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I have a set of tools (handplanes, to be precise) currently swimming in a molasses bath for de-rusting. I can find a lot of comparison data with other de-rusting methods, but not so much information on what to do after the treatment.

It appears imperative that the tools be treated immediately after being rinsed out to prevent flash rust from forming. But with what? Seasoning them with oil (like a cooking pan)?

In case it matters, the tools in question have cast iron main bodies with nickel plating, some brass hardware, some screws and spurs/knickers which are possibly steel, and some plane blades.

References:

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not so much information on what to do after the treatment.

I think part of the reason for that is that it's simply not that critical. Just as with removing the rust in the first place there are multiple approaches to cleaning up the tool or part after the major rust removal. First step is drying off by some method, including speed-drying with a heat gun or in a low oven1, drying off with rags or paper towels or blowing dry using compressed air

Any or all of these can be followed by spraying on some WD-40 or similar, wiping over with an oily shop towel, or nothing2.

For some background I've been using molasses solutions for rust removal for about five years now. I don't have a particular problem with flash rusting, although I used to and still do get it sometimes. Regardless and more importantly it's not a problem, even cosmetically, because flash rusting is extremely superficial and the very next step, that you're probably going to be doing anyway, can easily take it off and stop it from recurring.

Since things generally come out of rust-removal soaks dull and grey if only rinsed off almost everyone will next do something that will shine up the metal to some degree, and this should simultaneously stop the potential for flash rust. This can be brushing over with a wire brush, rubbing with steel wool, using a wire wheel in a grinder or drill, and sanding of course, including any lapping of reference surfaces you might be doing.

Once you've exposed fresh steel or iron that hasn't been acted on chemically flash rusting stops happening in most cases. The naked metal obviously needs to be protected but you're going to oil or wax it either right now or in just a few minutes anyway...... right?

If you intend to paint
For anything that you want to paint the dull original surface might be just what you want since this provides a superb 'key' for paint or primer to cling to. In this case I think you need to alter your approach:

  • as soon as they're out of the molasses solution scrub the parts thoroughly either under running water or in soapy water (rinse well after);
  • dry off as quickly as possible (do one part at a time, don't put one part off to one side wet and start scrubbing the next);
  • if necessary wipe over with denatured alcohol (UK: methylated spirits), acetone or lacquer thinner (UK: cellulose thinners) to drive off any remaining water that you can't see;
  • prime or paint ASAP.

1 Does not stop flash rusting (!) but it prevents it building up to a heavier level. Heating the wet steel can instantly cause it to go slightly brownish in front of your eyes. But as I say, this is really not a problem.

2 Doing nothing can surprisingly be OK, but the more humid your environment and the longer you intend to leave the piece before doing anything else to it the more you should apply something to the surface.

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  • Thanks! great extra on repainting. Your list excludes turpentine which is not per-se a cellulose thinner, depending on definition. But I suspect it might help drying too. Steel wool might be the only option to clean things up in my case, as I think anything more abrasive will damage the coating, which already seems fragile in spots. very comprehensive answer.
    – ww_init_js
    Apr 12 '18 at 21:28
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    Japanning and the paints that replaced it later tends to be very very tough so some rubbing with steel wool shouldn't harm it (if in good shape). But other paints used elsewhere on some planes aren't as robust so sometimes there's no option but to accept that some, most or all of it will come off during the restoration process. You can then leave the metal unpainted or repaint as you see fit.
    – Graphus
    Apr 14 '18 at 9:07
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After cleaning the parts and drying, a light coat of oil will prevent them from rusting. The application of phosphoric acid is to convert any remaining rust to a more stable compound.

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