Painting over OSPHO: Here’s What You Should Know

Protecting metal is an art form in itself. One of the best ways to protect the metal from corrosion and damage is with Ospho, a special substance meant to protect from rust and age. Is it possible to paint over Ospho once you’ve applied it?

Yes, you can paint over Ospho. Clean and dry your metal area thoroughly before applying the Ospho, and make sure that the Ospho is set and free of phosphate residue after it dries. Oil-based paints work best over Ospho, but there are some tricks you can use to help other paint types stick as well.

Keep reading to learn more about Ospho, how to paint over it, and how to make your finished project look its best.

What is OSPHO?

Ospho is a substance that is specially formulated to protect the metal from rust, corrosion, and elements. It combines phosphoric acid, dichromates, wetting agents, and extenders.

It is also used to eat away and eliminate existing rust deposits, and prepare that metal area for future protection and paint. It contains phosphoric acid and is a high-quality and easy-to-use product. It does need special care when handling and should be handled with PPE in a well-ventilated area.

It is typically applied on bare metal surfaces, such as fences, sheds, vehicles, outside tanks (such as septic tanks), and metal roofs. It is not paint but can serve as a helpful, protective layer underneath the paint.

How Does OSPHO Work?

Ospho provides an extra layer between the vulnerable metal and the final layer of paint. Paint can flake and peel away over time, leaving the metal underneath at risk of rust and corrosion.

Ospho helps to protect the underlying metal and prolong the life of the fence, tank, roof, boat, or vehicle. Rust can easily corrode and even destroy metal structures. Applying Ospho is a great way to help get even more life and service out of the metal components you rely on for everyday life, protection, transport, and safety.

Priming for OSPHO

Priming a metal area for Ospho application is not difficult. Simply scrape away any old paint and rust scale with a wire brush before applying Ospho. Make sure that the surface you’re about to work on is free of dust, dirt, contaminants, and excess moisture.

Be careful to not apply Ospho over paint, but only over bare metal. It’s specially formulated to protect metal, and won’t do its job properly if applied over a layer of paint. Sand down to the metal base of your project, and apply Ospho to those areas.

Stay Safe

Ospho is a great product but can be toxic if inhaled. Please be careful with Ospho, and use goggles, a face mask, and gloves when working with Ospho. Be sure to paint it in a well-ventilated, dry area with plenty of fresh air.


Applying Ospho

For best results, make sure that you apply Ospho in a dry and temperate environment, and feel free to use a regular brush for application.

Ospho can be applied in a single, thin layer and must be allowed to dry for at least 24 hours. If you are treating the new metal with Ospho, you may notice a white or grey, dust-like substance on the surface of the metal. This is a normal by-product of the conversion of ferrous oxide to ferrous phosphate, but you’ll need to remove it before painting.

Once the Ospho is dry, simply wash down your surface with moderate water pressure and scrub this dust-like film away with mineral spirits, and you’re ready to begin painting. Failure to scrub this fine layer away may make it more difficult for paint to stick to the Ospho.

Oil Based Paints Work Best

So what kind of paint works best on Ospho? Oil-based paints work especially well on Ospho, and bond nicely to the material with minimal peeling or flaking.

Why oil? Some materials simply adhere better than others. Ospho already provides a layer between the metal and anything you’re going to paint it with afterward, and the viscosity and composition of oil just seem to like adhering to the texture of dried Ospho.

So as long as the Ospho layer is dried and free of contaminants, your oil-based paint should stick just fine. What about other paints?

Can You Use Epoxy?

It is possible to use epoxy-based paints on Ospho, too? Yes, but you’ll have to give the epoxy-based paint something to adhere to. A common method is to gently re-wet the Ospho and lightly sand it with medium-grit sandpaper. Let this secondary job dry before applying epoxy-based paint.

This creates a slightly rough, pebbly surface on the dried Ospho and helps the epoxy-based paint stick and cling far better than if the Ospho were smooth and unsanded.

Modern epoxy-based paints are typically easier to work with than the paints of decades past, but the safest way to apply an epoxy-based paint on Ospho is the wet-and-sand method.

Making Sure Your Paint Sticks

The most important part of your painting job is going to be ensuring that your paint adheres to the layer of dried Ospho. Anything that is going to get in between the paint and the dried Ospho will cause issues here.

This includes grease, dirt, and water. If your surface was clean last night, it may not be clean in the morning when you come to paint it. Do a thorough check of the surface you’ll be working with to ensure that it is 100% clean and dry. This includes noticing any humidity or moisture in the air that could interfere with this process.

Allowing your Ospho to dry in a well-ventilated and dry area is going to be important here. If the air surrounding your project is heavy with moisture, your paint may not stick.

Another pro tip; give your paint plenty of time to cure. Just because your paint is dry doesn’t mean that it’s fully settled into the surface beneath. If you try to “test” the dry paint too soon, you could wind up with the paint peeling or flaking off. Give the paint at least two weeks to fully cure, and settle in before trying to see if it’s secure.

Best Uses for Ospho

So where would you most benefit from using Ospho? Anywhere that metal needs protection and extra durability. Farmers and ranchers use Ospho on fencing, mechanics use it for the metal underside of vehicles, folks use it to restore old RVs or hotrods, and you can even use it on metal boats or metal sheds.

It’s great for restoration projects, too. It turns rusty spots into a deep grey or black, helps make these spots less unsightly, prevents future rust development in these areas, and helps protect the metal from moisture better than a layer of paint alone.

Anywhere you need your metal to last and hold up to the elements is a great place to apply Ospho. And if you need to paint the metal item after application, it’s a paint-friendly substance.

It’s also great for steel parts such as motors, engines, steel piping, and metal tanks. Use it to get the most life out of your metal components, and give new life back to old vehicles, boats, trailers, and collector’s items. It even works great on old coins and metal furniture.

Common Mistakes

Some common mistakes that many do-it-yourselfers tend to make are to apply Ospho over paint. Ospho is not meant to go over paint, and won’t help protect metal or eliminate rust if it’s not in direct contact with metal.

You’ll need to strip the item down to the bare metal before applying Ospho will help your project. You’ll also need to be ready for a noticeable layer of dried, grey film or dust that will result from the dried Ospho. Make sure that all of this gets washed away, or whipped away as needed.

Be careful to begin and end your project in a well-ventilated area and make sure that your eyes, nose, and mouth are protected. Wear gloves, and avoid touching your eyes, mouth, nose, or sensitive skin while using.


Ospho is a protective substance that can help eat away at rust as well as protect metal surfaces from future rust. It can be applied with a brush to any bare metal surface that has been first cleaned, dried, and cleared of any flaking rust and old paint.

Make sure to adequately protect your nose, mouth, and eyes when applying, and work in a well-ventilated and dry area. Be sure to wear gloves, and avoid touching eyes or sensitive skin during the painting process.

Painting Ospho can be done once the Ospho fully dries. Wash away any grey residue that will result from the Ospho drying, and then apply a layer of oil paint to the dried Ospho. Epoxy paints may also be used, but it’s a good idea to wash and sand the Ospho beforehand so that the epoxy paint has a grainer surface to stick to.

Make sure that the pain has not only dried but is fully cured. This curing process can take up to two weeks, so be careful with your painted metal for a while after the paint job.