filling cast iron blow holes

I have a 12"x12" cast iron engine base that, in addition to a couple of chilled hard areas, has a series of small 'blow holes' on one flank. This do not create a problem to the structure but don't look nice so I want to fill them before prime and finish painting - but with what? The base is far enough away from the working parts not to suffer any great temperature gradient but it would be embarrassing if bits dropped out.

Tony

Vehicles have had bodywork filled, after accidents or corrosion, for decafes. Doesn’t fall out when properly applied.

A suitable key, for adhesion, is what is required.

Best stuff ive used is Plastic padding bodyfiller

I use JB weld, strong and a good shelf life. Can be machined/drilled/tapped when fully cured.

Chemical Metal is the best solution. Countersink the holes both sides so the filler is wedged in.

I would go along with JB weld but have also successfully used Araldite loaded with with cast iron filings / linishing dust. Warm the resin to help keep it fluid during filling of smaller holes.

Ideally use a carbide burr to clean and roughen the hole and clean with acetone and blow ou with compressed air (a bit like dental treatment!) and trim off any excess with a sharp knife or trimming tool when it is set but still 'green'.

Final flat into surrounding area will require filing and wet and dry sanding once the resin is fully cured. This treatment also works well for filling 'oops' holes in pillar drill tables.

I don't believe resin is strong enough to refix damaged items like snapped off mounting lugs, despite what the adverts say - happy to be proven wrong by others on this!

I'm with Vintagengineer for Chemical Metal from the Plastic Padding manufacturer, also follow John Paton method of cleaning before application.
Best to overfill a little as the material shrinks slightly after cure, saves applying another thin coat.

Emgee

Tony, before you fill the blow holes - you might think about treating the casting for those hard spots.

I've not done this with a casting as large as yours (12" x 12" ) but have done so with smaller (wheel etc) castings that seemed to have hard spots. Basically they've gone in a garden incinerator when I've had wood to burn (once it's well alight - with additional wood added for an hour or so afterwards) and have then been left buried in the hot ashes overnight. They then turned/machined well. I do this to some steel parts (to de-stress) them too... I can't think of any reason it wouldn't work for you.

I've used JB Weld to fill blow holes but it does 'shrink' - so usually needs a top-up. If you can pop it in a low heat oven that helps it cure. I've also mixed cast iron 'dust' with Araldite, which works well for cosmetic purposes (filling small external blow holes) but JBW would be a better choice where heat is involved or you need to machine it...

Regards,

IanT

Edited By IanT on 27/03/2018 11:04:05

If you want to be authentic the old boys used Iron-Filings with a dash of Ammonium Chloride mixed to a stiff paste with a little water. Inside the blowhole the mix corrodes and expands whilst bonding to the casting. Three problems: I don't believe it works particularly well as a filler; it takes a long time to set, and it's not compatible with many paints.

Dave

As it is only cosmetic I wouldn't bother with JB Weld or Chemical metal as they are harder to rub down. Car body filler will be quite adequate for this. I tend to use U-POL but whatever your local car shop has will do.

Once that has been applied and rubbed down very small imperfections are best dealt with using a high build filler primer, again I use U-POL "High5" and again rub that down when dry. This may also show up areas than are deeper so spot fill with your body filler or knifing putty.

Finally pirime with teh required colour to suit your topcoat, denib and apply topcoat.

Few pics of the process below.

Over fettled casting and roughing in of mismatched parts

First application of filler

Rubbed down

Filler/primer

Colour after grey primer

This post shows a similar process on another engine but also includes the prep of the raw castings

Production castings were always filled with a two part filler, very often the whole thing was filled to get a good finish. That was the difference in days now gone by between UK manufactured machine tools and East European imports.