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Filing finish on castings.

To file further than the flash?

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James Hall 314/01/2022 22:38:13
76 forum posts
11 photos

Just removed parts of my first model from castings - a Stuart H10 - from its bubble wrap.

Now I know that it's necessary to file flash and any significant irregularities, and the worst has already been done by Stuart. Where the castings are filed they are, of course, relatively smooth and these areas contrast considerably withe the rough 'sand finish' on the rest of the same surface. I'm assuming that this difference in surface texture will show considerably after painting. Do people, on the whole, worry about this, or smooth by filing/sanding/whatever as much as possible - this would be very difficult on some of the more complex shaped surfaces.

Advice and experience would be much appreciated.

Paul Lousick15/01/2022 01:00:03
2009 forum posts
711 photos

It depends on what type of surface finish that you are after. Rough casting or Smooth painted.

Sand blasting with a big grit size will roughen smooth areas to look like a casting.

Use a Dremel type grinder with different shaped wheels to get into complex places. Fill cavities with a suitable filler. (I use JB Weld or a 2 pack filler that contains fibreglass strands). Spray on putty is good as an undercoat if you want to smooth a rough surface for painting. (Available in pressure cans and easy to sand)

Thor 🇳🇴15/01/2022 06:36:30
1598 forum posts
45 photos

Hi James,

I have just accepted that the areas of the casting that has been filed or sanded down may have a different appearance. Spray on putty works well for me so if you want a smooth surface follow Paul's advice. Good luck with your 10H. Check Harold Hall's website about machining the Stuart 10.


Edited By Thor 🇳🇴 on 15/01/2022 06:39:26

JasonB15/01/2022 07:08:08
22560 forum posts
2634 photos
1 articles

I'll usually run a Dremel with orange teardrop shaped stone in it lightly over the surface to remove and high spots which is usually enough to get a paint finish that still looks like it was a casting as there will be some slight surface irregularities. You can do more if you want a mirror smooth painted look. Scale also comes into it as a casting from a half scale traction engine will tend to need less prep than a 1/12th scale model to get the same size original cast finish to look right as the same sand is used for both castings

I did a Stuart 7" flywheel last weekend this shows it before. A good casting with little flash and well lined up halves, flash was filed off before machining, dremel after as I wanted to also blend in the turned surfaces that will be painted with the cast ones.

And after

That will most likely get a coat of high build primer, rub down and then final prime and colour.

This thread goes into a bit more detail of how I finish castings and a few of their "features"


Edited By JasonB on 15/01/2022 07:19:47

noel shelley15/01/2022 08:39:17
1278 forum posts
21 photos

Hi James, High build primer or spray putty are available in aerosol cans from most car spares shops. Fettling castings is a fine line between just enough and not to much. If the 2 halfs of the pattern were lined up spot on then there may not be much to do, or you may have to try and blend 2 poorly lined up pattern halves, the smaller the part the more difficult it will be. It all depends on what you want - show case model or just a working model ? Best wishes and Good luck. Noel

Circlip15/01/2022 11:21:31
1499 forum posts

Full size textile machinery, "Exhibition finish" cost far more than "standard finish", same paint but surface prep different. Customer decided how much they would be prepared to pay.

Regards Ian.

Ian Johnson 115/01/2022 11:29:38
370 forum posts
102 photos

Real sized engines have sand castings which are usually painted to give a very smooth finish, so on a scale model the castings should be very finely finished and painted.

It is difficult if not impossible to scale casting sand, so there is no reason why scale models need to show any gritty sandy surface.

If sand was scaled up, each grain would be half an inch diameter or so.

Ramble over!


Bazyle15/01/2022 14:12:09
6295 forum posts
222 photos

We sometimes talk about the scaling effect on the gaugeone3Dcircle (3Dpriting forum on If you look at an N gauge wagon you can see the planks delineated along the bottom but in full size they were not even chamfered and fitted flush to avoid trapping coal dust etc. But people expect to see the planks so they are modelled with effectively 1 inch chamfers or it doesn't look 'right'. The brain-eye interface does odd things.

James Hall 320/01/2022 18:50:55
76 forum posts
11 photos

Thanks to everyone for your advice.

I had initially been mainly worried about the contrast on the same surface between smoothed areas as a result of removing flash and the rough sand finish elsewhere. It now seems that more work is really needed to smooth off/fill the rest of the castings if a good-looking model is required.

I'm interested in having models that actually work but would also like their appearance to be as good as possible.

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