Painting aluminum castings problems and questions.

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hopperwhistle

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I recently finished this Ericsson model which I also posted under finished projects. As a former automotive painter I painted this exactly as I had been taught to paint aluminum, first using an aluminum etching primer then a high build primer. The resulting paint job chips right down to bare metal with even the slightest bump, not peeling or flaking just chips incredibly easy. Does anyone have any suggestions for a durable painting process to use on aluminum. Also does anyone have any thoughts on an efficient way of stripping the paint back to bare metal without many hours of hand sanding little parts? Ive had great success stripping paint using an ultrasonic cleaner but Ive also had aluminum badly etched and pitted from the wrong cleaner in an ultrasonic machine. Im a little hesitant with how to proceed with preparing to repaint.
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Hi !
Before painting aluminum I usually clean the surface with acetone - must be very clean . Do not leave the aluminum surface for too long after sanding the surface
About paint remover, you can buy a paint remover
Note : Use a small piece of aluminum for testing !!!
 
The surface was cleaned with with lacquer thinner then automotive grade grease and wax remover. It seems the etching primer just didnt work. Im hesitant to use any stripper unless someone can offer their own experience regarding a specific product that has worked for them. I also try to avoid harsh chemicals in my shop if it can be avoided. The paint was a single stage paint with no hardener so I wonder if the parts could be soaked in something like alcohol or thinner without being harmed?
 
I rubbed my aluminum parts with household rubbing alcohol, and then painted them with two coats of Rustoleum (no primer used).

I have never had any chipping problems.



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Very nice paint job Green Twin. I never even considered not using a primer. I did use the same Rustoleum paint and was impressed by the high gloss and smooth finish I was able to achieve. I did put a skim coat of body filler over all the castings to smooth out all the imperfections, I wonder how the Rustoleum adheres to unprimed body filler?
 
I talked to the guy who reproduced Cretors popcorn wagons (Bob Pearson in Kansas), and he described how he made the reproduction glass that has superficial surface chipping.
He said the trick was to use a glue that contracted when it dried, and then it put a horizontal force on the glass, causing it to make decorative chips in the surface.

I would guess that the cracking may be due to srhinkage of the paint, or differential shrinkage between types of paint, ie: primer and final coat.

I have purchased house paint from Sherwin Williams made specifically to cover plastic siding, and apparently it has some flexibility when it dries.

The Rusoleum seems to dry hard, but it does not ever seem to crack or peel off.

You may not get as good of adhesion over the filler material, but I would think it would not crack unless you bumped it or otherwise disturbed it.

Just guessing.

I like the Rusoleum because it seems to level out to some extent.
I don't put an automotive finish on my models because I want them to look like the original engines from the 1800's, and the old engines always had some roughness on the surface from the foundry sand.

The second coat seemed to really help give the model a full and glossy look, for some reason.
Perhaps the paint is somewhat translucent, or perhaps the eye can see the slight increase in thickness and a slight convex affect.

Here is an example of a filled and painted 3D printed pattern.
The look and feel of a filled pattern is very noticable, compared to just a flat 3D printed surface.
It is the convex nature of a filled surface, along with the gloss that reflects light, that gives it eye appeal.

The old foundry patterns had convex surfaces to allow them to be pulled from the sand mold, and I think inadvertently that added to the look of an old engine in a positivve way.

The ancient Greek and Roman temples had curvature added to the horizontal surfaces to corrrect distortions by the eye.



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My experience is that paint does not stick to aluminum reliably without being cleaned thoroughly and primed. Additionally, the only primer that I know of that actually works is zinc chromate.
This is a very nasty carcinigen, and must be treated with the most care and respect. Don't get it on you and don't breathe the fumes. I do my spraying outside and make sure I'm upwind. Light coats work well, don't spray it on too thick. Zinc Chromate primer is not widely available, but may be purchased from outboard marine engine supply and repair places. They use it because it works on outboard engines used in salt water, and is reliable and lasts a long time.

After priming you can use a brush on oil base paint or a spray paint for a nice reliable finish.
Hope this helps

maury
 
Maury-

Nice to see you hear.
Maury is the master of pattern making and casting kits, for those who do not know him (ie: Lone Star Ball Hopper Monitor kit).

Its like I tell people who do foundry work though, don't solve problems that you do not have, such as degassing aluminum melts when you are not having gas issues.

I have not had any problems with Rusoleum delaminating or chipping off of aluminum surfaces, and I just did a light rub with denatured alcohol as a prep.

So before going to any extremes, I would suggest Rustoleum on cleaned aluminum (cleaned with rubbing alcohol), perhaps on a test piece, and if that works, then problem solved.

I would love to see Maury post about his development of the Ball Hopper Monitor patterns and kits.
That would be a fascinating story for sure.

Maury basically inspired me to start making my own patterns and castings.

.
 
You offer some interesting insights green twin, however in this case differing paint shrinkages seems unlikely as the layers were each allowed to fully dry between coats. I have seen shrinkage problems with paint before and it causes different problems mainly surface cracks and wrinkled finish as it dries. This problem is an adhesion issue.
Thanks for the zinc chromate tip, I vaguely recall that stuff from my painting days but never actually used it. I painted aluminum bodied vehicles on occasion but always had good results with an etching primer as the first coat. I’ll probably try the rustoleum on bare aluminum, sounds simple and easy to sand and respray if needed. Now to figure a simple way to strip it all again...
 
The surface was cleaned with with lacquer thinner then automotive grade grease and wax remover. It seems the etching primer just didnt work. Im hesitant to use any stripper unless someone can offer their own experience regarding a specific product that has worked for them. I also try to avoid harsh chemicals in my shop if it can be avoided. The paint was a single stage paint with no hardener so I wonder if the parts could be soaked in something like alcohol or thinner without being harmed?
Hi,

Be sure not to use any Alkaline product (such a Caustic soda or washing soda) on aluminium, it will remove the paint but attack the surface of the metal.

TerryD
 
Aircraft paint shop for stripper. I've stripped a couple of a/c for painting, the stuff was formulated for aluminum. Friggin expensive in a 5 gallon pail but you might be able to get a donuts worth at the airport. Zinc chromate can be sourced there too. On line supplier Aircraft Spruce will have good info and products too.

John 🇨🇦
 
Hello,

I had the same problem when I painted my Ericson engine. Are your castings from Myers?
My castings has some zinc added to aluminium. They feel very smooth even when sanded with emery paper and paint does not want to stick to it.
I used a normal PU primer and paint. It did not work. Etching primer is very expensive in India about 60-70USD per spray can.
I am thinking about using a two part epoxy primer to paint the engine.

What will happen if I dunk the parts in mild acid solution before painting? My thinking is that acid will eat into the part making it easier for paint to adhere.

Nikhil
 
The surface was cleaned with with lacquer thinner then automotive grade grease and wax remover. It seems the etching primer just didnt work. Im hesitant to use any stripper unless someone can offer their own experience regarding a specific product that has worked for them. I also try to avoid harsh chemicals in my shop if it can be avoided. The paint was a single stage paint with no hardener so I wonder if the parts could be soaked in something like alcohol or thinner without being harmed?
About paint remover, I only used it once but It was quite a while ago, and I asked for some from a friend
Find a few paint removers you can buy and ask your supplier for more information about it
If you can buy a small amount, enough for a few uses, that's fine, otherwise...maybe use your hands. 😅

Try soaking it in Butyl Acetate / Acetone
 
If you are going to take the engine apart to remove the paint perhaps you could grit blast it? I don't mean the sort of grit used on rusty metal, but some sort of plastic material. It might even leave a surface that will take paint better.
 
Hopefully, you are sanding or ruff scuffing all of the surfaces that you want to paint.

Alcohols are not always going to get all of the cutting oils out of the pores and pockets in aluminum castings.

A soaking in Lacquer thinner or other strong solvents, then heating to dry is very helpful, if heavy cutting oils are used.

Body fillers do not like to bond directly to bare aluminum.

As mentioned, a etching primer is used for a bonding agent, a zinc chromate is best, Epoxy paint is also used as a base and sealer. Just like everything else, the set-up, or prep is the key to success.
 
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Wow tons of good insights. Interesting someone else had the same issues with the same Ericsson castings. I like the idea of aircraft stripper and the blasting with plastic media or perhaps soda blasting. I can reiterate and stress Terryd’s comment, dont use washing soda in an ultrasonic cleaner with aluminum parts... your parts will not be happy when they come out. Responding to chiptosser, I personally have never had any bonding issues with filler on aluminum, if that is of concern you can use an all metal filler with high aluminum content, but its unpleasantly hard to sand. Just for fun I sprayed a bare piece of scrap aluminum with Rustoleum as Green Twin suggested, its surprisingly tough and adhered very well, its not chipping, seems pretty scratch resistant, even when flexed the paint doesn't crack or peel. Im impressed.
 

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