View Full Version : Color-matching filler for cast iron?

Doc Nickel
01-06-2014, 05:57 PM
I know this has been asked before, and I've read some of the older threads, but I'd like to hear if anyone's found anything new. Basically looking for a filler, epoxy or epoxy putty (etc.) that is a reasonably close color match to machined cast iron.

In this case it's just filler, so strength and wear issues are secondary to color. I know the patch is unlikely to be "invisible", but it'd be nice if it didn't stand out too badly.

I've looked at most of the local stores, and all their stuff is either "dark grey" or a "wood tone". Anybody have any good suggestions or recommendations? If you've tried it, have any pictures showing how it came out?


01-06-2014, 06:27 PM
A few years ago I used JB Weld to fill some dings on my newly acquired lathe's compound - it lost a fight with a chuck during some violent incidents with its previous owners. Along with the oil etc. that has come into contact with the filler, it has become slightly darker than the cast iron - but has been a non-issue for me from a cosmetic point of view.

There are other iron-filled epoxies available that may do a better job but I wasn't prepared to check them out. Perhaps a clear epoxy and some cast iron filings would work.


01-06-2014, 06:28 PM
Closest ive seen is that belzona, however an idea sprung to mind, i have seen a casting resin used for chess sets and models that was a metal loaded epoxy that was really close to the colour of a milling machine table, suppose its got a hole or two, mine has, annoying.

Mark Rand
01-06-2014, 08:15 PM
Probably not what you want to hear, but I found that mig welding with carbon steel to fill marks followed by surface grinding, produced a completely invisible fill for battle scars on a drill table. Obviously, strength and stability weren't guaranteed, but it looked and measured as good as new afterwards.

Doc Nickel
01-06-2014, 08:26 PM
Actually it's for a lathe. There's a casting flaw- a shallow crater- on the shift quadrant of the QCGB. The surface was originally ground somewhat smooth, but then painted. I'd like to leave the ground (raised) area as somewhat-shiny bare metal, to offset the new paint, a darkish grey. The 'crater', however, looks like damage- almost a small bullet hole- so I'd like to fill it in. If I were going to paint the area, I'd just use Bondo or whatever metal putty I have laying around, but since I'd like to leave it bare, it'd be nice if the patch was relatively close in color.

I do have some Muggy cast-iron welding rod, and I'd considered trying it, but it's just a cosmetic issue. I'd prefer not to risk cracking or warping the gearbox casting just to make it look a little better.


01-06-2014, 09:32 PM
Do not know if this will suit your needs/tastes, but
Seymour markets a paint called Cast Blast which
auto enthusiasts find useful enough that you
ought to have an easy time finding it locally.



01-07-2014, 06:16 AM
It is possible to get a reasonable colour match with some of the plastic fillers, but only initially, with clean cast iron. When you apply the filler, you naturally have the cast iron as clean and oil free as you can. After machining /filing down, the match may look pretty good, but as soon as oil touches the surface, the colour of the cast iron and the plastic look different again. The surfaces just absorb oil in different ways and this shows up. Perhaps you could mix a filler that would not behave like this, but this has been my experience. Personally I think the best reason to apply a filler is to stop holes filling with chips and causing problems, so colour match is a secondary issue.

01-07-2014, 07:23 AM
IIRC, "Devcon" makes a few variations on fillers specifically for metal and, based solely on photos, one of them maybe close (I have not used them but have seen all over the years...they have ones meant for steel, titanium and aluminum, perhaps more variations, and all are slightly different "grey", point being I guess is they are trying to color match but as stated it really depends on how clean the base material is regarding color).
Some of the other two part epoxies you maybe able to get closer by varying the amount of one of the parts (Permatex used to have one that was a mix of grey and white components...now how that may or may not effect the bonding, I don't know...YMMV)

01-07-2014, 10:20 AM
I've used the coloring dye that Brownells includes with its Acra-glass epoxy products to darken JB Weld. A little dab will do it and if you sneak up on it you should be able to get a fairly close match. The dye does not affect the performance of the epoxy and I think you can buy it separately.

01-07-2014, 11:38 AM
If the color is really important, and in this case it is important to you. My suggestion would be to use an endmill to create a shallow flat bottomed hole and then plug it with a turned plug of cast iron. The hole and plug could be threaded or jus a nice tight "Loctite fit". Experiment on some scrap to see if it works before trying it on the machine.

Doc Nickel
01-07-2014, 02:53 PM
Thanks for the suggestions, but as I stripped more of the paint and the ... filler, I guess, like a really thick primer layer, the casting turned out to be rougher than I thought. Nothing major, it's not any kind of a structural issue, it's just that I hoped it'd be smoother than it was. Looks like it'll have to be painted anyway...