I fell in love with a 640 PRO, and will be using Mother's Mag Polish to bring the externally visible parts of the frame, barrel and cylinder to a high luster polish.

Question is, can the MIM trigger be polished too? What product is best?

I use an extra fine (gray/green) cratex wheel to break the edges and clean up the face, followed by a buffing wheel on a dremel tool..... to which I apply either an emory or stainless polishing compound. If you want a "mirror", follow with Mothers Polish.

If abraisives are used, it's best to remove the trigger from the gun to prevent the airborn grit from contaminating the surface and interior action parts of the gun. Also, be sure to clean/flush the trigger assembly thoroughly before re-installing.

And.....if you are using power tools, wear safety glasses. Also, remember to wear a mask in order to prevent airborne particles from contaminating and damaging your lungs.

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Ret. LE, FA Instr, S&W Armorer

And take your time because once you take the metal off you can't put it back on.

I'm not sure, maybe armorer951 knows better than I, but I think the MIM parts are heat treated which means that it is possible to polish through the hardening making the part more susceptible to wear.

... and why would anyone want to do this....?

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Army veteran, NRA Life Member

That was always my belief. A thin layer or hardening on the outside with the inside being softer. That is the way it was with sintered and cast steel parts before MIM became a thing.

MIM may be different, but I would still be cautious messing with it.

I'm certainly no expert on MIM steel parts, but I think you are correct in your assessment at least in terms of corrosion. The concern is not so much about exposing the polished area to increased wear, but polishing would certainly seem to make the area more vulnerable to oxidation.

I'm not sure about the harness of the steel MIM internals in the S&W revolvers, but unlike typical case hardening, the hardness of MIM parts is consistent all the way through the part, not just on the exterior surface. Don't know about everyone else's perceptions, but from working with MIM parts over the past few years, they seem to me to be harder than the OEM forged parts. Time will tell, I suppose.

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Ret. LE, FA Instr, S&W Armorer

I was thinking about getting my 66-5 high polished. The hammer and trigger are MIM. I got a e-mail back from the polisher telling me that if he polishes the MIM hammer or trigger, one they will rust, two he would be polishing the hardening layer from the parts and they could wear in critical areas faster. I would hope he would stay away from the critical areas in the first place. He recommended I buy stainless parts as he did on his own 66, and he could then polish everything. Now the cost of the polish job , and the cost of the new hammer and trigger made it not worth it to me.

I read somewhere else that this "not polishing MIM parts" is bogus. It's not like the parts are chromed or something. I mean, how would you heat-treat just the surface of a part anyway?

Actually, read the last two posts of this thread for the answer to your question:
And fire your gunsmith, Apollo99!
Polishing MIM parts

I just spoke with the custom shop last week about a trigger job on my Trophy Match which came with MIM parts. Yes they can do MIM, break would be 4+ to 4.5+lbs best case. However, if they replace with tool steel components (hammer and sear) they can give you any pull your would like. Additionally with the tool steel components the trigger only gets even better with time and will last for a very long time. About $125 more. I am opting for this as soon as they reopen the shop.

The 'break' on my 617 is under 3lbs and took me about 20 minutes to do. MIM parts are much smoother than their machined counterparts and thus require almost no work. Nearly all of my time was spent on the frame.

About the polishing: case hardening is just that, a thin case or layer around the part is hardened. I've zero experience with MIM parts, but we used to case harden various metals by either soaking them in carbon or cyanide. The thickness of the case was dependent upon the soak time, with diminishing returns, meaning it may take an hour to get a .005 case but two hours would only get you .007 and three only .008. I don't know if this applies to MIM parts, but strongly suspect it does. Any way, let's say for the sake of discussion, MIM hammer and triggers have a .005 case. Simply polishing the surface to a shine is likely to remove less than .001. That's because it's hard (remember?) and unless you break out the power tools and get really silly, you're going to rub a long time before you do any damage. If you can't resist using your brand new Dremel, remember this in spades: Speed kills. Speed creates heat and heat can be the enemy of any heat treated part. Heat can change the heat treating properties anywhere from minor in nature to making them softer than before heat treating. So use that Dremel at your own peril but for your part's sake, keep it slow and keep the part cool. If it gets too warm to touch, it's too warm - period. At least that was our motto for sharpening drill bits and tool bits. Even stones, which do remove metal, can take a long time to remove that much metal, unless it's from a small area like a sear surface.

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So many S&W's, so few funds!!

instead of polishing I'm thinking about hard chroming the trigger and hammer. I've been told no problem with hard chroming MIM parts.

Just for clarification, I was only quoting SA custom shop. I have no issues with MIM. They are fantastic parts with reduced cost/labor to fabricate and much closer tolerances from the get go. I myself have never done a trigger job although I have spent several hours researching it is not something I want to do myself to this particular firearm. I want a professional to cut the angles and fit it perfectly.

I appreciate the fantastic feedback and many different perspectives and opinions!

To be clear, my goal is cosmetic improvements... the internals are not on my to do list at this point.

All the best!

I used Mothers on my MIM trigger using just 'finger pressure'. No difference at all. So, I'm satisfied!

I have polished both the 617 and 686 stainless MIM triggers and hammers to a high luster. I am absolutely not gentle to these guns and they show no signs of rust.

- One you disassemble, there is no need to polish the internal sides,
only the tips/exposed portions of hammer and trigger.

*Ever done or had an action job performed on you blued/stainless gun? Guess what, its performed by stones on your "Gasp" internals...
Keep your dremmel off these parts, just grab an old tshirt, Mother Mag polish, and some episodes of The walking Dead.