Rustoleum Rusty Metal Primer On Clean Metal?

[jcmiller]

I'm not ready to repair the rust in the floorboard yet, but I would like to do a relatively quick treatment to try to stabilize the rust as much as possible.  My plan was to sand the rusty areas with a 3M wheel and then brush apply Rustoleum Metal Primer and then a topcoat.  But on Rustoleum's website, they caution against using Rusty Metal Primer on clean metal because it contains "fish oil" which supposedly will float to the surface and cause problems with topcoat adhesion.  They make a clean metal primer in that case.  The problem is that the floor will have some areas of clean metal, some areas of existing intact primer/paint, and some areas with rust.  What's a good primer in that situation?

[55 Fargo]

I'm not ready to repair the rust in the floorboard yet, but I would like to do a relatively quick treatment to try to stabilize the rust as much as possible.  My plan was to sand the rusty areas with a 3M wheel and then brush apply Rustoleum Metal Primer and then a topcoat.  But on Rustoleum's website, they caution against using Rusty Metal Primer on clean metal because it contains "fish oil" which supposedly will float to the surface and cause problems with topcoat adhesion.  They make a clean metal primer in that case.  The problem is that the floor will have some areas of clean metal, some areas of existing intact primer/paint, and some areas with rust.  What's a good primer in that situation?

Clean metal, in this case is brand new steel, without any corrosion. I take it you have surface corrosion on the floor metal, clean it up, you can use a Phosphoric acid treatment, then scuff with 3M pad, and apply Rust Metal Primer, followed with Rustoeleum Black Rust Enamel for example.

I have used rusty metal primer on pretty clean metal, and topcoated without problems, but in those case also allowed some longer curing time, as this was mixed and shot out of a spraygun, and coated over with more agressive products like 2 k urethane primer/surfacer for example.

When in doubt, do a small area, and see how it turns out. The Rusty Metal primer will adhere to rougher, pitted metal,much better than on clean new steel.

Good Luck

[Plymouthy Adams]

The caution on the fish oil on clean metal is there for the reason to let you know why the paint will stay tacky feeling for a few days..on smooth clean metal the oil has no pits or rust to convert/adhere to thus it has to cure back to the surface and be slowly evaporated...that is why if you use the rusty metal primer one must show patinece and allow the product to work in the manner it was manufactured.  I assure you that if you allow this top evaporation process to be allowed in a timely manner, you will find the primer is not only there to stay but very sandable indeed and further if you hve to come back and weld..you will find that very little primer is destroyed in the process.  You cannot be in a hurry and use this product..if you want instant on and abiltiy to sand and block you must pay dearly for a prodcut designed for that application..there in lies the higher cost in most all of modern body working products and primers..

[DCurrent]

You didn't mention what the condition your floors are in. Are they cancerous or just covered in surface rust? In my opinion it all comes down to how far down the road you plan to repair the rust or holes. Cost of the materials is another factor to consider. If you plan to repair the rust properly soon, I wouldn't do anything. Knocking down the rust and painting over it with anything will hinder your welding and aggravate you to death because you aren't getting good penetration. On the other hand if you are years away from repairing, you might want to consider Por15. It's a excellent product designed for painting over the top of rust with a brush on new and old metal. Basically it sticks to anything. I have witnessed how it works and during a period of time. It will also strengthen the metal in your floors. No not to the point you won't have to replace them, but  But it is expensive! An automotive spray able epoxy is great for such repairs, but again it is more expensive and not really brush able like you had mentioned doing. Buying a good grade of paint is best and worthwhile, but you don't want to spend the money just to grind it off at a later date.

 

Darren

[Plymouthy Adams]

if you think rust is a problem welding and getting penetration try to do the process with the POR15....you are compounding your work to no end...if you wish to extend the metal life prior to do the work..a good phosphoric etch will do your deed...it will also hold out rust if not in direct exposure to the elements..welding over a painted surface is not the best way to go but a quick hit with a sander will be helpful unless you oxyacetylen welding where it is a smooth process with the rusty metal primer as you go..if MIG welding..well you got enough issue of dirt, rust, dissimilar metal thickeness and such to start with that hinders this process as it is...paint is just one more complication you adding to the mix

[falconvan]

I'd have to go with Tim on this; POR-15 is a great product and I've used a lot of it on frames, floors, inner panels, plus a lot of large commercial HVAC applications at my job. I showed it to the vendor who I use to repair cooling towers and they dropped their standard products for it.

[TR Waters]

The caution on the fish oil on clean metal is there for the reason to let you know why the paint will stay tacky feeling for a few days..on smooth clean metal the oil has no pits or rust to convert/adhere to thus it has to cure back to the surface and be slowly evaporated...that is why if you use the rusty metal primer one must show patinece and allow the product to work in the manner it was manufactured.  I assure you that if you allow this top evaporation process to be allowed in a timely manner, you will find the primer is not only there to stay but very sandable indeed and further if you hve to come back and weld..you will find that very little primer is destroyed in the process.  You cannot be in a hurry and use this product..if you want instant on and abiltiy to sand and block you must pay dearly for a prodcut designed for that application..there in lies the higher cost in most all of modern body working products and primers..

 

The primer is always going to cure from the same direction. It doesnt change directions because you are painting over rust or not.

[55 Fargo]

The primer is always going to cure from the same direction. It doesnt change directions because you are painting over rust or not.

Well it is "air dry", not 2k catalyst cure.

I think what Tim is getting at is, with pitted rusted or rough metal, the "Fish Oil" , tends to settle into the pits, pores etc, but if the substrate is smooth, then it may rise to the surfce.

Curing is curing, once the solvent vents off, then the curing process can begin, with the Rusty metal Primer, a full cure takes a while.

This is why production shops, car manufacturers do not use synthetic enamels anymore, well there are many other reasons too.

As far as POR 15 is concerned, I have tried it, and not saying it is a bad product, but for the price, it is overrated, I like a product called "Zero Rust" good quality, and much less money than POR 15.

I have also used 2 k epoxy primer, with much success on rusty metal that was prepped, epoxy seals well and has good corrosion resistance.

Edited March 3, 2013 by C38Spitfire6

[Plymouthy Adams]

The primer is always going to cure from the same direction. It doesnt change directions because you are painting over rust or not.

 

As per Fred's post above, it is not a tend to settle ,it is the design of the paint that if the pits and rust is not present it has to evap from the surface..while the paint curing process is cure from the top on any non catalized paint as TR says....it is the long time to evap due to the type oil that is the concern and the time factor envolved when the paint is over solid clean metal...I suggest you read on the Rustoleum website as this is explained there...that is why the paint is not recommended for clean metal..it takes so long for the surface to be ready for a topcoat thus the other primers are more effective..it is called

RUSTY METAL PRIMER for a reason

[Scruffy49]

Hit the floor pans with rust converter of choice, then a standard brush on primer. If you treat the metal before painting, standard rustoleum primer is fine. It's floor pans, they'll eventually be covered with a carpet or a mat anyway, right? The Krylon version works well too, as does Majick from Tractor Supply. Once the floors are fixed, reapply your brush on primer, let cure, top coat with whatever color brush on is on clearance. Since it will be covered afterwards, who cares what color the top coats are...

[jcmiller]

Thanks everyone.  To clarify a bit, by stabilize I meant to extend the metal life until I can properly repair it, rather than improve structural integrity.  It is mostly surface rust, but there are some areas with complete rust-through.  It will probably be several years before I repair it correctly.  I work slowly and want to focus on the mechanical systems.  I really want to drive this thing around a bit and the floor can wait.  (Every time I want to buy a part or bring up buying a second old car, my wife brings up the fact that I've had this one for almost two years and it hasn't left the garage.)

 

Based on the responses, it sounds like a reasonable approach is to do a phosphoric acid treatment, which doesn't even need to be primed unless it will be exposed to the elements.  If I want to prime and paint, I can use a standard primer after such treatment, rather than the rusty metal primer.