On 13/04/2020 08:58, The Natural Philosopher wrote:
On 12/04/2020 21:50, Paul wrote:

https://www.hunker.com/13415944/how-...th-naval-jelly

An experiment I did with this years ago, when you rinse
off the rust, any shiny metal underneath *rusts almost instantly*
after the rinse. It takes a very short time for the surface
rust to come back. This has to do with the naval jelly etching pores
into the metal and increasing the surface area. You cannot
possibly apply a finish fast enough, to stop that from happening.
You would need a finish that can be applied right over un-rinsed
pure naval jelly (which is not going to happen).

I don't know what 'naval jelly' is, but phosphoric acid is not like
that, and phosphoric acid is what the OP was asking about.
IME phosphoric acid immediately converts any iron oxide to iron
phosphate, which is stable, and impervious to air and water.

And black, not shiny.

Once rinsed and dried its a good base for zinc primer.


That's my understanding too, hydrochloric acid gets the rust off faster
(tried it after advice from here a year or so ago) but the iron chloride
it is converted to is soluble and the fresh iron underneath is very
reactive in air, as you say the phosphate is insoluble and stable for a
while before you paint.