I sprayed a handguard, upper, and lower with moly resin. When I assembled the rifle, some chips occurred which took off the moly resin and the anodized layer underneath. How could this happen?

I've done 3 uppers, 2 lowers, and 2 shotguns in moly resin with the same process and tools and everything's been fine before. The lower is one I sprayed with moly resin about 2 years ago in a different color, the upper is new, and the handguard is used but basically new. So far the handguard hasn't chipped, but both the lower and upper have spots missing. I'll post up photos tomorrow.
I've no idea what's going on, but it doesn't seem good.

There's a lot of factors that could be in play.
From lack of cleaning prep (pre-anodizing) to improper sealing (post-anodizing) or just simply thermal expansion (aluminum oxides, formed during anodizing, can react to heat differently than the base alloy).

Here are some photos. Keep in mind this is just from assembling the rail and handling parts on a clean bench. Wherever the hex wrench rubbed against the upper when tightening the handguard cap screws, the finish came off. Same with the lower, you can see where the cap screw rubbed on the lower and removed the finish.

Both upper and lower are Aero Precision, so that much is in common. However I purchased the lower about 6 years ago and first sprayed it with moly resin about 2 years ago. The upper I got from the EE a few weeks ago. Maybe Aero's anodizing process isn't that good, but I've done several of them before with no problems.

The finish came out harder and more brittle than usual; this is aside from the obvious flaking. A couple things come to mind:
1. These baked longer at a slightly higher temp than usual. Normally I cure for 70 mins, these went 90 minutes due to something coming up that I needed to deal with at the time. I always cure with an analog oven thermometer (checked against a digital thermometer) inside with the parts.  About halfway through the cure I noticed the temperature had crept up to 325F, which I corrected back down to about 310F for the rest of the cycle. According to the written instructions and the Moly Resin website, neither the extended time nor the temp should be a detriment.

2. I degrease parts with a non-chlorinated brake cleaner. Maybe something in the brake cleaner is reacting with the anodizing and causing it to delaminate. Again, I've used this same product a half dozen times before with no ill effects.

At this point I'm thinking I should take everything apart, sandblast, and have the upper/lower cerakoted.

@Feuerhand - is it possible that the hex wrench and screw head simply gouged into the finish enough to get through the anodizing?  If that screw is making contact right in front of that take-down pin for some reason as you tilt the upper forward, I don't see how any finish would hold up to that.  Why was your hex wrench making contact with the upper?  Same question about the screw head and lower...

Who makes this moly resin paint product?  Just curious.

Unlikely. The wear is from the sides of the hex wrench simply being in contact with the finish when turning the screws. I'm talking turning the short side with two fingers to get the screws seated. The wear near the take-down pin is from trying to get the handguard even; it would rub on one side, then the other. The handguard is a Daniel Defense MFR and those screws are really tight to the upper, there's not a lot of clearance for your take-down pin lug either.

I'm not sure how else to describe it, I was not handling these parts roughly. I've got multiple anodized flashlights that show less wear after a decade of weekly use.
Here's the URL for the Moly Resin website: https://www.molyresin.com/

Another possibility is the moly resin didn’t polymerize correctly. This leaves it more brittle than normal. When it fails, it takes the anodizing with it because it adhered to the substrate well, but didn’t create the linked resin coat like it should. Think of it like a fiberglass boat that the resin coat was badly mixed, then cracks.

It may be more the resin coat to blame, nothing to do with the anodizing. If I am reading the original post correctly, the lower was in service for two years with no issues prior to this. Have you use this same batch of Moly resin on anything else?

Hi. I'm watching with interest as I just started using Moly Resin.

A couple of thoughts; Aero sells uppers and lowers that are anodized, and uppers and lowers that are Ceracoated. I wonder if the Ceracoat is applied over anodizing, or if its applied over bare aluminum. Are you sure yours was anodized?

I have done a fair bit of anodizing and as I understand it the aluminum oxide layer “grows” equally both on top of the surface and down into the surface so it is actually an integral part of aluminum (if that makes sense). It can be scratched or worn but it should not be prone to flaking. I imagine something could go wrong during the anodizing. I can't image what, the process either works or it doesn't as far as I know.

Anyway, I'm watching in hopes of learning.

edit to add, after looking again I see black around the edges of the flaked coating so yeah I think yours was anodized originally.

It's definitely anodized. I have one of their cerakoted lowers and it looks to me like it's applied over bare aluminum.
Do you do the anodizing yourself? If you know of a shop that does small-batch anodizing I'm interested.



This is what I'm thinking too. What would cause the moly resin to polymerize incorrectly/incompletely? This batch is getting a bit old, about 2 years, but it's been stored in the refrigerator the entire time per their instructions.
I've done a bunch of other items with the same batch of moly resin and they've all be fine; the most recent was about a month, maybe 6 weeks ago.
This particular lower's been in use since 2016 or 2017, but was first coated 2 years ago and had no issues in that time.

I do not know of any shops that are willing to do it. I do it myself. It's the only way if I need something anodized.

I don't know moly resin at all - but have used many things like it/same principals.  IME, it usually came to down to two things:

a) Water. Either too high a humidity, or water contamination some how (it can happen in ways you do not suspect).  For example, if you stepped out doors to spray it, and there was much of a temperature differential, condensation you can't see can do it. or water into the equipment (from the compressor if used, etc.)
b) Oil contamination. A surprising amount of oil can stay behind even with some solvent cleaning.

I've added a few steps to my prep process and have had no issues since then (knock on wood):


a) dryer/trap in line if using air from a compressor.
b) Bake in advance any part that has been heavily oiled AFTER degreasing.  Oil can be in the metal that this will help bring up.  You can get something grease free (you think), bake it 300F for 1/2 hour, and oil can still seep up.  Really important with bare metal or parkerized surfaces. Probably not in play here.
c) thoroughly wash with TSP as very last step.
d)  heat with a gun all surfaces just before spraying.  This will drive off any barely visible condensation.


None of that is to say that the moly resin could not have gone bad, but there are some small "gotchas" that can creep in once in a while.

Norrells suggests to warm the parts prior to spraying, you didn't mention if you heated the parts up prior but if the surface temperature is too high the mr will flake.  Even though kept in the fridge, mr has a shelf life and it seems to vary between colors.  I'd usually do a light sand blast, carburetor cleaner, heat it and then submerge it in acetone as more oil will be lifted, then put it in a fresh batch of acetone a second time.

Without going into all the details, I have a water/oil separator before the airbrush, so unless that's stopped working it's probably not the issue. I degrease, heat parts in the oven for  15mins @300F, degrease a second time, preheat a 2nd time. Then I spray the part and periodically reheat with a heat gun until it's ready for final curing. If it's a larger part, like a shotgun barrel, I'll spray it until the moly resin stops flashing off when it hits the barrel, then reheat for 15 mins and spray more.

As an experiment, I resprayed the upper and lower and ran another cure cycle. I checked the finish with duct tape per the instructions, and little pepper-flakes of MR are coming off, but the anodizing is staying behind. This is still not "correct" behavior but I'll take it since at least the anodize is staying on the gun.
I think the Moly Resin has hit its expiration, finally, and it's time to move on.

('cept you did) ;)

But seriously, it sounds like you know what you are doing so it's highly unlikely technique. Sounds like the material degraded.

The mr could be past its date, when you heated your parts prior to spraying what temp did you bring them up to as I seem to recall 100+/- being the optimal temp?  For fun years ago I sprayed a kershaw knife blade that was always rusted on me as a test with the temp being right at 300 as someone on a forum suggested that temp as a flash cure or something, that coating or at least the color portion of it failed in short order flaked off with a finger nail rub, odd thing is that blade never rusted again so something was left behind.

I have never seen real anodizing "chip" off; it just doesn't do that.  It's a chemical conversion process, not any coating.  The best explanation here is your upper or lower were bare aluminum and then coated with something, before you coated it again with moly resin.  The reaction you got fits with what I've seen when one type of coating is covered by another type of coating.   Anodizing does not "flake" off (well real anodizing that's properly applied).  I think it is highly likely that someone screwed up in the supply chain and sold you coated, not anodized parts.  

I'd burn it all down to bare metal and start over.  Get the upper and lower actually anodized if you want, or just coat it, with one product.  

I'm not sure why more guys don't do this but if you have bare aluminum that you want to coat, but can't get any anodizing done, then give it an Alodine bath.  Used extensively in aviation to pre-treat bare aluminum before painting.  Check out Aircraft Spruce for price and availability.  Or go see any competent A&P mechanic locally and see if they can do it.  It's a very similar surface conversion coat to anodizing.  Gives you an excellent paint base, and good corrosion protection.  Not as durable as anodizing therefore makes a better "primer" or surface prep than a top coat.  But it will get in all the little nooks and crannies that paint can't reach.

I've also never seen anodizing chip, but here it is. When I used duct tape to test the adhesion of moly resin, it pulled bits of anodizing off the upper and lower; plus there was the chipping that happened when assembling the handguard and I even got flaking on the shell deflector side of the port door. I'm certainly willing to accept that the upper could have been poorly anodized, but also a lower that I purchased 6+ years earlier and had been previously coated without issue? I certainly don't have any answers, but to me the evidence points to the moly resin.



I'll probably send it to my buddy with a cerakote setup; I'd prefer anodized but not sure who will do just one part?

I've never heard of Alodine, but that's potentially really useful. I looked up the product (now called BONDERITE M-CR 1201 AERO), and I even have a local-ish airport. Thanks for the info!