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Clear coat with Penetrol - Discoloration


rstor

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Hello,

I was reading on the forum about the use of Penetrol to clear coat bare steel. Someone had mentioned earlier:

Quote

We use a product called Penetrol which is actually a paint additive but it does give a fairly durable clear finish, even outdoors it lasts a couple of years before needing re-coating.

https://www.iforgeiron.com/topic/48866-clear-coat-sealer-question/

I had purchased a metal drawer cabinet that were in rough condition and had someone blast it with copper slag. The cabinet chassis  in the attached pictures was then sanded using an orbital sander to smooth the finish. (Outdoors) I had cleaned the surface with isopropyl alcohol and then proceeded to apply Penetrol. Outdoor humidity was about 70% and it had just rained. On the attached pictures you see some brown discolorations. I am curious to know is this rust or something else?

 

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56 minutes ago, John McPherson said:

Isopropyl alcohol contains 10-30% water, which is why no one who does car restoration uses it. Far better to use mineral spirits or acetone as a final cleaner/degreaser.

 

Thanks John,

The IPA that I used was 99%... I think I have some acetone so I can give that a try on the drawers.

Are there any issues with washing blasted metal with a degreaser and regular water, drying, then cleaning with a solvent before applying Penetrol ?

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No water, water based, or water containing compounds ever on carbon steel. Alcohols will actually draw moisture from humid air. Maybe not much, but enough. And at $30 US per gallon, there are cheaper alternatives for metal finishing.

Once you thoroughly clean the surface, it will flash rust, as in right before your eyes. Using a heat gun will not be fast enough to dry it before it starts to rust.

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36 minutes ago, John McPherson said:

No water, water based, or water containing compounds ever on carbon steel. Alcohols will actually draw moisture from humid air. Maybe not much, but enough. And at $30 US per gallon, there are cheaper alternatives for metal finishing.

Once you thoroughly clean the surface, it will flash rust, as in right before your eyes. Using a heat gun will not be fast enough to dry it before it starts to rust.

I am guessing that is what happened as I used lots of IPA and it just rained... so it probably drew moisture from the air before I could apply the Penetrol

I'll use acetone in this case.

For the drawers (attached picture) the individual who blasted it mentioned it is not steel like the chassis but an alloy... would you be able to advise if this rusts in the same way as steel?  

drawer.jpg

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I am going to assume that where the rust appears now was already rusty before you started to work on it. Maybe not bad enough to really see but it was there.  To stop rust(oxidation) you have to 1.remove oxygen or 2. Remove moisture. In your case I think what happened was with the copper blasting was not aggressive enough to actually remove metal to get to the bottom of the rust pits and remove the rust. And your sander only smoothed off the top not removing those pits. Those little pits still have rust in them and absorb moisture and oxygen. When you then coat the surface with penetrol it might block further oxygen and moisture from getting to the surface but does not remove the the moisture and oxygen in those tiny rust pits and they keep oxidizing even though they are sealed from the elements. In the body shop industry the only way to fix rust is to replace the rusty metal. The next best "fix" is to aggressively grind the area to remove all those tiny rust pits.  40 grit on a grinder.  80 grit on an orbital is not aggressive enough. And those pits can go more than half way through the metal. I work in a body shop and to really get those out takes alot of careful attention(magnifing glasses and a good light) to get all the pits. Now you have to remove those aggressive grinder marks(alot of work) to get it to look like the un rusted areas. Then apply the top coat for protection.  Then on the other hand. If the metal was perfect, you have you be faster between sanding and top coating.

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I went to my local auto paint dealer and they gave me a two solution process for preparing the metal.  One was used to remove finger print oil and other oils.  The solutions were mixed with water, and I had to dry them immediately after washing off the solutions.  The times I did this I was lucky to have very low humidity.  Even taking lots a care to scrub all the surfaces really well to remove all the oils and never touching the metal without gloves on, I still found little spots of rust that had grown when I just clear coated the bare metal. That was many years ago, and when I went to the auto paint dealer recently, they gave me one spray can to use.  No washing or two solutions.  Good luck.    

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I use acetone, and only acetone, for cleaning any possible oil/grease contamination even on freshly ground or polished steel prior to arc welding or coating bare steel. Acetone used even after rust removal. Never use any form of alcohol.

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I ended up spray painting over the Penetrol on the chassis and front of the drawers with a metallic look paint.

For the handles, I am removing the existing paint to bare metal and would like to keep the bare metal look on the handles. If one had to choose between Penetrol or clear acrylic spray paint, which one would it be?

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