Been doing this on all our trailers for 35/40 odd yrs. Scored 2 X5L containers lanocoate grease way back then. Our boat trailer is 40yrs old yet to have a weld repair other than couple mud guard support ripped on a fence post.
Trailers are well washed and water blast then let dry for few days.
If any surface rust, wire brush and phosphoric acid (rust kill) without the white epoxy additive.
Do not put the acid on any good galve areas, it eats the galv.
I 1/2 fill a jam jar with lanocoate grease and near fill with turps/ kero. With old brush slowly build up a soln of grease about 1/2 the jar.
Then paint well into springs back of wheels hubs etc. Remove the wheels , makes access for painting and spraying easy later.
Using a elcheapo super cheap panel beaters body under spray gun.. has a spay tube about 10/12".... 1/3 fill gun with lanocoate soln and top off with turps....I have topped off with fisholene fliud before.. cause 'was there'
My box chassis chassis rails are open each end
Important: hand a rag over the other end of the chassis rail from end going to spray do not then lanocoate will end up over the cars, neighbours windows.
Spray up one end the from the other, doesnt take much.
Spray chassis cross members , springs etc. forcing what hasa been painted further into springs and crevasses.
Whole process takes about 20 min not including painting springs.
Do not spray winches .
It remains tacky for a while..
After couple days in the sun its just tacky...doesnt pick up dust etc, but do not go over dusty metal roads for a week or so.
It leaves a slightly brown ting to the trailer.. not pretty but on the other had becomes a point of conservation as what it is and that the trailer is actually cared for.
I have tried the lanocoate in a can .. you will use a lot cans and just doesnt get deep in places really needs it, like a gun and compressor...and stays tacky far longer
Down side.. bit of over spray on the hull.. not really noticeable, but if fussy wipes off easy with bit turps and rags.
So thats 30 odd yrs of actual real hands on personal trial and error experience, and results, originally based on my grandfather spraying his farm machinery with waste oil in the 1950s and before.
Hope some may find it usefull.