Naval Jelly is just phosphoric acid. There's a chemical reaction
whereby phosphoric acid reactos with iron oxide to produce ferric
phosphate, which is a black material that you can paint over.
'Phosphoric acid - Wikipedia, the free encyclopedia'
You don't have to buy Naval Jelly. Gelled phosphoric acid is the active
ingredient in most toilet bowl cleaners. If you go to your local home
center, and read the contents label on the toilet bowl cleaners, you'll
certainly find several containing phosphoric acid.
However, there's a better way in my opinion to fix a rust spot on
The first thing is to dissolve the rust using hydrochloric acid.
Hydrochloric acid is the active ingredient in toilet bowl cleaners.
Most toilet bowl cleaners that don't use phosphoric acid will us the
very much more aggressive hydrochloric acid as the active ingredient.
You dissolve the rust with hydrochloric acid.
Then you wash the bare exposed metal with water.
Now, to remove the water COMPLETELY, dry with a paper towel, and then
rinse the exposed bare metal with any rapidly evaporating solvent that's
soluble in water, such as isopropyl alcohol, methyl hydrate or acetone.
When you introduce the alcohol, methyl hydrate or acetone, any water on
the surface of the metal will dissolve in the alcohol, methyl hydrate or
acetone. When you then absorb that solvent with a paper towel, any H2O
molecules dissolved into the solvent will be absorbed into the paper
towel along with the solvent. So, rinse 2 or 3 times with alcohol,
methyl hydrate or acetone, dabbing up the liquid with a paper towel
after each rinse, and then let the solvent evaporate. Now you have the
bare steel without any moisture on it at all.
Now, mix up some epoxy, and put it on a small piece of cling wrap. Turn
the cling wrap over and center the epoxy over the bare steel and set the
epoxy down onto the bare steel. I prefer using marine epoxy because it
has a 60 minute working time, and that gives me plenty of time to work
Now, dip a finger in dish washing detergent and smooth the epoxy patch
under the cling wrap. The soap will act both as a lubricant and a shock
absorber so that you can form the epoxy under the cling wrap into a very
smooth patch. If you feel the patch is too big or thick, simply pull
the cling wrap off (and some epoxy will come off with the cling wrap)
and put another piece of cling wrap down on the remaining epoxy and keep
smoothing with a soapy finger.
Then, allow the epoxy to cure with the cling wrap still on it. Once the
epoxy is fully cured, you should be able to peel the cling wrap off of
it easily. If the cling wrap doesn't come off the epoxy easily, just
scrape the cling wrap off the hardened epoxy with your fingernail.
Now, paint the hardened epoxy.
The above is how I repair chips in enamel steel bathtubs in my building
that are starting to rust. Bathtub chips are critical to repair well
because that chip could eventually rust through and cause a leaking
So, phosphoric acid will work, but you can do a better job if you use
hydrochloric acid to dissolve the rust and then use epoxy to coat the
bare steel to prevent further rusting.