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Do we need Primer on Bare Cast Iron?

Rick Strobel

New member
Howdee all:wave: and sorry to awaken this dead horse again.:bonk:

Do we really need to use primer on bare cast iron engine and parts? I use the good top coat (3 pax) with reducer and catalyst.

Thanks for your time
RickinMt.
 

PTSideshow

New member
Re: Do we need primer on bare cast iron ?

I would say that if you are using a top coat/sealer other than a rubbing oil to protect the item form rust. It would depend on what the manufacture of the top coat/sealer says. And the second part would be if you haven't has the top coat/sealer failed or have you had to redo it?
Like they say "there is the right way, the wrong way and the (insert military, or company name here) way" I know that I have done things with paint that was told by people you can't do that!
Well it turns out that you can :brows:
As with any other painting project, it is all in the surface prep before top coating sealing! Clean, clean, oil free and dry.
 

PaintChemist

New member
Re: Do we need primer on bare cast iron ?

Cast iron is a very tricky substrate to paint with good long term results. Cast iron is both very porous and contains a lot of graphite, neither of which are friends to a paint coating. The preferred surface preparation method for cast iron is abrasive blasting with steel shot to an absolute SP-5 white metal finish. Power tools can be used to roughen the surface but don't ever polish the surface to be painted with a wire wheel. You'll have coating adhesion problems for sure. I would only consider an epoxy coating to prime cast iron. Don't use lacquer thinner or acetone to clean cast iron. It will lodge itself in the pores of the steel and cause your paint to wrinkle or orange peel. Get rid of the oil and grease before you do any abrasive blasting or power tool cleaning otherwise the grease and oil will be forced into the pores and you'll never get it out.
 

Mike in NC

Moderator
I use self-etching primer on all bare metal, then apply epoxy primer over that. I have had good results with it.
 

s100

New member
The world won't end if you do not use a primer. Auto manufacturers squirt thousands of naked engines every day with cheap enamel every day, with moderately acceptable results under very adverse conditions. If you have clean cast iron and if you squirt it with an appropriate finish under favorable conditions (temperature humidity properly thinned etc) then you too will probably get acceptable results.

That said I would not paint cast iron without a good coat of primer/sealer. The primer will seal and to some extent fill the pores in the cast iron, resulting in a much more attractive finish. If you are only concerned with durability and not appearance, the primer is also a benefit as you will have a much more uniform film thickness. The rough cast iron, under magnification, looks like a mountain range, with peaks and valleys everywhere. The paint will run into the valleys, creating little puddles and will leave the peaks with a very thin coating. Anything you can do to alleviate this roughness will help achieve a more uniform film thickness and a more durable finish.
 

sdowney717

New member
I would use a primer. Simply it seals the surface better from water, I think paint can let water through. I have heard tiny microscopic holes smaller than you can see are in the paint, when the solvents evaporate out from paint. And primer holds on to metal and paint better.
 
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Merv C

Member
I use A Wattle product, You may not have that brand. The manufacture recommends that it can go straight bare metal, even if it has light rust. I apply it by brush usually two coats. I have been very impressed with the result and the best part is that after a reasonable curing time gasoline doesn't seem to affect it.

I usually use electrolysis to deal with the rust and sometimes use caustic soda in boiling water to get rid of the hardened oil and grease. If I want to preserve the original paint I don't use those methods.

If I paint I do like to have a very clean surface as I believe the paint sticks better.

Merv
 

Andrew Mackey

Moderator
I have painted many engines with both Krylon rattle can and Dupont Dulux by brush, and both have turned out well with no primer on cast iron. Proper preparation is the key. Making sure the cast is oil free is a must, if you don't want the paint to peel off. Before they came out with the E-10 crap gas, a stiff parts cleaning brush and a gallon of gas in a steel wash tub did the trick. Once all the surface grease and oil was washed off, a second good rinse and a couple days in the sun had the iron good and dry, as well as clean. I have one engine done that way, and painted with the Dulux, that still looks good 40 years later! :D Several, done with the Krylon still look good too, although Krylon has changed their formula to meet new EPA BS rules. Don't know how good it is now. I did buy it by the case load, when a friend who has a hardware store told me to stock up. He told me Krylon told him the change was coming, and it would not only change the colors, it would also affect the durability. I still have a few sealed cases! As for Dupont Dulux, again due to EPA, no longer available. I still have a few gallons of that too! Last year I finished a 1 HP IHC Famous that didn't have a spot of paint left on it. I found it 1/2 buried in a man's back yard. It looked like brand new, when I got done with it! That engine was cleaned first with gas and then Braklean auto brake cleaner. The parts were left to dry in a heated garage for a week before painting.
 

cobbadog

Active member
This topic has come at the right time for me. At the end of this month I am having our David Brown Cropmaster sandblasted for painting. I have already decided to use a metal primer and an etch primer on the alloy bellhousing and top of the gearbox. Top coat will be 2pac paint so I want to ensure good adhesion of the paint as I only want to do this once.
As for using Rustoleum. I have used it for a few years now straight over bare metal. It does bond well but what I like even more is the coverage it gives. It does cover well. Only slight issue I have is that it looses a slight amount of gloss over about a year. So I am now going to try an epoxy gloss black made by White Knight called Rust Guard here in Australia.
 

IronworkerFXR

Subscriber
Nason Ful-Cryl®
II 2K Acrylic Enamel Topcoat

Is what I have been using for several years now , depending on the application , acid prep, self etching primer, epoxy primer, then top coat. no sun fade for several years at this point .

again depending , I will use epoxy primer then paint. I have had very good luck and material saving by doing a lot of brush work, just do not add much reducer.
and always follow proper PPE protection.
 

cobbadog

Active member
Well I have finally got the blasting done on the tractor and everything is in Hi-Chem Super Etch Primer except the fuel tank which ended up with some pin holes so I am soldering them up tomorrow. This primer does the job of etch and base primer especially for 2pac paint to go over the top of so one step less than normal.
I am happy that I have used the etch primer over the bare metal both cast iron and mild steel and the finish is brilliant.
Again the Rust Guard paint is designed to go straight onto bare metal and so far the items I have painted show a great finish, good high gloss and is an epoxy paint that should stand up to UV light but only time will tell.





i
 

IronworkerFXR

Subscriber
When you go this far.... its worth the extra effort and $ the finished product is worth it .

now if I can get some good lead paint ..... ..... ... of course I won't let kids touch,eat or look at it...
 

cobbadog

Active member
At the end of the day you get what you pay for. If you are happy with a general tidy up use what ever paint you like and do the preparation you are happy with.
When you see an engine presented at a Rally in a paint job that your average Rolls Royce would be envious over you have to step up to the plate and do all the right prep, cleaning, and using high quality paints.
Ordinary enamel paint will look nice for a few years but don't spill any fuel on it as it will go nuts and wrinkle up and then it looks less than nice and that is when you say to yourself, "if only".
I have found after doing a few projects in 2pac paint that it stands up nicely in the full sun and handles fuel spillage well but I still wipe it off when I have finished filling the tank but it is not a race to do so now.
Here is my current project in etch primer waiting for the next step.
 

Attachments

dkamp

eMail NOT Working
I'm certain that no matter WHAT you choose to do, in the end, you will have done much, much more than the factory ever did.

They came down the line, and got sprayed or dipped. If it stuck, and was the right color, it kept going.

My dad used to have a large collection of antique cars... two were restored, the rest were original, and as I child, I never really concerned myself with the difference. Now, I see things that have been 'restored', and same that were 'original', and realize that most who 'restore', are so involved in the detail of something, that they forget how limited the original production attention was.

So if there's any wisdom here... it would be that... you should paint it however you want it to be, and realize that you cannot make it 'original' (it's only original once). When you paint it, you'll be using modern products and techniques, some of which are substantially better, but some are worse. No matter HOW you do it, you will not have an 'original', you'll have 'nice'... and while it'll last a long time, it will not last forever.
 

PaintChemist

New member
The world won't end if you do not use a primer. Auto manufacturers squirt thousands of naked engines every day with cheap enamel every day, with moderately acceptable results under very adverse conditions. If you have clean cast iron and if you squirt it with an appropriate finish under favorable conditions (temperature humidity properly thinned etc) then you too will probably get acceptable results.

That said I would not paint cast iron without a good coat of primer/sealer. The primer will seal and to some extent fill the pores in the cast iron, resulting in a much more attractive finish. If you are only concerned with durability and not appearance, the primer is also a benefit as you will have a much more uniform film thickness. The rough cast iron, under magnification, looks like a mountain range, with peaks and valleys everywhere. The paint will run into the valleys, creating little puddles and will leave the peaks with a very thin coating. Anything you can do to alleviate this roughness will help achieve a more uniform film thickness and a more durable finish.
S100, I don't mean to be argumentive but much of what you posted is somewhat erroneous. I've done much paint consulting work for the "big three" automakers and they spend enormous sums of money to make sure their painting is to the highest industry standards and done by the book. The reason is that warranty claims are even more expensive to resolve. I can tell you first hand they don't squirt cheap enamel on anything. The automakers all use the very best coatings and the best application equipment money can buy and they also spend the most effort to achieve the highest quality painted product. It may of been the way you describe years ago but it's surely not that way today. Next, poor performance will always result without using the correct primer, especially with a cast iron substrate. Topcoats aren't formulated to act as primers and primers aren't formulated to be topcoats. Both together, work as an effective system. Next, a rough surface has no effect on a coating's durability. Cast iron typically does not have peaks as does a deep sandblasted surface but does exhibit pits or voids that can be of concern. The pits can be filled in a variety of ways to achieve a better looking finish but surface roughness by itself has no effect on the coating's durability as long as the surface is fully coated. Paint adheres better to a roughened surface much better than a smooth surface and is the reason surfaces are sandblasted. Paint thickness will indeed be more consistent on a smooth surface but not necessarily more durable. The easiest way to achieve great results when painting is to read and follow the paint manufacturer's technical product data sheets. They all have them and most are posted on their websites.
 
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