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Epoxy

Intro

A short pause from the woodwork before initiating the first steps with epoxy. There are two large families of resins used in boat making: polyesters and epoxies. Epoxies are maybe 5 times more costly than polyester... so why?

I won't enter in technical details here - there are plenty sources of information on the 'net. My advice: use epoxy. Mechanical properties are vastly superior, adherence and protection of the wood is much better and justify the investment. Also, epoxies are much more versatile (see below). If you want to keep cost down, using polyester, at least try to obtain the marine kind!

IMPORTANT: Don't let them sell you polyester instead of epoxy! At least here, there's a lot of confusion regarding what's what. Try to find a good, reliable source, else you may even find a salesperson who's convinced both are the same.

Globally, only a few sources for epoxy exist. The different brands formulate their own products, mainly differing in the accelerants. The add other products to control times, temperatures and several other problems.

Accelerants normally are hygroscopic - and humidity absorbed causes problems. Keep those containers closed!

Basics

Before use, epoxy is mixed with it accelerant, following the instructions of the manufacturer. That's it. No discussion. This is not like polyester, different ratios do not change hardening times, just ruin the mechanical properties.

Hardening times are specified by the manufacturer at a certain temperature, frequently 25 C. Approximately, this time halves each time the temperature increases 10 degrees C. By the same rule, times double with a 10 C decrease of temperature.

Epoxy always hardens if the mixing ratio is correct. This may take a week if it's really cold, but don't dispair... Try to avoid working below 5 C.

Mix well. Repeat after me... mix WELL. The mix is a very thick liquid, which like to adhere to the sides and in the corners of your container. Use something wide to mix - like those wooden icecream sticks. And mix till the liquid is completely transparent.

Wait a few minutes before applying. I try to detect an increase in temperature before starting to apply. My (little) experience indicates that applying without the wait increases greatly the hardening time afterwards. Of course, if the manufacturer indicates otherwise, believe him, not me...

Applications

Basically two types of applications: covering/filling fiberglass, and filleting/filling/glueing.

To cover and use with glass cloth, use epoxy without additives. That way it penetrates the wood, and produces an excellent protection and adherence.

If used to glue or fill a space, it's important some epoxy remains between both parts. Too much pressure eliminates the resin, and produces a weak joint. One trick is to mix a bit of filler, eg. 10 % of talc powder. The talc particles maintain a minimum separation. Mind: first mix the epoxy to the manufacturer's instructions, then add the rest!

If you add - up to 100% of the weight of the resin - of talc, you'll get a putty which is easily applied to fillet the stitched panels and stays in the shape it's apllied. It becomes very hard, but is easy to sand.

Note: Recent studies show that talc powder contains trace particles of asbestos, a cancer producing product. I've read lots of different opinions, but try to avoid breathing the powder!

If fine wood powder (flour) is used instead of talc, you get a light mixture, good for repairing and filling visible spaces. Mind, epoxy slighly darkens the wood powder, so mix and test before applying.

Problems

Applying epoxy seems easy, but has a few secrets. These are some of the problems I experienced:

Amine blush

If you work in a space heated by gas, or simply the weather is humid, the hardener may react with the water and produces a sticky fluid (amine) on the surface of the hardened epoxy. So, before applying the next layer, check the surface, and, if necessary, clean with a soap and water... Special boat resins may have additives to reduce the possibility of amine blush. Not much of a choice here though.

Bubbles

If epoxy is applied in the morning, and temperature rises later on, it's quite possible that any humidity remaining in the wood tries to escape, and leaves small bubbles in the coat. These are very difficult to remove, so try to paint the epoxy on the warmest moment of the day. Or heat the place while painting, the cut off the heat.

Milky color

Applying the epoxy, and trying to 'even out', with a lot of passes, I found that sometimes the resin takes a milky color. It seems to disappear on hardening, but try to avoid it at all by applying the resin with as few movements as possible.


(c) John Coppens ON6JC/LW3HAZ mail