How to Seal Plywood for Outdoor Use

How to Seal Plywood for Outdoor Use?

Plywood is one of the strongest and long-lasting materials used for house building and remodeling. Many DIY workers also prefer this wood because of its longevity. Plywood is also affordable compared to many other kinds of wood. Unfortunately, it is a known fact that exterior plywood is vulnerable to moisture.

To prevent it from surface cracking, plywood demands sealing which not only saves it from water damage and decay but also ensures its long-term performance and endurance.

After sealing, the woodwork is finished with paint or some kind of strong resin such as polyurethane. The required tools for sealing of plywood can be easily found in any hardware store. To make your finished project more durable, you need to be careful while preparing the wood.

However, the sealing project can be sometimes tricky when it comes to sealing rough surfaces. Don't worry! Our step by step guide on how to seal plywood for outdoor use will assist you even with sealing rough surfaces.

Benefits of Sealing Plywood

Benefits of Sealing Plywood

Sealing plywood has any advantages. It will save you from extra expenditure and your wood from decaying.

1. Prevention of Rotting

All exterior woods are prone to gel mold due to its nature and other factors like humidity. a rot makes a wood damaged and also destroys its appealing look. Rot can be developed on ply-wood in various ways. The best way to protect it from any type of mildew is by sealing it.

Sealing ply-wood properly and efficiently will save you from future trouble.

2. Protection from Sun and Moisture

If water penetrates plywood, it just not only cause rot. Extra water penetration can cause wood to break. This can largely damage your plywood material. To prevent it from happening, seal your wood as soon as possible.

Sunlight rays also damage the wood. More intense exposure to the sun will result in the twisting of wood. This can cause wood to lose its natural beauty and structural integrity. Sealing plywood will help to maintain its structure and appearance.

3. Balance Sheet

Sealing wood will reduce its chances of splitting. Sealing keeps plywood strong and prevents it from shrinking, expansion and twisting. A balanced wood surface will reduce the chances of wood warping.

How to seal plywood for outdoor use: Step by step guideline

Following are some steps taken during sealing of plywood:

  • First, all sides of plywood are sanded by using sandpaper of 80-grit measurement. It is better to sand in back and forth motion and along the natural grain of the wood rather than across the natural grain.
  • To get a smoother finishing touch, sanding is repeated with 120-grit sandpaper. Sandpaper of higher grit can also be used for an even better smooth finish but in the case of plywood, it is not required.
  • By using a clean cloth, plywood dust is wiped off. For applying a sanding sealer, a dust-free and clean area is required. You also need a clean paintbrush to apply the sealer.
  • With the help of a clean paintbrush, sanding sealer is applied on the plywood in even and leveled brush strokes to coat the surface. Unevenly coating saturates the surface making it rough. The essential element for a proper and smooth finish is even coating. Let the coating of sanding sealer dry, as instructed by the manufacturer. If you wish to seal both sides of the plywood, do it in steps. First, finish one side and let it dry. Then seal the second side.
  • Now, the sealed surface is again sanded using 120-grit sandpaper. This light sanding removes the bumps caused by the first sanding and cause airspaces (left from sealing) to diminish. Sand till you get a smooth touch but without damaging the sealing. This causes the plywood to appear waxy.
  • In this step, a final coating of the sanding sealer is applied and allowed to dry. This gives plywood a long-lasting finish.

3 Common Technique For Outdoor Plywood Sealing.

Plywood Protection Tips

To ensure a long life of plywood outside, it is necessary to weatherize it. You can simply do it by yourself with the help of the following methods. This whole project will just require only 1-2 days.

1. Polyurethane Varnish

One of the best methods to seal plywood edges outside is by applying exterior polyurethane varnish. You will require the following materials.

  • Small brush
  • Varnish
  • Masking Tape

Apply a coat of polyurethane varnish to the edge of a wood with the help of a brush. Avoid it using on the surface of the plywood. Cover the surface with masking tape. Do a full coverage with tape to block varnish.

If varnish gets a hold on the plywood surface, it will not allow the wood to absorb any future wood treatment. Now after 1st coat, wait for 30 minutes. After that, apply several layers of varnish for the best results.

This treatment of plywood edges will prevent it from water penetration. It also helps to save it from dreadful weather conditions.

2. Water-Based Stain

Cover the entire surface of the plywood with a coat of any exterior water-based stain. These stains are very light in color and do not show any bright shade. These work best for protecting the surface against strong UV rays. Grab the following material

  • Pump-up Sprayer
  • Cloth rag
  • Water-based pigment stain

With the help of a garden or pump-up sprayer, apply the stain. Allow it to dry for several minutes. Drying time will always be mentioned in the stain container. For best results, follow the mentioned timeline.

After spraying the whole surface, use cloth rage to clean any excess or dripping stain. Keep in mind, this water-based pigment stain is not proposed to completely paint your plywood. They will give a light color and protection to your wood from sun damage.

For the best effect, leave your wood for at least 24 hours after applying the stain coat.

3. Water Seal

This sealer is great for guarding plywood against water penetration. It is a good wood conditioner and doesn't block all pores of the wood.

Things you will need are as follows.

  • Water sealant
  • Garden sprayer

Take a garden pump-up sprayer. Apply a coat of water seal to your plywood. After smoothing the surface, apply another coat. Use several coats of sealant to get a maximum result against water penetration. It is best to reapply sealant after every 2 years.

A wide range of water sealants can be easily found in any kind of paint store. They usually come under manufacturer's names.

Which Plywood to Choose For Outdoor Use?

There is a variety of exterior plywood and it is important to know which one will benefit you the most. If not selected properly, sealing will not be useful. It will be prone to become rotted, soft, and break. The most common types of exterior plywood are CDX, marine-grade, pressure treated, OSB, and T-11.

Marine-grade exterior plywood has knot-free sheets and is mostly used for decking, porches, planters, and arbors. CDX is the most commonly used plywood. It normally comes in piles of 3, 5, 7, 9, and the lowest number indicates the uppermost class of a CDX plywood. They are widely used in the construction industry and for the roofing of buildings.

Which Plywood to Choose

Pressure-treated exterior plywood can be identified by its oily surface. It is prepared by infusing regular plywood with different chemicals under a certain pressure. These chemicals treatment makes this wood strong against rotting and mildew.

The main purpose of OSB plywood s is roof decking. It is manufactured with a water-proof glue and provides safety against excessive moisture. T1-11 is mostly used for sidings.

The best thing about this plywood is that it can be painted with natural color and can be well-matched against different color varieties.

  • If you have a limited budget use a CDX plywood. A Urethane coated CDX has very strong resistance against weather conditions.
  • While working with horizontal installations, pressure-treated plywood is a good choice. This wood lasts longer even in unfortunate drainage systems.
  • If you are working on a project that includes outdoor vessels for water flow, prefer marine-grade plywood. This type of wood is expensive but strongly resists moisture. It is perfect to use it for underwater applications like boats.
  • If you are working on any siding project, choose a T1-11. This type of wood is also good for big exterior wooden boxed. It can be easily stained for any desired color and appearance.


Now you have become familiar with our guide on how to seal plywood for outdoor use. Always select suitable plywood for your exterior project and then seal it step by step. The above techniques are used by professional woodworkers to extend an exterior plywood life.

Above mentioned methods and tips are also valuable for protecting edges of the wood. Sealing edges of the wood are significantly important because they are more exposed to weather conditions. If you seal plywood flawlessly, it will enhance your wood resistance capacity.

We are sure that you have got complete knowledge of sealing plywood and will be able to seal your wood like a pro!

Recommended To Read:

  1. User guideline for sealer for cedar swing set
  2. Cedar wood sealer: which one you should use?
  3. Top 6 industry quality sprayer for Thompson water seal
  4. A guide to choose right Sealer for cedar fence
  5. 6 steps to apply Thompson water sealer

28 thoughts on “How to Seal Plywood for Outdoor Use?”

  1. I want to make a small extension to my existing shed what plywood can I use please?, and what proofing should I use? The extension size is 6ft long and 5 foot high, and by 2foot wide.thankyou.

  2. Hi There, I would like to use 18mm ply for work tops in the utility room and paint the tops with F&B.
    Is it best to use Polyurethane Varnish on the edges and perhaps the underside of the edges?


  3. Hi – Thanks for the article.
    I am using plywood for an exterior skateboard ramp. There are 3 layers of 6mm ply (2 cheap pine layers and top layer is Marine ply). I have already sealed all the ply (including edges) with diluted Bondcrete. I am painting with a water based paint on top of this (both sides of bottom and middle layer), but for the top layer of ply I’d ideally like to leave the wooden colour/grain visible. Is there a suitable clear product you can recommend that will go on top of the marine ply that has been sealed with the bondcrete that is hard wearing and weatherproof? Thanks, James

  4. We built a kicker ramp for out 9 year old grandson out of 1/2″ plywood. I want to put decals on the side and then then varnish the entire kicker ramp, leaving it in a natural looking finish. Can I use polyurethane varnish on top, sides and bottom, and are all brands of varnish consider equal? OR would you recommend I finish it in some other way?
    Thank you,

  5. I used 1/2โ€ ply on the walls of my wood shop. Would sealing the exposed side with 1:1:1 (thinner, spar varnish, and linseed oil) cause the plywood to delaminate? I want to keep the natural color but also want to seal it.

  6. What would you recommend for a chicken coop? It was built about 6 months ago with non marine grade plywood, but nothing was ever applied to the wood so now it is starting to expand and bow a bit, nothing drastic but I would like to remedy it before it gets worse. We plan on painting it in about 6 months as well

    1. For you situation, you can try this process but please be careful. If you cannot observe the board continuously then you might create another problem.

      Step 1: Put or spray hot water on the concave area of the wrap plywood.

      Step 2: Immediately put the board on under the sun placing the wet side down and also put weight in the middle of ply so that it can return back to its previous position.

      Step 3: Keep an eye on the board, whenever it starts straighten up. Also put weights on the both end so that it does not bend on the other side.

      Hopefully this process will help. ๐Ÿ™‚

  7. I want to make a cover for my pool using plywood. Can I just put waterproofing roofpaint directly onto the plywood to keep it waterproof?

  8. John Schweighardt

    I built a “box” around my Generac home generator with 1.5 inch treated lumber to eventually reduce the decibels when it operates. I now want to cover with plywood that will be screwed onto the box (about 5 foot panels around the top and two sides. I can buy treated plywood, but only with two-quarter or three-quarter sheets. This is more expensive and heavy to use. I can get three-eighth sheets that are not treated and with a better result. Can I utilize your process and when dry, paint over with a water-based primer and/or a water-based fortified acrylic house paint? I built the box on a treated wood rectangle filled with small stones, and did use the the primer and paint. It’s a year old and still looks very good. Any thoughts for a major senior?

  9. I’m building a bird aviary in my back yard and would like to make the floor out of plywood with a drain in the middle so that it can be washed/rinsed out as needed. The floor will be about 4 to 6 inches from the ground. What would you suggest to seal the floor with so that it last.

  10. Iโ€™m making an insect hotel with 6mm plywood. Should this process still be used to preserve the wood?

  11. Hello
    Iโ€™m making an outdoor bar.
    What would you recommend for sealing, staining, and protecting the plywood from moisture and sun?
    Also which plywood would you recommend? Iโ€™d prefer to see the wood grain.
    Thank you!

  12. I am making a workbench in a new shed. 22mm marine ply 60cm x2.3cm ,I have sanded top smooth and wish to seal and colour it a medium brown colour with varnish. Can you please recommend a hardwaring surface.

  13. I am constructing a plywood cab-back for a truck that will see 100% weather for max. one year.
    I must have a finished product that is white.
    What is the best process to weatherproof and appear white?

    1. You can use the 3 steps in the article above:

      1. Apply Polyurethane Varnish to the edges of the plywood.
      2. Apply Water-Based Stain to the entire surface of the plywood.
      3. Apply Water Seal to the entire surface of the plywood.

      Before proceeding to step 3 you have to wait for the underlying finish to be absolutely dry.

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