Surface Rust

I have been working on my International KB1 pickup truck project. The hood and top of cab where covered in thick heavy surface rust. I wire wheeled it first to knock of the loose stuff. I tried taking the rest off with an angle grinder and 80 grit but it was taking off more good metal then rust. I tried to use phosphoric acid but it was too slow. I ended up spending hours with a small rubber sanding block and 60 grit sand paper. Lots of elbow grease. It took about 10 hours to clean off the hood and cab top. I sanded each rusty spot until I saw brite metal sanding grooves through the rust. That told me any remaining rust was below surface level. I then hit it with 15% phosphoric acid and scotch brite pads. I now have tons of very small dark pits. The pits are only the size of the sharp end of a needle. Is this rust dead and am I ready for epoxy? I plan on removing the white film with a scotch brite pad, wipe on wax and grease remover, and then use Omni 170 epoxy primer. This is way too much work to do over again. I have been reading a lot of stuff on the internet about surface rust removal with acid and epoxy primer. A lot of conflicting information. Thanks for the help and advise.

If this is a keeper then get it blasted. Plastic media or wet sand blast or any of the other non-warping media. Epoxy will not neutrallize anything. Black or brown spots WILL come back to haunt you especially on flat exposed sheet metal. RB

Google it. The black is iron phosphate. It is inert. Sand blasting will still leave tiny bits of rust at the bottom of the pits. Any brown pits need another hit with phosphoric. You are supposed to keep the surface wet with the phosphoric for 10 minutes in order for it to give it plenty of time to penetrate. If it is all either shiny or black you have it as good as you can get it.

Thanks for the replys. This is the same split I find when just looking at car restoration sites. Some say blast it and some say acid etch it. I do think the comment about blasting not reaching the bottom of the pits has some truth. The pits are very tiny so to get the blast media in contact with it the blast media would first have to open up the pit more. I have been removing the paint with a wire wheel and finding small rust spots burried underneath the 66 year old enamal. So it does make some sense that small acid nutralized rust pist locked under epoxy will take decades to come through the paint.

I agree that it is now changed from oxidation (rust) to being inert but it still is not a part of the base metal and is not technically attached to the base metal and as such , is just a filler of the pit. It's your call if you want to take the chance on it. Also blasting correctly and with the correct media will not leave rust in the pits. Any procedure done in a marginal way will produce marginal results. Anything left in the botom of a pit will eventually let go. Guess I'm just "old school".

B, you just gave the reason I don't use phosphoric acid, except in areas I can't get to mechanically. I simply don't trust it. And I have never had trouble with pits, rust, or anything else after sand blasting. If not sandblasted I use a fiber wheel on an electric drill for sheet metal. Slow and leaves deep scratches that a surfacer takes care of -- there is no rust left. But, I have used Picklex 20 or Must For Rust as a protectant on bare metal. Since then I have found I can leave bare metal inside for months with no rust, obviously I live in a dry climate. Different situation with Showcrop in NH, who lives in salt air.

My understanding is that epoxy primer needs some extra surface scratches to mechanically lock into. Most people seem to recommend sanding with 80 grit before epoxy. I would think the rust pits have jagged sides and the epoxy will lock into them like a cork in a bottle. Doesn't sand blasting actually smooth out the metal surface? This is a good discussion I am learning as I go. Thanks

(quoted from post at 17:28:15 07/09/13) B, you just gave the reason I don't use phosphoric acid, except in areas I can't get to mechanically. I simply don't trust it. And I have never had trouble with pits, rust, or anything else after sand blasting. If not sandblasted I use a fiber wheel on an electric drill for sheet metal. Slow and leaves deep scratches that a surfacer takes care of -- there is no rust left. But, I have used Picklex 20 or Must For Rust as a protectant on bare metal. Since then I have found I can leave bare metal inside for months with no rust, obviously I live in a dry climate. Different situation with Showcrop in NH, who lives in salt air.

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What is salt air? I googled it and got just places and adjectives. Air here is the same as air in Kansas. I looked it up. True we probably have generally higher humidity, but the salt doesn't get into the air except in salt water spray right at the ocean, which may travel 50 yards in a really strong wind. I have a set of Oliver sheet metal in my shop that I sand blasted four years ago and it has yet to rust. I just don't trust sandblasting to get down into pits. I have, like every one else, found a lot of rust under some pretty decent looking paint, and I don't believe that the people that painted 25 years ago just painted over a lot of rust. I think that they cleaned it up shiny clean, except for the pits, which they probably figured were too small to bother. It really bothers me to see my professionally painted panels bubbling up where there were pits before the paint was cleaned off, and it was perfectly cleaned to shiny metal. Now I do it myself, and anyplace that there are rust pits gets acid wash, just like the counterman at the automotive paint store told me to.

The PPG Omni epoxy's tech sheet says no finer than 180 grit, not 80 grit. Used by itself 80 grit scratches may show through the topcoat if no surfacer is used. I usually use 150. I have never noticed epoxy lifting on sheet metal or cast. If there is a nick that removes the topcoat, the epoxy usually stays intact, that is what it is supposed to do. For hoods, gas tanks, and grilles, the initial paint and rust removal, the fiber wheel I use is much coarser than 80 grit. I attempt to flatten it by using 100 and 150 grit, then I apply epoxy followed by surfacer. If I look closely I can sometimes see minor scratches through the topcoat. For things like battery boxes, seat frames, etc, which is smooth metal, but not sheet metal I use 150 grit and epoxy with no surfacer, any light damage doesn't chip the epoxy there either. As to sand blasting, I sand it afterwards to smooth it out and also to remove any sand that may (or may not) be imbedded in the metal. However it is probably smooth enough to begin with -- I just like a smoother surface.

I used salt air as a slang term. However salt, whether on highways or just close to the coast has an effect. I grew up in south Texas, about 200 miles west of the Gulf, I went to college in Kingsville Texas in the 50's about 40 miles from the Gulf when cars and trucks had virtually no rust protection. It took only a couple of years for rust to start -- so the salt air does make a difference. All of my rust is gone, pits or not before I paint. I have only done this a little more than 10 years and have seen absolutely no paint lifted -- chipped, yes, but not rust. However all my tractors are inside, but I would expect the same outside. Before I paint the rust is mechanically removed. I do live in a different climate and I assure you the air is different, salt or not yours simply has more water in it. Some days here the humidity is as low as 10%, 15-20% being more common. High temp so far this summer is 110. I did a little painting in my building at a temp of 95--I don't particularly like doing it when it is that "warm". If I remember some of your earlier posts, the rust in the pits that lifted the paint was done by a not so professional painter, not you.

This is an old pickup so I plan on using high build primer and block sanding it out. That is after I get all of the small dings and dents filled with body filler. I have most of the dents worked out pretty well with a cheap set of body dollies and hammers. My goal right now is just to get all of the metal work done and get everything in epoxy primer. I also want to get the frame cleaned up and painted this summer.

If you do a bright-metal blast using standard Black Beauty grit (not sand) there will be no rust remnants left in any pits. The resulting blast profile will be more than sufficient for priming directly with epoxy without sanding. Two coats of epoxy will fill the profile well.

Remember that epoxy has a window -- about 3 days to a week,(depending on the brand) that you have to apply topcoat or surfacer, etc on. Outside the window you have to scuff/sand the epoxy and apply another coat -- scuffing without another coat is not enough to make the epoxy bond to the paint. As to high build primer, urethane surfacer (uses hardener) is best because it doesn't shrink and expose the scratches.

My plan has been to scuff the epoxy and apply the body filler and sand it out. This will be after the epoxy has fully cured. Could be next summer before I get to it. Then apply another coat of epoxy followed by the high build primer. Will the body filler stick to the cured and scuffed epoxy or does it need fresh epoxy? Also does the high fill primer need to be sealed before final top coat? Again thanks for the help.

Some controversy about your question. I like body filler on bare metal, some like it over epoxy, probably doesn't matter, but I put it on bare metal. As far as fully cured, epoxy can have something over it in 30 minutes, so curing doesn't matter as long as it is in the window. But, I can't answer whether or not the body filler sticks to old epoxy or not--I would sand it first. There is a good chance when you sand that part/all will go to bare metal. Rod(NH), far as I'm concerned, along with Glennster, B-maniac and Gordo and a very few others on this forum are the experts on epoxy--I have my own opinion which sometimes disagrees, but one of them will give their opinion. With PPG products you do not have to seal the high build primer, some other brands may say you do. Some say that applying epoxy over the surfacer, etc makes the topcoat adhere better--that is a possibility, I have not tried it.

Here's what I have done successfully in a situation like that: I like putting epoxy on almost immediately after blasting to bright metal. I do all my work outside in the summer and live in a fairly humid area. Even fingerprints on a bright-blasted steel surface can start the rusting process. Getting epoxy on as soon as possible protects the metal for additional work later on. I've also used Picklex 20 for that purpose but prefer epoxy if I can accomplish it at the time. If there are some areas that need plastic filler later on (hammer/dolly work should have already been completed), I'd rough the area with a course grit (say 80). That will go to bare metal in the heavy scratches. I then would apply a fresh coat of epoxy to the area and apply the filler within the epoxy window. After sanding the filler as needed, there may still be spots where sanding went to bare metal. Scuff the area at that point (Scotch-Brite is good for that) and apply a final coat of epoxy. You then could go directly to the topcoat within the epoxy window if the surface is smooth enough at that point or apply a few coats of surfacer. You then could delay the topcoat for a time of your choosing without worrying about any further time windows. You'll need to final sand the surfacer anyway and that will provide the "tooth" for the topcoat.

I have never had a problem applying filler over epoxy that way. I'm not questioning that filler to bare metal with a heavy scratch is "better" - and I've seen some test data that so indicates. However, I remain unconvinced that it's necessary in the vast majority of cases. So I prefer epoxy first, then filler.

I know that B- prefers to add a fresh coat of epoxy between the final-sanded surfacer and then topcoat within the window. Not as any kind of sealer - no sealer is required. The coat of epoxy at that point provides for a chemical bond with the topcoat and is "better" than just the mechanical bond using only the scratches from the surfacer sanding, usually with a final 320 grit or so. I've never had a problem skipping that step and feel that while it is "better" it's likely not necessary considering the more applications you do, the more chances for something messing things up.