CPES – Smiths Clear Penetrating Epoxy Sealer

What Is Clear Penetrating Epoxy Sealer™?

Smiths Clear Penetrating Epoxy Sealer™ (CPES™) is a thin epoxy that will penetrate deeply into the surface of wood. It is the original product of Smiths and Co. and has been restoring and protecting for over 40 years now. It is also known as Lignu in Scandinavia and MultiWoodPrime for new wood applications.It is often abbreviated to CPES.

There is no substitute that performs as well. It is not cheap, but it is significantly cheaper than wood, repainting and maintenance generally. Do it once, do it right.

Clear Penetrating Epoxy Sealer is an epoxy product, supplied as Part A and Part B in two tins. It has to be mixed before use. It is mixed in a 1 to 1 ratio, and is tolerant of minor mix errors. If you wish to mix by weight, both Cold and Warm formula are mixed at 98 parts A by weight to 100 parts B by weight.

Packaged between the two cans is a very comprehensive instruction leaflet. Please read this prior to use if you are unfamiliar with this product.

Smiths clear penetrating epoxy sealer

Curing Rot With Clear Penetrating Epoxy Sealer

Clear Penetrating Epoxy Sealer is capable of restoring rotten wood and allowing it to resist further attack by rot. Often used in conjunction with Fill-It for this purpose to restore the original section of the timber. It’s use is well established in the marine world, where the cost of replacing timbers in a wooden boat give a significant incentive to ‘Restore not replace’, however it can be used to good effect to repair structural or cosmetic timbers in many other environments. Details on restoration use are found here. Because Clear Penetrating Epoxy Sealer and Fill-It work so well together, we sell repair kits containing both products at a discounted price too.

Coverage of CPES on rotten timber

This unfortunately is very hard to estimate:

On badly rotten timber, which you can crumble to dust with your fingers, you will typically use 2 or more times the volume of rotten timber in CPES to harden your wood dust. You will however end up with a piece of plasticised wood that you could beat someone to death with. I will often harden wood like this as it can make the restoration of a profile far easier than removing the dust and then filling, even though it is a little more expensive. The solvents can take a long time to evaporate from timber in this state if the sections are large.

CPES will find rot that you were not even aware of in less rotted timber; the safest way to estimate requirements is to actually try a section of your rotten timber. If you have a lot to do, I normally recommend you buy a 1.9 litre pack and use that. After treating as much as you can with that you will have an accurate idea of how much you will need for the whole job, if it is not sufficient to do the whole job.

Priming Wood to Make Paint and Varnish Last

Clear Penetrating Epoxy Sealer penetrates deeply into timber, especially so at the end grain. This penetration strongly resists the ingress of moisture, protecting joints and end grain particularly thoroughly. It also promotes adhesion of the top coat, which will bond far more strongly to CPES than to the wood itself, thus significantly increasing the life of the top coat and reducing maintenance costs. See here for full instructions on its use as a primer. Clear Penetrating Epoxy Sealer can be used without a top coat to achieve a natural surface finish on timber whilst protecting it from the elements, see here for instructions for this application.

CPES Coverage on new wood

CPES use on new or sound timber is far easier to estimate. As all timber varies these figures are only estimates:

Treatment is for two coats on NEW timber:

  • Softwood or plywood: 5 sq metres / litre
  • Hardwood: 10 sq metres / litre

If the wood is old, porous or has suffered significant exposure to sunlight or the elements, it is normally safest to halve these coverage figures (ie you will use double the amount required for new timber).

Curing characteristics: Which Formula CPES?

Clear Penetrating Epoxy Sealer is available in two formulations, a warm weather and a cold weather formulation. We stock and supply both formulations. For use in the UK Cold Weather Clear Penetrating Epoxy Sealer is recommended, Warm Weather Clear Penetrating Epoxy Sealer should be used if the temperatures will exceed 25 degrees centigrade. Check which temperature CPES you require if you are unsure.

Clear Penetrating Epoxy Sealer CPES working and curing times

Working With Smiths CPES, Clear Penetrating Epoxy Sealer

The two parts of Smiths CPES are supplied in separate tins, and the materials are mixed 1 part to 1 part by volume prior to use. I find the easiest way is to take a parallel sided jam jar, decide how much you wish to mix, and make marks to suit, say at 4 and 8cm up from the base on the jar. Pour to the marks, and shake gently. Should you wish to mix Smiths CPES by weight the ratio is 98 parts of A to 100 parts of B. Smiths Clear Penetrating Epoxy Sealer is very forgiving of minor mix errors and the only real penalty will be more of one can left than the other when you have finished the job.

If you like video’s, there is a short one on this page showing the mixing procedure outlined above.

Prior to mixing you will find two metal inserts under the child proof caps. these inserts tightly seal the cans against solvent evaporation. I find these easiest to remove by hammering a small screwdriver through a cap, and prying it out.

Solvent evaporation is the main means by which CPES ages, store the cans in a temperature stable environment such as a cellar, with the caps tightly screwed on, and 1.9 litre packs are useable well over a year later, bigger cans even longer. The resins do not go off or set in the cans.

ALL epoxy resins can cause skin sensitisation. Please always wear rubber gloves when working with CPES.

If you have some left over mixed CPES and wish to use it for the next coat, lightly cover the container in which you mixed it (Do not tightly screw the lid back on the jam jar, once the solvents have evaporated the resin is hard to the touch, and will glue the lid back on), keep it in a cool dark place (fridges are not recommended as sparks can ignite the solvents). It will remain useable for a surprisingly long time. Brushes likewise will soften again once placed into the container prior to applying the next coat.

Priming for Caulks and similar

Modern polyurethane caulks, such as 3M 5200, will bind extremely strongly to CPES, as will the hybrid caulk Everbuild Stixall.  This capability can be used to very good effect to achieve permanently sealed boat hulls, and permanent flexible sealed joints in timber framed houses etc.

More Information

FAQ – Frequently asked questions, with answers, about CPES

Creators Notes – From the pen of Steve Smith

Six simple steps to restoration

Penetration testing with the Rot Doctor – The best vs the Rest

How to create the perfect varnish finish

How to restore rotted wood, the science and the practice

CPES Reviews

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 22 reviews
Deck Repair

I used 3 products to repair the delaminating deck on my 33 year old boat. Rain water had found its way into the balsa sandwich between the grp layers and soaked and partially rotted some of the wood. This required drilling numerous holes in the affected part of the deck. After allowing a few weeks for the balsa core to dry out I used a syringe to inject CPES into the affected area to consolidate the wooden core and protect from rot. Layup and laminating epoxy with added silica was then injected to fill any voids. Finally Fill-It was used to fill and fair the holes. I then painted the deck with anti-slip paint. This process seems to have done the trick. The deck is strong and stable. It just remains to be seen if the repair stands the test of time. I found all 3 products extremely easy to work with.

Indoor Planters

Instead of lining all my orangery planters with plastic, I decided to use CPES. It was very easy to apply, and so far has worked brilliantly. My planters are on a wall inside, so I did not want water dripping down the wall from my automatic watering system. So far so good, I do keep checking the underside of each planter, and to this point no water is seeping through at all. I should just mention that I did apply a silicone sealant to the joints between each wals and the floor first as well as the CPES. Whilst I was applying the CPES I had a question about multiple coats. I posted the question, and within hours had received a phone call and was talked through exactly what to do. So all in all, for the product and the customer service, I would heartily recommend CPES.

Smiths CPES

I can only comment on the short term at this stage. Steve is a very helpful chap - call him with any questions and he always gets back to you. I used CPES on the teak frame on my boats superstructure prior to varnishing, and it has worked a treat. Very happy. Also has worked wonders on repairing some marine ply parts of the superstructure. Recommend both the product (though expensive for sure) and the vendor - top people.

Yacht rebuild

Have used CPES as a primer prior to painting on ply interior refit of small yacht. Application is simple and effective, a couple of coats sealed the good quality exterior ply I am using. Paint took well to the sealed wood. Obviously yet to see if it helps the wood last longer in the marine environment. Will update when I know!


I use CPES as a primer on new wood prior to varnishing with Epiphanes. The CPES is a doddle to use, I use medication measuring cups to achieve the correct mix and just paint it on, it goes on like water. Keep applying until you achieve a shiny surface then apply the varnish before the primer fully dries. I have used this method on many wooden items on my boat, whether used inside or out and have had fantastic results and good longevity of the coating. I use CPES on new items as well as recoating old ones, it is expensive initialy but will reduce the amount of time, money and effort needed to keep wood looking good in the future.

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