Find Surface Scratches BEFORE Finishing

Hi All,

I make artsy brass and bronze things so they are often machined and hand finished, little grinding here and a little more there. Things don’t really get measured, but they have to look great, and final surface finish is everything.

Yesterday, brazed up the part, cleaned it up with belt sanders and wheels, looks really good, put them in the tumbler for a few hours, still look great. Complete sandblast, look great, acid wash, still good, patina to a deep brown, looks good. Put on a clear coat, one of them has a pretty good scratch on it, looks like Scarface…..AHHHHHH.


The problem is, when sanding and such the little scratches all blend, so to say, the tumbler makes a perfect matte finish, no contrast to see a scratch. Sandblast, still a matte finish. Patina, now it’s a brown matte finish. And it doesn’t have any contrast to “show” the scratch that’s actually been there the entire time, until you apply the clear finish. And then all the blemishes come out….On the final step.

The Question: Do they make some type of contrast powder to brush on? I was thinking colored chalk and a big soft make-up brush. Give it a quick dusting, brush it around real quick, and see if that contrasts the flaws and scratches. Once it’s good, give it a quick final sandblast and off to the finishing process.

Thoughts and comments?

Thanks

Hi Aaron:
Have you tried a solvent soaked paper towel with some Dykem on it.
It should show up scratches just like a Magnaflux test does.
It will make your hands awfully blue though, so you may have issues touching your wife for intimate playtime.
Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining

It will make your hands awfully blue though, so you may have issues touching your wife for intimate playtime.
Cheers

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John Stevenson said he often used a felt tip pen instead of markup blue because - in his immortal words - "You don't get a blue hampton when you go for a jimmy riddle"

Both of those are excellent ideas in a traditional environment, but unfortunately you can't use those types of products with patina. Patina is just pretty rust, so it's very delicate to get on, surface contamination like that will not really come off.

Once patina is on, it will last for decades and just age gracefully.

If we had something acid soluble that would be nice, parts are tumbled in citric acid, and rinsed before patina with phosphoric acid.

I was thinking chalk for a few reasons, it's cheap and I think it will desolve in the acid. The lampblack in the black might cause a problem, so I might go with a different color. Just for reverence, a black sharpie, is a braze and patina resist, but red is ok for braze, but not patina. so I have no black sharpies in my shop, red yes, but they need to be removed with acetone and then sandblasted A LOT to remove the residue.

Just wondering this there is dry chalk in a can. and no I'm not going to try dry graphite spray. I have that too, it is an excellent braze resist!

I'll keep looking, something will turn up.

Thanks
Aaron

I just bought some blue chalk powder and a blush makeup brush. See what that does.

I also bought Red and Blue tempera paint, I can thin that and put it in a spray bottle, see what that does too.

We'll see.

Thanks
Aaron

I use raking light a lot to find flaws in woodworking as I sand. It is very effective to show even very small scratches. Get a light at a very low angle to the surface (essentially almost 0 degrees - parallel to the surface)and it will show small defects like they are mountains. the more polished the surface is, the better flaws show up. Just be prepared to have to back up and fix them with coarser grits (or coarser processes, whatever you are using).
-Dave

Never though about light, my surfaces aren't flat, but the idea is good. Wonder if there is a polarized inspection light or something. I'll look into it.

It's funny, from Dykem/sharpies in the first comments, now we tempura paint. Chalk was my idea. Now with the light I'll look for some type of specialty light. Just the chain of thoughts to get to a solution, and often you end up with many solutions and they all work in slightly different situations. But they are ALL extremely useful in that one particular case.

Keep the ideas coming.

Thanks
Aaron

dstig said:

I use raking light a lot to find flaws in woodworking as I sand. It is very effective to show even very small scratches. Get a light at a very low angle to the surface (essentially almost 0 degrees - parallel to the surface)and it will show small defects like they are mountains. the more polished the surface is, the better flaws show up. Just be prepared to have to back up and fix them with coarser grits (or coarser processes, whatever you are using).
-Dave

Click to expand...

Works really good when you're finishing sheetrock too.

My experience with musical instruments (trumpets, trombones etc.) is that you need to bring the part up to a high bright finish before you do any "special" finishing like a patina.

If you first use a fine (600) grit sandpaper, you will find most of the divots. Then a quick light buff with tripoli will show up a few more. Eventually you can color-buff with rouge and it will be good. It will be an iterative process. With experience, you get better at seeing the imperfections earlier in the process.

After the part is perfect at a high shine, you can do other finishes, like scratch-brushing or a patina.

-Jess