Surface rust, how much do I need to remove?

[tamo3]

Hi, I'm working on stripping paint and fix rust area.

 

Yesterday, I use AirCraft Paint stripper and wash and sand the hood.

There are several surface rust spot.

 

I got POR15 tryout kit and use "Prep-Ready" for those surface rust spot.

Some spot bobble up and some are not.

 

My question is is it OK to leave this as is and apply Epoxy primer? or do I need to sand down completely to remove rust black spot to the same color with other bare metal?

 

Thanks,

tamo3

Edited July 27, 2013 by tamo3

[morbias]

You need to remove all of it unless you want it coming back! Use a wire wheel drill attachment to get the rust in pitted areas, then use prep-ready / phosphoric acid to kill off any microscopic rust. Make sure there is no acid left when you prime though, in fact I would wire wheel the whole thing over again to make extra sure.

[tooquick260]

As morbias said , wire wheel as much as you can then use some type of rust inhibitor/remover. I have seen my dad use ospho. You brush it on, turns black. This stops the rust process. Something about anodotic/cathodic effects of metal. The rust spots are area's that are giving up or sacrificing itself. 

 

After using the ospho spray with etching primer and your good to go. Or atleast for 20+ years.

[tamo3]

Thanks for your advice. I will do that!

[clarkspeed]

I have a question. What do you do to clean away the excess leftover ospho before you primer. I have some large areas to treat and there always seems to be some residue in difficult to reach areas.

[tooquick260]

This is from Ospho web site...

 

 

RUSTED METALS - OSPHO is a rust-inhibiting coating - NOT A PAINT You do not have to remove tight rust. Merely remove loose paint and rust scale, dirt, oil, grease and other accumulations with a wire brush - apply a coat of OSPHO as it comes in the container - let dry overnight, then apply whatever paint system you desire. When applied to rusted surfaces, OSPHO causes iron oxide (rust) to chemically change to iron phosphate - an inert, hard substance that turns the metal black. Where rust is exceedingly heavy, two coats of OSPHO may be necessary to thoroughly penetrate and blacken the surface to be painted. A dry, powdery, grayish-white surface usually develops; this is normal - brush off any loose powder before paint application

 

 

Link: http://www.ospho.com/directions.htm

[clarkspeed]

Yes, I've read it before. And I've been using ospho for many years with good results. But especiallly when applying 2 or more coats, I sometimes end up with a black Goo that it highly acidic lurking in hard to reach corners.

[NgoZ]

When I use Ospho, I try scaping the goo off then apply one more time before I prime/paint/whatevertodonext.

[socorob]

You're only supposed to put light coats of ospho on. I put it in a spray bottle and mist the surface then wipe it around with a rag. If you put it too thick you get that black goop and I heard you should grind that off before painting. I do a couple very light coats. I think it steals an oxygen atom from the rust so it's no longer rust.

[djbit]

I did this on my floor panels that had been sitting in my garage for three years  and it worked pretty good. I did run an orbital sander and drill with wire wheel before I applied the vinegar + water (1 to 4 ratio) mix. I applied some edging primer after wards since the panels are still sitting in my garage.

 

 

http://www.youtube.com/watch?v=m8_STIIF_x0