Filling a blow hole in aluminium casting - Help!

[Mike Ginn]

I have some blow holes and a depression in a web on the Kiwi crankcase casting. Normally I would fill this with xxepoxy based compound and when painted it wouldn't be noticed. For this model I would like to have a sand (glass) blasted surface which looks really good but-but-but I have these small holes. I thought I could mix some aluminium powder with clear epoxy into a paste and use that as a filler but it is far too dark and sand blasting simply shows it up. I concluded that the resin was not allowing the fine powder to become visible. The next step was to file a block of aluminium and use the filings to make a paste. It worked and could be filed into a slightly aluminium type surface but when blasted it darkened unlike the casting which turned a nice silvery colour.

Do any of you guys have a magic way to fill al castings which could then be blasted. I am dismissing any form of welding or even the low temperature solder sticks as they have the potential to wreck the casting.

My current fall back is to paint the crankcase- probably dark green, leave the cylinder polished and the head black. Brass parts sand blasted and clear mat varnish.

I would appreciate your views
Mike

 

[SmithDoor]

I have some blow holes and a depression in a web on the Kiwi crankcase casting. Normally I would fill this with xxepoxy based compound and when painted it wouldn't be noticed. For this model I would like to have a sand (glass) blasted surface which looks really good but-but-but I have these small holes. I thought I could mix some aluminium powder with clear epoxy into a paste and use that as a filler but it is far too dark and sand blasting simply shows it up. I concluded that the resin was not allowing the fine powder to become visible. The next step was to file a block of aluminium and use the filings to make a paste. It worked and could be filed into a slightly aluminium type surface but when blasted it darkened unlike the casting which turned a nice silvery colour.

Do any of you guys have a magic way to fill al castings which could then be blasted. I am dismissing any form of welding or even the low temperature solder sticks as they have the potential to wreck the casting.

My current fall back is to paint the crankcase- probably dark green, leave the cylinder polished and the head black. Brass parts sand blasted and clear mat varnish.

I would appreciate your views
Mike

View attachment 149064

I never use epoxy on casting.
It will let down soon or later.

Use MIG or TIG it will hold for ever.

I can tell how many time I would run cross a good engine but there was hole in casting.
All had to do was remove the epoxy from casting and weld the hole. Now I have a rebuild engine.

Dave

 

[abby]

Actually there is a low temperature aluminium solder which would not harm the most delicate of castings .

 

[SmithDoor]

Actually there is a low temperature aluminium solder which would not harm the most delicate of castings .

I like

I have welding and aluminum solder is more costly.
It is great if do not have welding and fixing holes.

Dave

 

[Scott_M]

Hi Mike
The problem that you will run into with any kind of fill, be it welding or some other kind of fill is that it will be a different hardness which will sand blast a visibly different shade or color. Something very soft may even get dirty from the blasting and any weld will most likely be a different grade of aluminum and will show after blasting. Even if you use the exact same grade of aluminum, after welding it will be a different hardness in the heat affected zone and will not take the impact of the blasting the same as the rest of the part and will show.
I do not think you can get an invisible repair after blasting and will have to paint it.
I do a lot of TIG welding and have been dealing with this problem for many years. The only way I have gotten around it is to weld with the exact same material then anneal the part and re heat treat if necessary. It can be a real pain.

I think your Kiwi will look very nice painted

Scott

 

[Jasonb]

I assume Mike said he does not want to weld/solder as the crankcase is already machined.

As said any for of filling is likely to be a different colour once bead or sand blasted so paint may be the best option.

I quite like the look of a black or red crankcase particularly if not bright gloss and possibly a bit textured, have a look at some of the engines in this album for the kind of finish 1 Cylinder Model Engines

Another option although slightly darker is Rustoleums "cast iron" which has a bit of texture and is not to shiny, I used it here on my RMC typeB

 

[Mike Ginn]

Thanks Janson. I have concluded that painting is the way forward and I really like the look of "cast iron" paint. Never used it but its now on order. I also have some matt silver paint so I will experiment with that and others. Fortunately blasting gets me back to the start.

Thanks for the link to the photo archive - great pictures and it's good to see the paints people use - fascinating engines!

Thanks Guys for your comments on hardness - I hadn't really thought of that but it clearly would be an issue.

Mike

 

[MacKendrick]

Hi Mike
The problem that you will run into with any kind of fill, be it welding or some other kind of fill is that it will be a different hardness which will sand blast a visibly different shade or color. Something very soft may even get dirty from the blasting and any weld will most likely be a different grade of aluminum and will show after blasting. Even if you use the exact same grade of aluminum, after welding it will be a different hardness in the heat affected zone and will not take the impact of the blasting the same as the rest of the part and will show.
I do not think you can get an invisible repair after blasting and will have to paint it.
I do a lot of TIG welding and have been dealing with this problem for many years. The only way I have gotten around it is to weld with the exact same material then anneal the part and re heat treat if necessary. It can be a real pain.

I think your Kiwi will look very nice painted

Scott

Drill a

Thanks Janson. I have concluded that painting is the way forward and I really like the look of "cast iron" paint. Never used it but its now on order. I also have some matt silver paint so I will experiment with that and others. Fortunately blasting gets me back to the start.

Thanks for the link to the photo archive - great pictures and it's good to see the paints people use - fascinating engines!

Thanks Guys for your comments on hardness - I hadn't really thought of that but it clearly would be an issue.

Mike

drill a hole and press in a pin of the same material.

 

[TonySteamHobby]

You might look into powder coating. I have done many colors which look like bare metal, iron, aluminum, copper, silver, gold…
I use an old oven, but I also have an old, large toaster oven which works very nicely.

 

[ranger]

In the UK, have a look on “Frost Automotive Restoration” website. All types of paints, coatings, etc. many high temperature. Some quite expensive, but there’s just about everything you could possibly need, paints, powder coating kits, anodising kits, electroplating gear, etc. well worth a look! I believe that they offer a ‘Cast Iron’ effect paint that can even be used on turbo chargers and exhaust manifolds!

 

[Mike Ginn]

Ranger - thanks for that link. I have not come across the supplier before and apart from some expensive stuff they have a range of "ceramic" type paints which look very good . I favour spray finishes and I have the equipment. I considered powder coating but its new to me and I therefore don't understand its characteristics. I have 2 types of paint on order and will try them out for both application and looks.
Mike

 

[Bentwings]

You might look into powder coating. I have done many colors which look like bare metal, iron, aluminum, copper, silver, gold…
I use an old oven, but I also have an old, large toaster oven which works very nicely.

I’ve seen my fill f powder coating any repair is complicated by trying to get the coating off so a repair can be made as far as I kno nearly all racing organizations have banned powder coating as it often covers critical cracks and defects . I’ve repaired aluminum casting and matching a casting texture seems more like bling to use a dumb expression . I’m more interested in a quality repair . And functionality .

 

[Lakc]

I would say you have a perfectly practical form, put a thick coat of paint on it and use it to make a new casting.

 

[Charles Lamont]

I’ve repaired aluminum casting and matching a casting texture seems more like bling to use a dumb expression . I’m more interested in a quality repair . And functionality .

But in the OP's case the problem is not a structural repair, it is a purely cosmetic issue with a casting that is functionally sound. He wants the casting to look like a pristine casting because he wants to take a pride in making a nice looking model.