Need some do's and don'ts for sandblasting

30cabriolet

30cabriolet

Senior Member
I am getting ready to sandblast the fenders on my Cabriolet. I have sanded them down as best as I can, but there are still plenty of areas that I can't get into with a sander. I plan on blasting just the corners, the flanges, the inside bead area, and the deep rust spots. I plan on staying away from flat metal so I don't warp it. I need to know what the best way to sandblast is without doing any damage. What pressure, tip size, media, speed, and any other things you can think of?

After blasting, I am going to follow it up with Ospho to convert whatever rust I missed. I will then follow it up cleaner and epoxy primer.

I appreciate any help you can give me.
 
I did my whole body including the fenders, aprons, skirts, hood, everything. First Sand blast, then Ospho, Sit for three days, Brush or sand off the Ospho residue, then Epoxy primer, Then filler primer within the 72 hr epoxy window..

Blasted with 60 to 70 lbs pressure, and keep the nozzel moving. To much pressure or stay in one spot for too long and the metal will warp. It is the pressure and heat generated by blasting that warps. I use sugar sand Grade 5) as it is more gentle than grade 4 sand. But down side is it is slower. Use a 3/16 inch dia nozzle and it is worn (and needs to be replaced). Keep nozzle 10 to 12 inches from metal.

Last, be sure to use a good respirator as sand (silaca) willl kill you - so says the government. Black Beauty is more aggresive but coarser. Suppose to be safer but still I dont want to breath the dust.
 
A typical home setup is less likely to warp things than a bigger commercial setup.

I had already gotten a supplied-air system for painting, and I used it for sandblasting, too, sticking the air hose into my sandblasting hood. This made the job MUCH less unpleasant!

If your hood has a replacable rectangular clear plastic window, go buy a piece of plexiglass and cut up a bunch of replacement windows. They will frost up pretty quick. It's handy to have about half a dozen spares on hand.

Try not to work on hot humid days (like today in NJ!). You have more problems with water in the system, and you won't be sweating to death in the suit and hood. March or October is a nicer time of year to sandblast...

To prevent flash rust, I used Zero-Rust Prep-Step. It puts a phosphate coating on the metal, but it is not acid-based, so you can be liberal when spraying it around, and not worry about it getting on your skin, clothes, floor, paint, etc.

Doug
 
Having only sandblasted 4 whole cars with no panel warpage...

Warpage comes from too much energy (NOT heat) from the sand hitting the surface of the metal. These little "hammers" stretch the surface of the metal causing a difference in length between the top and bottom of the metal.

Too much energy comes from too much pressure or too much material.

We blast with 40 PSI with the sand valve turned until you can just see sand in the stream. It would be very hard to warp a panel with this combination, I tried and couldn't.

A siphon blaster can not control the amount of media in the stream. This will allow for too much media in the stream and lead to warpage.

When blasting it is very important to get the dark spots out of the bottoms of the pits. These brown spots will rapidly expand into more rust even if treated with a acid as the acid can not convert much past the surface.

In some cases, it is recommended that you blast, clean the metal surface and then spray with epoxy primer direct to metal. Follow the instructions that come with the epoxy. You need to be aware that paints stick by attaching to scratches made by sanding or chemical etching. Many epoxies are NOT compatible with acid conversion products so keep this in mind.

You might also keep in mind that electrolytic rust removal is an alternative to sandblasting. It will remove the rust from the bottoms of the pits, but takes more time.
 
On a similar subject, I will soon be sandblasting using a small glove-enclosed cabinet. There was some discussion on a previous thread about arcing. Should I ground the unit somewhere, or nothing to worry about? Thanks.
 
Kevin in NJ said:
Having only sandblasted 4 whole cars with no panel warpage...

Warpage comes from too much energy (NOT heat) from the sand hitting the surface of the metal. These little "hammers" stretch the surface of the metal causing a difference in length between the top and bottom of the metal.

Too much energy comes from too much pressure or too much material.

We blast with 40 PSI with the sand valve turned until you can just see sand in the stream. It would be very hard to warp a panel with this combination, I tried and couldn't.

A siphon blaster can not control the amount of media in the stream. This will allow for too much media in the stream and lead to warpage.

When blasting it is very important to get the dark spots out of the bottoms of the pits. These brown spots will rapidly expand into more rust even if treated with a acid as the acid can not convert much past the surface.

In some cases, it is recommended that you blast, clean the metal surface and then spray with epoxy primer direct to metal. Follow the instructions that come with the epoxy. You need to be aware that paints stick by attaching to scratches made by sanding or chemical etching. Many epoxies are NOT compatible with acid conversion products so keep this in mind.

You might also keep in mind that electrolytic rust removal is an alternative to sandblasting. It will remove the rust from the bottoms of the pits, but takes more time.
Click to expand...

Kevin, if a siphon is not recommended, what type of delivery system is? I am just finishing up a home made blast cabinet and would like recommendations for delivering the sand to the nozzle. I'll be using a heavy duty compressor(200 psi).

Scott
 
Kevin in NJ
Enjoyed your posting on your restoration, I love to see someone do their own instead of paying big money to someone else than saying " I restored this". I can appreciate all your work and thoughts. Thanks again
 
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Blast cabinets are different, though you still need to consider the part.

The media in blast cabinets is usually a much lighter material so they will not have as much energy as what you would use for bigger jobs. That being said, it will warp thin sheet metal like the piece that goes around the ignition switch as I found out one day.

The problem is when you use one of those simple siphon units with some sand or other heavy material. They usually go through a lot of material fast. They can do the job, but you have to be on the ball with them. One trick is to always keep the stream at an angle to the surface. I have one for doing quick small jobs at home. I got it cheap and have used it a couple of times. I am more likely to use other means, but once in a while it is the right tool.

The siphon unit that you get from TPtools is a good unit. I have found the cost of the carbide nozzle is well paid off over ceramic. I get years on a carbide nozzle and months on a ceramic. Plus the ceramic nozzles do not work as well as they wear so your cut rate slows dramatically.

I live in NJ so I get a lot of humidity. I do not have a water problem, but I have two water separators. One at the tank and they other at the blast booth. I also use 1/2" copper pipe at each end where I have the valves and 3/4" pipe running across the garage with a steep slope to the tank. The 1/2" pipe keeps the costs down on the valves, the expansion into the 3/4" should only help remove the moisture.

You also want to get the 3/8" hose couplers over the 1/4". The 3/8" couplers have twice the area for air flow over the 1/4". This helps with your HVLP guns as you do not have a subtle pressure drop as the air flows. I have a hard enough time getting paint to lay right and I found I was doing better after I changed over to the 3/8" couplers.

As far as media goes, I bought some Harbor Freight stuff to try and did not like it. It seemed to break down quicker then the stuff I get from TPtools. I am still using it, but I have also had to cut my car budget back a bunch and I am not in the hard clean up phase. I also cut my time in the booth by putting parts though the electrolytic rust removal process first.
 
Kevin,

Thanks for all the details. It helps to get some specific recommendations from someone with experience.

Scott
 
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