Should Epoxy Primer be Sanded?

Whether you’re covering the walls of a water tank or are looking to keep a metal surface from rusting, epoxy primer is one of the most cost-effective solutions. Applying epoxy primer is easier than painting a surface, but the primer itself doesn’t have a color, which means it often requires a topcoat.

Epoxy primer can be sanded if the topcoat doesn’t have enough adhesion, but this isn’t mandatory for standard coats. Most paints can be applied over unsanded epoxy primer without adhesion issues. However, hand sanding the primer with fine paper can improve bonding.

In this article, you will discover the best practices of sanding an epoxy primer alongside other relevant information like how necessary the sanding process really is, what to expect when you sand a primer, and the alternative uses of said primer. Let’s start with a brief overview of the subject.

Epoxy Primers: a brief overview

You should sand epoxy primer when you wish to coat it in a material that doesn’t otherwise bond with the primer. As it is, primers are usually conducive to bonding and can be painted over, coated, and sprayed on with ease, but the amount of primer applied also matters. If the primer is too thick, it becomes unstable because of vertical layering. 

Sanding can then be used as a corrective measure to thin the layer. But if the epoxy primer is applied too thin, then sanding can lead to the actual primer getting scrubbed off entirely. If you are sure that the coat you’re looking to apply over the epoxy primer isn’t going to bond with it unless you sand first, make sure that the primer isn’t laid on too thick or too thin. It should be applied just thick enough that sanding making it rougher without baring the surface under it.

Sanding Epoxy Primer – Recommended waiting period

Another factor that affects the degree to which sanding can improve or damage a project is how long you wait after you’ve applied epoxy primer. The most common error happens when you confuse dry-to-touch primer with fully cured primer. 

Epoxy needs to be fully cured before it can be sanded without making the coat uneven. If you’re familiar with resins, you know that epoxy takes a lot of time to cure completely. It is natural to wonder if that’s the case with epoxy primer as well.

You should wait two days before applying an epoxy primer. During these two days, the project should not be in a cold area. It should receive the standard heat of a coastal state’s daylight. In the absence of perpetual summer weather, you can use a space heater to bring the temperature of the coated surface to at least 50 F.

Avoid haste at all costs because if you do not wait 2 days before sanding the primer, the sandpaper will be blocked by the epoxy. More importantly, the primer will get chipped off of the surface, forcing you to reapply and start the waiting clock all over again. 

You might also wonder if you can turn up the heat well beyond 50 F to hasten the curing process. It is possible to increase the temperature to 85 F to facilitate quicker curing, but you cannot reduce the waiting period to below 24 hours.

In contrast, if you sand too late, there’s no issue. So when it comes to choosing between erring on the side of using sandpaper too early or waiting too long to sand the primer, I recommend opting for the latter. This is safer for your sandpaper and ensures that the primer will not need to be reapplied. Even if the curing temperature is closer to 85 F, you must wait two whole days before attempting to sand the epoxy primer.

Is Sanding Necessary?

Speaking of room for error, you should technically be able to apply most coats over epoxy primer without sanding. To confirm this, look at the product sales page of the specific primer you’ve used to coat the surface you wish to paint. 

If the package or the product page says “for LPU paint,” then that means the paint will bond with the primer without sanding. If a specific type of paint isn’t mentioned as compatible, that might mean you have to sand the primer but can also mean that it will not dry properly on epoxy primer.

Cost of Sanding Epoxy Primer

Since it is optional to sand epoxy primer, you should know the cost of sanding to know if it is worth the trouble. The upside of sanding the primer is that it guarantees that most paint layers will properly bond with the primer. 

The downside is that you can end up ruining the primer when trying to sand too early and will need to reapply a primer coat. This is a potential $10 to $50 investment. But if you wait more than 2 days in the average temperature, there’s almost zero risk of the primer getting ruined by sanding.

It takes one hour to sand a 28 square foot area. Since the process requires an intermediate skill level, it cannot be delegated at minimum wage. The cost of labor for this kind of work is slightly more expensive than minimum wage work. You can expect to pay $0.50 to $3.00 per square foot depending on the state you’re in and the labor supply available in the home improvement space.

Epoxy Primer Sanding Best Practices

Fully cured epoxy primer is hard to ruin, but that doesn’t mean any kind of sanding will work. You need to be careful when choosing both the mode and medium of sanding. The best practices in this section will help you figure out the best product to use and the optimal sanding method.

Choose the Right Grit

If the sanding paper you choose is too harsh, it will remove the primer and expose the underlying surface to the elements. On the other hand, milder sandpaper might fair to affect the primer. You should opt for a 320 grit sandpaper at a minimum and a 400 grit at maximum. Within this range, higher grit will require more work, while lower grit will require fewer passes.

Avoid Power Sanders

Regardless of the paper grit, you should use a sanding block and hand-sand the epoxy primed surface. Power sanders can be too harsh on the primer, and hand sanding gives you more control over the number of passes and the depth of impact. Sanding blocks are also cheaper than power sanding tools.

Wet Sand, Don’t Dry Sand

Wet sanding involves using water to soften the impact of sanding. Dry sanding leaves deeper abrasions and can remove the epoxy primer entirely. When wet sanding, you use water as a lubricant to ensure that the primer is sanded without being removed.

Best Sandpaper Product for Epoxy Primer

Dura-Gold Premium 320 Grit Wet or Dry Sandpaper

The Dura-Gold brand is one of the most trusted ones in the home improvement spaces. Its Wet or dry sandpaper works for both dry sanding and wet sanding. Since the sandpaper itself is extra fine, it has less impact per pass compared to lower grit papers even when dry sanding, but since the paper isn’t 400 grit, it cannot be used for dry sanding epoxy primer.

With over 2,600 reviews and ratings on Amazon, this product stands at a global average of 4.7 out of 5 stars. Its ease of use is rated at 4.7 stars, while its sturdiness stands at 4.6 stars, which is also indicative of it being less abrasive. The sandpaper’s amazon page features customer reviews with plenty of photos showing end results, including smooth metal and even sanded primer.

Warner Sanding Block Hand Sander

As mentioned earlier, hand sanding is the only acceptable mode of sanding primer. And Warner Hand Sander represents everything good about using your hands to sand a primed surface. It comes in two grip options, one with dimpled rubber and the other featuring standard rubber, and is dubbed “easy to use” by a majority of the reviewers discussing the product’s price. 

Of over 4,900 reviews and ratings, 68% have given it 5 stars, while 17% have rated this sander at 4 stars. This brings the global collective average of this product to a 4.9-star rating on a 5-star scale. The sander comes with a sanding sheet attached, but you can change the sandpaper.

Should Epoxy Primer be Sanded Before Adding Filler?

Filler helps strengthen a metal surface’s defense against abrasion and protects it from wear and tear. Epoxy primer also does this to some extent, and the two aren’t mutually exclusive. You can apply filler over epoxy primer without sanding the primer. In case that’s all you need to add over the primed metal, you can side-step sanding altogether.

Can You Double Prime With Epoxy Primer?

It is possible to apply two coats of epoxy primer over metal. You can also apply primer, followed by filler and paint. As long as the layers aren’t too thick or detached, the coats will remain relatively stable. When priming wood for interior use, only a single coat of paint is required. 

You can apply two coats of paint or primer for exterior wood. Since epoxy primer is most commonly used with metals, the general rule remains that applying two coats is acceptable but not necessary. Instead, choosing a high-quality filler can achieve better results.

Epoxy Primer: Alternative Uses and Benefits

Using Epoxy Primer as a Sealer

While epoxy primer is often used on metal surfaces like a car exterior, it is useful in woodworking as well. You can apply a modified epoxy primer over the top coat of paint on a wooden surface and effectively seal it, so the wood doesn’t incur water damage. That said, if you don’t feel comfortable bringing the primer down to 50% and using it as a sealer, you can opt for an epoxy sealer instead since it doesn’t need careful adjustment.

Pros of Epoxy Primer

  • Epoxy primer forms an appropriate base for smooth-finishing topcoats that would otherwise not stick to surfaces like metal.
  • It acts as an adhesive bridge that can anchor otherwise elusive coats to a low-traction surface.
  • Epoxy primer has excellent sealing ability and can effectively waterproof a surface.
  • The primer keeps the surface from coming in direct contact with the elements and extends an object’s lifespan.

Cons of Epoxy Primer

  • It takes time to cure – If you’re in a rush to paint a surface, waiting over 24 hours isn’t exactly what you might have in mind. 
  • Is bad for the environment – Epoxy might be one of the more resilient products when it comes to shielding surfaces from the environment, but that means it isn’t easy to break down and recycle.
  • Can require repetition – If the primer hasn’t fully cured before you start sanding or you sand with very abrasive sandpaper, you will need to coat the underlying surface with epoxy primer once again. This can be even more time-consuming.

Topcoats Conducive to Epoxy Primer

As we reach the end of this article, we must discuss coats that are conducive to epoxy primers. When you opt for one of these, you don’t have to worry about sanding as the topcoat itself has enough bonding ability to anchor into the primer layer. Sanding only adds to the process’s adhesion.

  • Clear sealer – Any transparent sealant can be applied over epoxy primer without sanding. 
  • Linear Polyurethane (LPU) – LPU paints stick to epoxy primers regardless of whether the primer is sanded or not.

The epoxy primer can be applied to bare steel, fiberglass, or even plastic. It is pretty resilient and can be used as a sealing agent in projects that involve these surfaces as long as they are meant to be covered by an LPU paint.

Final Thoughts

You should not sand epoxy primer if you don’t have two days to spend waiting for the primer to cure enough to be conducive to sanding. In case you choose to sand the epoxy primer, please keep it low impact by opting for fine sandpaper and use water to lubricate the process. Wet sanding a fully cured epoxy primer doesn’t affect the integrity of the protective layer.

Damien Madeira

Damien has been doing woodworking for the last 5 years. He began as a hobbyist with hand tools and slowly worked his way up to own larger machines and mill rough wood into beautiful creations. While still considering himself a hobbyist, he has a passion for woodworking and enjoys working with epoxy as well.

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