Thread: Molasses rust removal- post-tank treatment?

I've got hold of a number of old milk cans that have been sitting in a disused dairy shed for several decades; I plan to use one of them to replace our rusted-out milk can mailbox, will probably make gifts of a few more.
I have had very limited experience of molasses rust removal, used it on some superphosphate applicators for potato planters about 35 years ago and the post-treatment finish involved bolting the super feeders back onto the spud planter.
I got hold of a 200-litre plastic feed bin, diluted 20 litres of molasses into it, so very roughly 10;1 water / molasses. Pressure-washed the first milk can and then immersed it in the mixture, pulled it out and hit it with the pressure washer about 10 days later, not quite cooked so put it in for another 10 days. Pulled it out and washed it off again with the pressure washer. It came up beautifully, grey bare metal like it had been sandblasted.
The obvious problem, though, is that once I wash the molasses off, the thing is wet with water, leaving it to dry I come back and it is covered in surface rust.
I'm after some info on the appropriate way to treat it after the molasses, should I give it a coat of Penetrol straight over the surface rust, or is there some other thing that is usually done? I'm not interested in getting them bead blasted or some other expensive process, that would make the whole molasses thing pointless. What's the usual thing to do with parts between a molasses bath and a coat of paint?

I usually use compressed air for small parts to blow/dry the water off. For something that size a leaf blower might be better. I’m a Penetrol fan - so would use that followed by some epoxy enamel.

Steve

Compressed air is the go but getting the water out of the inside that way is going to be a PITA. If you heat them to dry they will rust immediately. Mind you it will be thin surface rust so a good coat of epoxy enamel will take care of it.

With something like vinegar or citric acid you don't have to wash them with water - blow dry what you can and then put them on a hot BBQ or shove a hot air gun up the opening and heat them up till they are bone dry and then paint them. Oxalic acid is even better. The problem is the large volumes of acids you will need.

Electrolysis might be better - washing powder and electrodes for the inside and outside. Use Carbon welding rods as teh pother electrode they make much much less mess.

OTOH I'd just paint them with a couple of coats of rust converter and paint directly over that.

Find out what people do after sandblasting. Because that flash rusts pretty quick as well.

If your definition of quick is several months then yeah after sandblasting it rusts quickly. I have parts I blasted while we locked up in November last year still sitting untreated that have not rusted. My work space is far from hermetically sealed.

Flash rust occurs in general because of exposure to oxygen with remnants of chemicals or incompletely neutralised chemicals on the part, not generally something you see after sand blasting.

The first of the cans was taken out of the tank when I only barely had time to rinse it with the pressure washer and then leave it sitting upside down to drain overnight, didn't get back to it for two days and it had a lot of surface rust, especially around the inside of the neck where the water from inside had run down- although looking at the photo now that is probably staining from the molasses residue running down as well. I think my plan of attack with the next one (in the tank now) will be to take it out when I have plenty of time to rinse it, blow dry it and then coat it with penetrol straight away. The first one can go back into the tank for a day or two. I found a few blogs with descriptions of using molasses for old car parts and such but no real details about what they did between molasses and painting.
This is the first can after two weeks in the molasses and four days out of it.
milk can surface rust.jpg

Originally Posted by .RC.

Find out what people do after sandblasting. Because that flash rusts pretty quick as well.

The bloke who has done sandblasting for me blows the things off and then primes them straight away. I guess the big difference is the molasses has to be washed off, so the part is wet when the metal is at it's most vulnerable.

I cleaned up a milk can and a cream can pretty much identical to that one pictured , the cream can being the smaller size. Immersed them in hot Citric acid , wash down with a cheapo pressure cleaner, used a hot air gun to dry off . Then went over them with a wire brush . Painted with epoxy primer .

Usually, those cans have the farm or property owners name hand stamped onto a metal tag . These cans were still in use into the 1970s around Gippsland.

Originally Posted by morrisman

Usually, those cans have the farm or property owners name hand stamped onto a metal tag . These cans were still in use into the 1970s around Gippsland.

This one has the farmer's name stamped into the angled part below the neck- was clearly visible when it came out of the tank, obscured again now by the surface rust. Not that there's any mystery about it, the (now retired) farmer is a family member And a hoarder.