Gray Coating after Sand Blasting

Mon Jun 03, 2013 3:24 pm

Hi all - is it necessary remove by extra sanding the gray that remains on steel and cast after blasting? Alternatively can it just be wiped clean with solvent and left to dry?

Regards, Ken

Re: Gray Coating after Sand Blasting

Mon Jun 03, 2013 5:05 pm

Ken, If you've sandblasted down to bare metal, there shouldn't be anything left to sand off. You don't need to sand the bare metal shiney if that's your concern. Wipe down until the cloth comes off clean and then it's ready to prime.

Bob

Re: Gray Coating after Sand Blasting

Mon Jun 03, 2013 5:07 pm

Ken I think the gray is baked on rust. I went thru the same thing when I first started sand blasting. It comes of real hard. Back in the cabinet for the stuff. Post pics if you need to and I can tell for sure.

Re: Gray Coating after Sand Blasting

Mon Jun 03, 2013 5:45 pm

Cecil wrote:Ken I think the gray is baked on rust. I went thru the same thing when I first started sand blasting. It comes of real hard. Back in the cabinet for the stuff. Post pics if you need to and I can tell for sure.


Thanks Cecil - I'm down in wonderful Paramus, NJ on business - I'll send some pix when I return - but not till Wednesday as tomorrow is my 36th Wedding Anniversary - Best be doing the right thing - which is NOT hiding in my shop :D :D

The coating comes on parts that still have a good coating of paint and no evidence of rust.

Regards, Ken

Re: Gray Coating after Sand Blasting

Mon Jun 03, 2013 6:15 pm

Don't know what kind of steel you are talking about, but if it is something made from bar stock, it could be "mill bark" which is just metal that has been decarburized in the hot rolling process. Wipe it down to remove the dust and paint it up.

Re: Gray Coating after Sand Blasting

Mon Jun 03, 2013 7:16 pm

that is probably rust as said before, many times you can give it a rap with a hammer and it falls off
tim

Re: Gray Coating after Sand Blasting

Mon Jun 03, 2013 9:30 pm

tst wrote:that is probably rust as said before, many times you can give it a rap with a hammer and it falls off
tim


I seldom disagree with the esteemed contributors to the site - in this case I will and her's why. Please all - take this the right way as in the fun of the forum..... :D :) :lol:

Take the valve cover. I had sandblasted it clean - it was not rusty at all, but the dull aluminum color (light gray) remained after sandblasting. I used my angle grinder w/3M disc, to remove the coating down to shiny metal. In a previous post I mentioned painting it but not liking the way it came out. So hears a piece freshly painted, shiny metal that I blasted again, so I could repaint it. It still came out a dull aluminum color (light gray). Sanded it again to shiny steel, painted it again now I am happy.

Are the operative words here "dull aluminum color", since I am using 70 grit Aluminum Oxide as a blasting agent? It does not seem to matter, what the condition the part is in - the final color after blasting is dull aluminum almost gray, but very smooth - with no signs of rust - especially on the valve cover.

Love a good conversation - as always thanks for all the input.

Re: Gray Coating after Sand Blasting

Mon Jun 03, 2013 9:46 pm

the sandblasted surface is perfect for the primer to stick, i would wash and prime it the way it comes out of the cabinet.

Re: Gray Coating after Sand Blasting

Mon Jun 03, 2013 10:38 pm

Ken:

I would listen to Rob .. he is after all a professional auto re-finisher. I have very little experience with sand blasters and have only done a couple pieces. I have had the same result .. thankfully for me, I just washed it down with lacquer thinners and went about my business. The parts I had done were not going to accept a wire wheel etc., otherwise it would have gone in the electrolysis tank and not the sand blaster.

Re: Gray Coating after Sand Blasting

Tue Jun 04, 2013 3:22 am

Color has everything to do with what light is absorbed, reflected, or refracted. A sandblasted surface scatters light. You mentioned blasting the valve cover again because you didn't like the results. Why go back to bare metal again unless something was wrong with the prep? Better to sand it smooth where you were not happy and degloss the rest with sandpaper or steel wool and put on a new topcoat.

Re: Gray Coating after Sand Blasting

Tue Jun 04, 2013 7:34 am

Hi Bob - good point - but cabinet is still new and I wanted to try out the new modification I made to the draft control. It was really just a good excuse to use my cabinet and be in my shop... :D :D :D

Re: Gray Coating after Sand Blasting

Tue Jun 04, 2013 7:40 am

I don't see any mention of a primer. The sheen of the topcoat depends heavily on the primer. Most people prefer sheet metal components to shine.
The sandblasting will leave your metal with tiny pitts. These pits diffuse the light and it doesn't reflect back the light as a shine.

The Ething primer is applied to stick to the metal. This can be lightly sanded to fill in those pits. Clean with lint free cloth & reducer.
If you want it to really shine then apply a filling primer and wet sand it with 1000 grit. Clean with lint free cloth & reducer, then when it dries, clean with a tack cloth.
Next apply a light layer of the color paying most attention to the difficult areas to paint (like creases and pockets that are hard to get to). This gives the final topcoat something to stick to. Don't let this coat get completely dry. Apply a wet finish topcoat and allow to dry completely. Wet sand and buff sheet metal.

Re: Gray Coating after Sand Blasting

Wed Jun 05, 2013 10:22 am

I think you're right about the dull gray coating being aluminum oxide from the sandblasting. Perhaps a different blasting media would be in order?

Re: Gray Coating after Sand Blasting

Wed Jun 05, 2013 11:11 am

outdoors4evr wrote:I don't see any mention of a primer. The sheen of the topcoat depends heavily on the primer. Most people prefer sheet metal components to shine.
The sandblasting will leave your metal with tiny pitts. These pits diffuse the light and it doesn't reflect back the light as a shine.

The Ething primer is applied to stick to the metal. This can be lightly sanded to fill in those pits. Clean with lint free cloth & reducer.
If you want it to really shine then apply a filling primer and wet sand it with 1000 grit. Clean with lint free cloth & reducer, then when it dries, clean with a tack cloth.
Next apply a light layer of the color paying most attention to the difficult areas to paint (like creases and pockets that are hard to get to). This gives the final topcoat something to stick to. Don't let this coat get completely dry. Apply a wet finish topcoat and allow to dry completely. Wet sand and buff sheet metal.


outdoors4evr is exactly correct. A sandblasted surface is not a "smooth" surface. When cleaned of any oil or moisture it makes a perfect surface for multiple layers of primer which will provide the "smooth" surface for your finish coat of paint. The primer will fill in the tiny holes and scratches and "grabs" the metal for a perfect base. The more primer, with sanding in between coats, the better and "smoother" the base. Think of primer as very, very fine body filler.

Re: Gray Coating after Sand Blasting

Wed Jun 05, 2013 11:20 am

Hi all - here is a headlight right out of the cabinet. I used emery paper in one small area with some very light sanding to expose the shiny metal. The ultimate question is - does this gray color need to be sanded off?

IMG_2236.JPG
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