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Penetrating Epoxy Questions and Answers

We regularly get asked about a series of things about Penetrating Epoxies, we are specialists of course. Here we present a set of Penetrating Epoxy Questions and Answers largely written by the man who invented the technology back in the 1970’s.

CPES, our Smiths Clear Penetrating Epoxy Sealer CPES FAQ. If you do not find an answer to the question you have please use the contact form to ask us. If we don’t know, we will ask the inventor; and if the answer isn’t somewhere in that pile of papers, it probably does not exist!

steve smith clear penetrating epoxy questions answers cpes faq
Steve Smith invented the concept of a Penetrating Epoxy in the 70’s

Penetrating Epoxy Questions

Why does Smiths Clear Penetrating Epoxy use solvents when some competitors don’t?

In recent months this has become one of the most common of the penetrating epoxy questions for us. The solvents are there to make it work properly. The performance of any penetrating epoxy that uses solvents to carry it will far outperform those that are water based.

Solvents are:

  • Expensive – water is free
  • Require metal cans to store and ship the final product in – water based epoxies can be shipped in plastic
  • Make the product expensive to ship – it’s now a dangerous goods class – water based epoxies are not
  • Make it more complex to use. Respirators may be required etc.

BUT – They do make it work and work far better than any competitors out there. It penetrates deeply as a result of the solvents – and that is what it’s designed to do and why it will continue to use a solvent blend to ensure penetration.

We will be doing some performance testing soon.

Penetrating Epoxy Brush Care and Choice

This is the most common of our Penetrating Epoxy Questions: How do I clean the brushes?

Smiths CPES is an epoxy resin (glue) in a solvent blend. Whilst we can sell you a solvent which will completely remove uncured epoxy resins from surfaces we advise against it. You run a real risk of rinsing a brush 6 times – and still having enough glue in it to make it unuseable.

Our recommendation:

  1. Use cheap brushes (CPES is water thin – it’s not hard to paint and doesn’t need an expensive brush)
  2. Prolong the use of the brush (see below)
  3. Discard them when they harden

Prolonging the useful life of a paint brush with penetrating epoxy resins

Now. This is not one of our common penetrating epoxy questions – but it ends up being offered as advice when we are asked how to clean brushes. Since that is the most common of our penetrating epoxy questions, we present it here.

Firstly here are a few facts:

  • CPES is an epoxy resin in a solvent blend
  • All chemical reactions are temperature dependent (hotter >> faster)
  • The curing of the CPES is slow – a chemical cure is 2 – 5 days by design
  • Curing of CPES or any other epoxy system is slowed down whilst the molecules are held further apart in a solvent or other diluent blend

So if you:

  1. Keep your brush moist in a little CPES left over from your first application (say in a jam jar, with cling film to reduce solvent evaporation)
  2. Keep it in a cold dark place (not your fridge – which is unlikely to be solvent safe) and away from direct sunlight
  3. It’s likely to still be soft when you go for your next coat of CPES
  4. Just dry the storage CPES from the bristles, and reuse the brush
  5. You will use less brushes and save both resources and money


What is the difference between Cold and Warm weather formula?

The solvent blend in Cold Weather formula is designed to evaporate more rapidly, making it suitable for use in colder climates and better for rotten wood consolidation. Once the solvents have evaporated the resins that are left are identical to the resins in Warm Weather formula. This is one of the most common penetrating epoxy questions that we get asked, so we wrote this article to show you how to choose the right temperature CPES for your job.

Will Paint or Varnish Stick to Smiths Clear Penetrating Epoxy Sealer?

Oh yes, and far better than to wood itself. The product is sold in America as MultiWoodPrime for this very reason. Wooden boats stay varnished for far longer, houses stay painted longer.

This is an interesting question, it’s another very common from our list of penetrating epoxy questions and the reasons behind it are quite complex as well. As such we have provided a reference page just to show how this keeps varnish finishes on wood for longer.

How much of the old paint will I have to remove?

Perhaps 95%. Clear Penetrating Epoxy Sealer will not soak through old paint, but if the remaining paint on the wood is in spots smaller than 3 – 6mm (1/4″ – 1/8″) the CPES will soak around and under the old paint. It may, however, cause the edges of the old paint to curl up. You may have to sand the wood and apply CPES again.

Removing paint is really good for two reasons:

  • Bare timber dries quickly when exposed to the air – the water will migrate quite quickly through it and dry wood is easy to treat with penetrating epoxy
  • Your new paint wil stick much better to the CPES than it will to the wood. The more you remove – the more will be properly bonded fter you ahve finished your repair

How much rot do I have to remove before applying the sealer?

Whatever can be scrabbled away with the bare finger tips should definitely be removed. Light brushing with a wire brush easily removes badly deteriorated wood.

Authors note here.

Another very regular one of our penetrating epoxy questions – Though I often offer a different answer to Steve Smith’s above. You can with CPES turn rotten wood dust, that can be crumbled in the fingers, into a block of solid plastic. You will use a fair bit of CPES to do this, but sometimes it makes sense to me to do so.

As an example if I am trying to repair timber with a tricky profile I may well choose to keep the rotten pulp, and harden it to useable, as it can make the job of filling far easier later. It’s a trade off with a bit more money on CPES to save a bit of time filling.

How soon can I paint the filler (Fill-It)? Do I have to prime it?

The filler cures overnight and should be sanded before painting. Paints really don’t stick very well at all to Fill-It – we really recommend you prime with a thin layer of CPES after finishing filling. CPES is an excellent adhesion-promoting primer for paint when the topcoat needs a primer, although most primers can be used.

How long is the shelf life?

Another of the most common penetrating epoxy questions this one. At least a year for the 1 litre cans (2 litre kit), if the containers are closed after use. Much longer for the 5 litre cans. See the next couple of comments

How does CPES age – when can’t I use it?

Smiths Clear Penetrating Epoxy Sealer – CPES, ages by solvent loss. The unmixed resins won’t go off in the cans – they aren’t mixed, but the solvent blend will change due to the more volatile solvents preferentially dissipating. This will of course affect the penetration of CPES into the fibres of the wood that is being treated. If you know you put away a half can, and there is only a third of a can left – it’s time to buy fresh product. If in doubt, mix a bit and ask yourself ‘Is this the same water thin consistency that I remember? Does it penetrate wood with the same speed and efficiency?’. As an aside, if you can’t use what you have left, dispose of it as your local regulations would have you dispose of any other solvent based paint.

How do I maximise the shelf life of Clear Penetrating Epoxy Sealer?

Smiths CPES is best stored with the cans tightly closed in a temperature stable environment. As the CPES ages by solvent evaporation, a temperature stable environment minimises the solvent loss caused by the cans ‘breathing’ as the temperature rises and falls, and the internal pressure rises and falls as a consequence. A cellar is good, a garage less so.

Can I use it inside?

Yes, but you must provide adequate ventilation such as an exhaust fan in the window. It is also a good idea to wear a mask with an organic vapour filter cartridge when working in confined spaces with chemicals or solvents.

If you are workign in a boat hull – try to arrange for a fan blowing air in, and another blowing out through a different area of the hull, and ensure you have a respirator with an organic vapour cartridge to hand.

Does it meet the Air Quality Management District regulations?


Will wood repaired with Clear Penetrating Epoxy Sealer (CPES) pass inspection?

The inspector usually stabs the wood with a screwdriver to see how easily it penetrates. After proper restoration the repairs will feel the same as new wood.

Smiths Clear Penetrating Epoxy Questions Evolution

If you can think of any answers that you would like to see on our Smiths Clear Penetrating Epoxy Questions and Answers page, please do let us know, and we will add them.