Does rust paint primer stop rust?

[DaveC426913]

TL;DR

Should I have ground the rust off this steel project before applying primer? Should I go back and do it?

I'm making a thing - a trailer hitch accessory - out of steel leavings (from a steel shop). (This is not a functional gadget; it is purely aesthetic. It is removed when the hitch is in use.)

The steel wasn't rusty when I brought it home, but it's acquired a few light patches of rust as I've worked on them over a couple of weeks. The worst spot is where two pieces rested on each other, and I guess moisture accumulated and made a patch of rust a half inch square.

I had intended to do a light grinding to remove oxidation and expose fresh surface before painting, but I got the two steps mixed up and foolishly decided to give it a light coat of primer in prep to grinding/rasping/sanding off any last imperfections. Now I can't tell where the rust spots might be.

I'm using standard Tremclad rust paint.

Now that it's kind of too late, have I doomed my project to rusting out? Should I take the time to grind/sand off the primer? Or am I OK?

(Wait, am I using the wrong term? This isn't rust rot; this is light surface oxidation.)

 

[Baluncore]

Remove loose surface rust with a wire brush or grinder.
Treat the surface with rust converter = phosphoric acid.
When it is dry, the rust will be converted to black.
Prime the surface with Tremclad rust paint.

If you did not prepare and rust convert the surface, don't go back.
Rely on the hope that Tremclad will cover and seal any surface rust.

If you can see rust flakes through the primer, you will need to go back to chip the flakes, and prepare the surface again.

 

[DaveC426913]

Thanks OK. It sounds like I don't have to start again.
I just ground off the spot where it was the worst. And even that was just surface oxidation.

Pics to follow.

 

[DaveC426913]

Treat the surface with rust converter = phosphoric acid.

Yeah. I used this stuff on a couple of boat anchors I salvaged from the junk heap at my sailing club. Pretty darned magic stuff.

(Now all I have to do is convince my kids that we want a nautical theme for the front yard of our new home.)

 

[berkeman]

(Now all I have to do is convince my kids that we want a nautical theme for the front yard of our new home.)

I think you should post a poll at PF to help you decide. Pink flamingos vs. gnomes vs. anchors...

 

[Baluncore]

(Now all I have to do is convince my kids that we want a nautical theme for the front yard of our new home.)

Just paint the Lloyds Register "standard load lines", low down on the front of the house, above a red Plimsoll line.
https://en.wikipedia.org/wiki/Waterline#Standard_load_line_marks

No obstructions in the front yard, nothing to steal, and it makes a great talking point during the floods. Maybe you could replace the drawbridge with a gangplank.

 

[DaveC426913]

during the floods.

You have no idea how accurate that is.
States of emergency all over the region after this or that storm. Half my friends have flooded basements at any given time.
End of times, man, End of times.

 

[Ranger Mike]

go to NAPA and get a can of rust treatment
spray on and stops rust gives protective coating no primer required the n spray on your paint
https://www.napaonline.com/en/p/PTX7651671

 

[DaveC426913]

go to NAPA and get a can of rust treatment
spray on and stops rust gives protective coating no primer required the n spray on your paint
https://www.napaonline.com/en/p/PTX7651671

I'm posting this because that particular horse has left the barn.

"...foolishly decided to give it a light coat of primer in prep to grinding/rasping/sanding off any last imperfections. Now I can't tell where the rust spots might be."

 

[sophiecentaur]

Just slap lots of primer and then two good coats of exterior metal paint. It will last ages as long as you don't use it on a boat at sea.

 

[DaveC426913]

I made a thing!

 

[Ranger Mike]

fyi for future ref...rust primer PREVENTS rust the NAPA stuff stops rust already present and provides primer base coat

 

[DaveE]

The most difficult question for engineers: "I did something the wrong way, how bad will it end up? ... we don't know, we don't study that much.

However, it's a cosmetic part. Why not wait and see, then fix it if it starts to look bad. What do you have to lose by fixing it later versus fixing it now?

My guess is that if it was really dry, and your paint can really (really, really) keep water out, even under mechanical and thermal stress, then the rust won't get worse since it's just surface corrosion (i.e. mechanically stable).

I think you'll be OK as is.

 

[sophiecentaur]

I think you'll be OK as is.

You can paint it again - perhaps an adventurous new colour - in five years time.
Or you may take it off because you're fed up with people asking you what it is. I like it tho'.

 

[DaveC426913]

The only reason it was a timely issue was that I would have wanted to do the grinding/sanding before assembling it. Because it's riveted it can't be taken apart once assembled.

But it's all fine.

 

[Dullard]

A kick-up rudder - nice. It seems a bit small for the size of the 'boat'...

 

[DaveC426913]

A kick-up rudder - nice. It seems a bit small for the size of the 'boat'...

I seriously ruminated over how hazardous it might be to have a steel blade hanging down from the stern of my car. There's a mall entrance nearby where cars regularly bottom-out on their tow hitches...