by Brian Verbeek
I'm looking into easy to use do-it-yourself chemical treatments and this is what I learned about chemically derusting and preparing metal for further treatment:
Phosphoric acid: It costs about $2.50us per Kilo (20 liters is about 31.5 Kilo). It can be thinned with water (I don't know the best ratio). It works best unthinned, but then you must keep watching the process as it eats metal. This is called "phosphating". VW does (or did?) it with most steel parts before they were painted--it gives an iron-phosphate coat on the part and, unlike galvanizing, it reaches all holes and crevices. It's a good thing to do to any steel part that will not receive any other treatment before paint or primer. Phosphating will disturb the galvanizing process--it gives marginal corrosion protection compared to galvanizing. Visible phosphating can be found on computer chassis, the flat gray metal color finish. A slightly better corrosion protection can be had by using a special phosphoric acid that contains zinc. The process is then called "zinc-phosphating".
If you want to remove the rust, or only clean the parts prior to galvanizing or any other electroplating, use sulphuric acid (I hope that's the correct translation, we call it salt acid) which does not leave a residue. Both phosphoric acid and sulphuric acid will remove a lot more than a regular solvent you'd use to degrease a part prior to painting. The risk of rust will be reduced by a lot.
Both treatments are very simple to do. Just take a suitable container or bucket fill it enough so you can emerge the part in the stuff and see how it goes. Both chemicals will not strip paint so you'd need to do that first. Don't put an alloy part in there of course :) and, needless to say, these are very corrosive chemicals, very nasty stuff. If you have a safe place to keep them you can use them over and over, but I wouldn't know when it's used up. In my humble opinion, phosphating is a lot better than sand blasting. Sand blasting increases the surface area (and thus the risk of rust) by a high (I forgot how high) percentage and it cannot crawl into a folded seam or weld.
I think it'll become problematic to use this
process with the fanshroud. I'll look for a company that can dip it
into something. I got a tip from my galvanizer. I asked what he
would prefer me to use on the bare metal to prevent rust and if he
had problems if I used motor oil. "No," he said, "if you're
stripping the paint you must rinse with a lot of water. After that
you fill the sink with water and a little dishwashing soap (a
little more than you use for dishwashing). Wash the part again and
let it dry. The soap residue is enough to protect against quick
rust, and it removes real easy." The galvanizer can emerge it in
sulphuric acid as he always does and the soap is
gone.
