The Detroit Post
Thursday, 21 October, 2021

How Much Rust To Remove Before Painting

Daniel Brown
• Friday, 06 November, 2020
• 8 min read

Restore Rusty Metal Before Painting (DIY) | Family Handyman Skip to main content Family Handyman The key is to properly prep the rusty steel before you paint.

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A little work up front will yield long-lasting results. That old wrought iron fence or steel gutter may look terrible, but with the right preparation and primer, most rusty metal surfaces can be restored to almost new.

Grind down heavily rusted metal spots faster with a wire wheel and a drill or angle grinder. Coat deeply rusted metal areas with a special primer that chemically converts rust to a printable surface.

Oxidation is a natural process that occurs when metal is exposed to oxygen and water. Unless it is galvanized, a metal surface will rust when exposed to exterior weather.

If you would like to paint a rusted surface, dedicate yourself to thorough preparation, or adhesion difficulties will inevitably result. Rust removal is essential to promoting a durable, lasting finish.

Eliminate as much rust as possible from exterior metal surfaces with a pressure washer. Wash the iron oxide primer from your brush with mineral spirits.

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Elegant and durable, metal makes an appearance on furnishings and decor in and out of the house, from entryway benches to patio chairs and fence posts. Paint offers one way to spare these rusted pieces from the junkyard and restore their looks while lending them a pop of color.

If rust has partially or fully eaten through the metal (i.e. pits or holes are visible in the piece) or the piece has structurally weakened to the point that you can bend it by hand, then paint won’t halt the corrosion and inevitable crumbling of the metal. Filling the pits with an auto body filler product (like Bond, available from Amazon) would be your best option for restoring it, or else you’ll need to replace the metal piece altogether.

This coating will chemically convert the rust into a flat, usually black, nonadjustable surface that’s ready to receive paint. You want to apply the primer as soon as possible after wire brushing and cleaning it since the metal will otherwise continue to rust with exposure to oxygen.

Add up the costs of a 12-ounce spray can of rust conversion primer and a 12-ounce can of metal paint that each cover around 15 square feet, and you’ll find that painting the rusty surface of a small metal piece such as an end table runs you as little as $8. Compared to the cost of scrapping the item and buying it new (which might start at $25 for a bare bones metal end table), you’re looking at a savings of at least $17 for a minor project alone.

The protective coating around galvanized metal (usually made of zinc) can corrode with exposure to heavy rain or an accidental splash of a powerful household chemical like Adriatic acid. With continued exposure to the atmosphere, the metal piece can form white rust.

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carmakers treat their body shells before painting by spraying them with a form of phosphoric acid, which is then thoroughly rinsed off. You apply the fluid, leave it to act for a certain time, wipe it off, then rinse carefully with clean water and dry thoroughly before priming and painting the treated area.

Some removers are in jelly form so that they do not drip on to sound paintwork, which would be damaged by the corrosive action of the acid. Use an electric drill fitted with a sanding disc or a flap wheel to cut away the surface rust.

Clean off loose rust or paint with a wire brush or coarse emery cloth, or use an electric drill fitted with a sanding disc or flap wheel. If there is any risk of rust -removing fluid dripping on to sound paint or other parts, cover these with plastic sheet.

Leave it on for the recommended time, and wash it off with water or methylated spirit or wipe it away with a clean cloth, as instructed by the manufacturer When the area is completely dry, apply primer paint and, if necessary, cellulose stopper to level the surface, as with smaller rust patches.

Clean up small chips in paintwork with a scrap of coarse wet-and-dry paper, used dry. Wipe the area and about an inch all round it with a cloth moistened with white spirit, to remove any way.


Scrape away all the loose paint and rub down the rusty area with a scrap of course wet-and-dry abrasive paper, used dry. If it is the type that does not dry on non-rusty areas, wipe off the surplus with a clean cloth moistened with methylated spirit.

When the primer is dry, use cellulose stopper to bring the area up to the level of the surrounding paintwork. Bring the damaged area up to the level of the surrounding paint by smoothing on a thin layer of bare-metal cellulose stopper, using a flexible knife.

Smooth the stopper with 400-grit wet-and-dry paper dipped in water so that it blends into the surrounding paint. When working on broad, flat areas, wrap the paper around a sanding block.

I know I'm asking in a way that is difficult to quantify, but “how good” will it get, compared to having all the rust removed? Even assuming the rust is properly sealed from the atmosphere it appears it can create new $\CE{H2O}$ under this protective layer.

As the acid is formed and the iron dissolved, some water will begin to break down into its component pieces -- hydrogen and oxygen. For example, water does not spontaneously decompose to the elements by the reverse of the reaction referred to above $$\CE{2H2O(l) 2H2(g) + O2(g)}$$ nonspontaneous.

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However still, from where does new oxygen get introduced into the system, assuming the atmosphere is actually completely isolated from this internal system taking place under the surface of the paint? Should it matter, assume the environment is approximately 1-atmosphere pressure and that the temperature varies between -10 to 40 degrees Celsius.

However, even without acid present, even in slightly basic conditions iron slowly reacts with water in presence of free oxygen, forming rust. The process is hindered only at pH above 10 or at absence of free oxygen.

Since natural water is usually slightly acidic due to presence of dissolved carbon dioxide, iron always rust on contact with water and atmosphere. This, actually, is a reason why concrete layer over steel frame is regulated: concrete always contains water, and surface layers quickly looses basicity due to reaction with atmospheric carbon dioxide, so steel near surface of concrete quickly rusts, increasing in volume and tearing the concrete from inside.

As long as the paint film over rusty part is broken, water and atmosphere contacts the metal and start to erode it, resulting in more paint film be destroyed. $\begingroup$Most paints will prevent oxygen and water to come into contact with the metal.

So I've found that one important reason to remove all rust, often by sanding, is to help achieve 'smooth and solid' ; in other words not to affect the chemistry of the surface, but to adjust the physical characteristics of the surface. I've found that in actuality almost all coats of paint have tiny holes in them that show up later.

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These tiny holes are perhaps from bubbles or debris in the paint as it is applied. A few years go by and tiny rust spots start to appear, then they get bigger with time.

By carefully preparing the surface each time before it's coated you can minimize the chance for these pin-holes. BTW, another source of holes in paint where rust starts to form are places where impacts occur, like where rocks hit fenders, or where surfaces connect and expansion and contraction can break the paint joint in time.

Those products contain acids that are supposed to etch, seal, and “convert” the rust. Other claims by Ceylon and Mausoleum are exaggerations in any climate but a high desert.

Conversion coatings do very well in salt spray tests but I can't remember and real numbers. $\begingroup$Best solution is use of a first-rate primer over clean metal, with all rust removed.

20 or 30 years ago the rust conversion COATINGS were mostly just visual effects. (SO there are rust dissolves, meant to be rinsed off, and rust converters (made with paint resins), meant to form a primer coat (or sub-primer coat).

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Chances are nothing will prolong the life of the part (or coating) any better than frequent stripping and recoating. Use outdoors or with plenty of ventilation (and away from heat, flames and sparks (like cell phones)).

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