I’m currently fettling a table lamp that has rust on the external side of the fount base plate. It’s been in evapo-rust for about 24 hours and with some light scraping away of the surface rust is starting to look quite good. Apart from the base which is rust affected, the fount is chrome plated. I’m trying to decide between coating the external side of the base plate with zinc galvanising paint versus converting any last rust spots with phosphoric acid and then coating with normal paint. I’ve searched here and CCF and can’t find an example where anyone uses gal paint on a baseplate. Is there a reason why it does not seem to be used?
I’ve used a relatively new Australian galvanising paint on lantern cages and chimney/globe base plates. It’s excellent. I’ll find it in the shed tomorrow and post a photo of the brand. EDIT: I found a photo: Tony
Regardless of which paint you use, the phosphoric treatment would still be a good idea to get rid of any rust the evaporust misses. If you get the baseplate free of rust it isn't going to make a lot of difference whether you use the galvanize or regular paint. Either way you will be sealing the metal and preventing contact with moisture which is going to prevent rust.
Basically, like what others had already mentioned. Honestly, I would suggest that you don't compare or decide between these as different options. The reason,...'cold galvanizing' or zinc-rich paints are very different from actual hot-dipped galvanizing or zinc-plating. Not merely different in the methods of which they're applied. More importantly for this purpose, is the way in which both offer rust protection. In zinc-rich 'galvanizing' paints, there is basically no cathodic-protection being effected as opposed to what would be possible with actual galvanizing. For cathodic protection, the zinc needs to be in direct galvanic contact with the underlying steel, something that's not possible in any kind of zinc-rich paints, due to the zinc particles being held and galvanically insulated from the steel by the resinuous binders in the paint. Hence, there is little difference between zinc-galvanizing paints and any other types of good paints. They only offer surface sealing or providing a barrier against oxygen and moisture. Whereas, actual galvanizing offers both cathodic and sealing protection for the underlying steel. Actual galvanizing is also much more resistant to abrasion. Since the base of the fount won't be heated, it would be advantageous to first apply phosphoric acid for residual rust removal. In addition, the phosphoric acid would also react with the iron to form a rust-retarding iron phosphate on the steel surface. This 'phosphotized or phosphated' surface is already a decent rust protection on its own. On top of that, it also offers an improved surface for paint adhesion compared to bare steel. Therefore, it'd be ideal to do both (phosphoric acid treatment, followed by a coat of zinc-galvanizing paint). You'd get better results of them in combination.
I wonder what is involved in zinc or manganese phosphating (parkerising) and if it has any useful application for lanterns ? - I know it is no good for stainless/nickel steels but for mild steel it should work.
There are many useful applications for those kinds of phosphating. Although in principle, they are somewhat similar in that phosphoric acid is used, they are done professionally in rather more elaborate, industrial-type processes where proprietary solutions based on the said acid are used. Be it zinc, iron or manganese phosphating, each would have their own areas of application, depending on the main objectives(corrosion protection, as a priming base for further coatings, wear protection, etc). I would not elaborate those here...way too lenghty and technical. I have seldom, if ever, come across any such processes being used for lanterns, especially classic ones. They would of course be irrelevant on stainless steels or brass. In my opinion, any parts made of steel or iron would benefit from such phosphate coatings...improved corrosion protection. However, they are not effective on highly heated parts because they need water in their molecular structural made-up. If the phosphate coatings are somehow dehydrated by strong heating, they'd loose their structural intergrity, strength, properties and usefulness. They are also not used where electrical conductivity on the parts is a requirement.
@MYN @Fireexit1 Thanks everyone for the advice. I think the next step may be removing some small remaining rust spots mechanically with a nylon thread abrasive wheel which I hope is gentler on the base than a wire wheel. I’ll then try the phosphoric acid and the zinc paint. I’ll post a photo of the finished fount once I’ve completed the steps.
