a common filter/moisture separator was all I needed. The reason this helps is that hot air out of the compressor can hold more moisture than the same air cooled by passing through a long pipe. My 15 cfm compressor feeds into a 50 foot run of 1 inch pipe that ends at a filter/moisture separator. Then there is another 50 foot run of 3/8 rubber hose to another filter/moisture separator on the pressure tank. I got a lot of water out of the first filter but I didn't get a single drop out of the second filter. I concluded that either the second filter wasn't effective or that the air was already dry enough when it got there. In any case, I never had any trouble with moisture gumming up the media.

Good Luck

[kyleb]

I don't think moisture is a problem for me. I have two water traps coming out of the compressor, a long hose run, and another water trap going into the sandblaster. The compressor is in an air conditioned space (so the air is pretty dry). I haven't found the first indication of water in any of the the traps.

I think it is either a material feed rate issue that I need to control better OR the media is too large for the equipment I have.

I think I'll go flying today, then come back and experiment more with the sandblaster this afternoon or evening.

Of course, the Mrs. has a few other ideas for today, like me hanging pictures, a quilt rack, and some other things. Being that she's 6 months pregnant, I may have to compromise a little...

[David Johnson]

Kyle, I finally got tired of the problems you are having several years ago. Siphon blasters are way slow. Bought a good pressure blaster. If you are up for a trip to Jasper, will be glad to loan it to you. Won't fit in a trunk, need at least a small truck. Just sitting in the shop gathering dust. You will probably save enough time to make up for several trips if you have a compressor big enough to pull it. My 7.5 hp compressor can't keep up with a big nozzle, but I have the small one on it now. David J.