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Harden Rotted Old or Soft Wood

How Can I Harden Rotted Wood

Smiths CPES™ is the perfect product to harden rotted wood. Use of the product on rotten wood will leave the rotten areas of the timber impregnated with a flexible yet durable epoxy resin which will effectively consolidate and harden rotted wood, leaving it with a tough surface that makes it far more resistant to damage with everyday knocks. CPES is absorbed by capillary action along the porosity which dry rot fungus creates within the wood, until all the affected areas are totally impregnated with the epoxy resin. CPES converts the cellulose in the wood to an epoxy-cellulose composite which returns strength and hardness to the wood, and strongly resists further attack by rot fungi or bacteria. The penetrating ability of CPES is second to none, and it is unheard of for rot to return to wood that has been treated with CPES.

Why can’t I use cheaper one part hardeners?

One part wood hardeners are typically made of an acrylic compound dissolved in a solvent. they are unstable in long term exposure to moisture, do not bond particularly well to the fibers of the wood. Their performance is marginal compared to two part epoxies, and they are primarily manufactured to meet a very low price point. They will not move with the wood, and the bond between wood fibers treated with them and any subsequent filler will be very poor compared to that between CPES and Epoxy fillers, either Fill-It or our Epoxy Adhesives such as Layup and Laminating adhesive. If you want long lasting repairs you must use two part epoxy products, and of all of those available on the market, these are the best.

Why is CPES the finest wood hardener available today?

Smith’s Clear Penetrating Epoxy Sealer™ (CPES™) consists of a tough, flexible resin system in a solvent blend which dissolves the sap, oil and moisture found in wood. The resin system is derived from natural wood resin and develops a chemical adhesive bond to the wood fibers themselves.
The wood is strengthened while allowing normal expansion and contraction with changes in temperature and humidity.
When repairing wood that has mild dry rot, the CPES will travel through capillary action along the abnormal porosity which dry rot fungus creates within the wood, as long as it continues to be fed in. Impregnation of wood with CPES changes the cellulose of wood (which fungi and bacteria find tasty and easily digestible) into epoxy-impregnated cellulose which resists further attack by fungi and bacteria while reinforcing the wood, accomplishing restoration.

OK, How Do I Harden My Rotted Wood

Prepare The Old Wood for Hardening

Remove any loose rotten timber, as a rough guide anything that can be easily removed with the fingers is not worth keeping. Anything much firmer will be hardened into something usable, certainly firm enough to take fixings, and adding some structural integrity. Rotten beams can be made firm enough to take joists again for example.

Brush dust and debris away to ensure it does not get glued to the surface with the CPES and become a permanent feature of the timber.

Mix the Clear Penetrating Epoxy Sealer (CPES)

For hardening rotten wood, especially large bits of rot in big sections of timber, we really recommend the Cold Weather formula.

Exceptions to this recommendation are:

  • you already have Warm formula around
  • you wish to use it for other purposes and live in a warm climate.

The solvents evaporate much quicker in the Cold Weather formula and evaporation times from big sections of rotten wood can be days or even weeks even with the cold weather formula. Either one will however work, but the job may take significantly longer.

The mix ratio is 1 to 1, so is very easy to get right. This short video covers mixing:

How to Mix Smiths CPES, Clear Penetrating Epoxy Sealer, Repair rotten wood, prime new wood

Apply the Clear Penetrating Epoxy Sealer (CPES)

Use a disposable brush or have some Smiths Epoxy Clean Up Solvent or lacquer thinners around to clean the brush with.

Apply CPES by brush to the wood, apply as much as the wood will take and allow the solvents to evaporate and the CPES to soak in. If you are trying to harden rotted wood which is in very large sections (beams for instance), drilling holes and pouring CPES into a funnel placed in the hole is an easy way of getting rapid penetration. The CPES will travel quickly through rotted timber, especially along the grain.

Harden Rotted Wood in large sections using holes and a funnell

Two or three coats can be done in one day if working on small sections of rot often from one mix of CPES, provided that it is kept in a closed container and at a low temperature between coats. if you are working on large sections of rotten timber it may take days for teh solvents to evaporate before another application can be made. Keep the timber dry whilst the solvents are evaporating.

A final coat is normally given the next day (or the day before the top coat is applied). The wood has had as much CPES as it will take when the final coat dries with a sheen. The top coat will chemically bond to the CPES if applied while the CPES is still chemically curing (one to 3 days after application, typically done 24 hours after the final coat of CPES is applied) and after the solvents have all dispersed (the surface will appear touch dry, and will no longer smell of solvents).

How Hard Will My Wood Become

Very is the simple answer. CPES will have penetrated a good distance through sound wood, leaving a durable surface which will be resistant to knocks. It sets to leave a hard yet flexible coating on the wood fibers.

It is very hard to describe the performance of a hardener on wood, so we have prepared a short video to demonstrate how hard the wood becomes. The below is a 130 year old oak windowsill hardened with CPES:

130 Year Old Timber (Oak) Windowsill Hardened With Smiths CPES: Clear Penetrating Epoxy Sealer

 What Now That I Have Hardened My Timber?

Your surface is now waterproof, but micro-porous (breathable).

You can now apply Fill-It filler to restore the surface if required, or just paint or varnish with your preferred top coat. As CPES is microporous your top coat can be one of the modern breathable paints.

Please Note: If your wood is exposed to sunlight, you MUST either:

  1. apply a top coat (paint or varnish), as CPES, along with all epoxy resins, has no UV tolerance
  2. rinse it off of the top surface to leave a matt finish, resembling natural timber. Notes for how to do this here
  3.  accept that the top surface will decay over time, going from relatively shiny to the natural surface described in 2 as the CPES weathers

Additional Reading

Further information can be found here:

The Rot Doctor in the states has conducted significant penetration testing comparing CPES to it’s competitors, the results are astonishing, read them here.