Solving epoxy problems: orange peel, sticky patches and heat resistance

Epoxy gives a beautiful result but is demanding to work with. Almost every problem that reaches our customer service comes down to four things: mixing ratio, temperature, humidity and a contaminated surface. Below are the most common problems and their causes.

Orange peel on my epoxy

Orange peel is caused by irregularities in the surface during curing. Two main causes:

• Temperature swings. Epoxy is sensitive to changes in temperature. Avoid direct sunlight and keep the temperature constant throughout the entire curing process — not just while pouring.
• Grease or dirt on the surface. Degrease thoroughly before pouring. Isopropanol works excellently; acetone or water with ammonia will do too.

A wavy surface

Waviness almost always comes from uneven application:

• Incorrect mixing ratio. See the step-by-step below.
• Not spread out enough. Work the epoxy out properly with a roller or brush.
• Too thick a layer. Roll it out evenly and pay particular attention to edges and corners, where the resin runs off.

Wet or sticky patches in my epoxy layer

This is almost always a processing error. Follow this step-by-step:

1. Make sure both components are at room temperature. The correct temperature is in the data sheet.
2. Calculate the quantity per component precisely and keep to the mixing ratio exactly. Check carefully whether the ratio is given by weight or by volume — they are almost never the same.
3. Use an accurate scale, especially for small quantities. A deviation of more than 1 to 3 percent per component already causes problems. If your scale or dosing syringe isn't accurate enough for the quantity you need, simply mix more epoxy.
4. Mix in a cylindrical mixing cup, not a container with corners. Mix very thoroughly, including the bottom and the sides. Then pour into a second clean cup and mix again. When pouring out, do not scrape the bottom and sides.
5. Make sure the surface is dry, dust-free and grease-free. Avoid silicone contamination and moisture.
6. Keep the ambient temperature constant throughout curing, ideally the temperature from the data sheet (usually 20–25°C).
7. Don't work in high humidity. Certainly not above 80 percent, preferably below 60 percent.
8. Only use colourants and fillers suitable for epoxy, and never too much. Over-colouring causes surface problems and lowers heat resistance.

Repairing sticky patches

Sticky patches are frustrating, especially after covering a large area. The thorough approach:

1. Pull the sticky, liquid epoxy away with a plastic spatula. Don't drag too far across the properly cured areas.
2. Scrape the remaining tacky layer away with a metal scraper until you reach the substrate or properly cured epoxy.
3. Can the spot be treated locally? Lightly sand that area including some of the adjacent good surface, remove the dust and pour fresh epoxy. If the damage is too shallow to repair locally, or you fear the new resin won't flow out flat, the whole surface will have to be lightly sanded and poured again.

For small, shallow sticky patches you can sometimes work faster by applying a 1-component PU varnish over the top, such as Yacht Varnish PU Satin. Whether that works has to be tested case by case.

My epoxy coasters go sticky under a hot cup

Assume you chose an epoxy for coasters that handles at least 80°C. Then check the following:

• Is the epoxy fully cured? At room temperature (around 20°C) that takes 2 to 3 weeks.
• Cure at a constant temperature, without large swings. You can speed this up by putting the piece in the oven at 60°C for 2 to 4 hours — preferably not an oven also used for food.
• Don't use too much colourant, and certainly no alcohol inks. They quickly destroy heat resistance.
• Measure the mixing ratio perfectly and mix in two cups: mix once, pour over, mix again.
• Never include the epoxy from the rim of the mixing cup in the final pour.

My epoxy coasters dent under a hot cup

Even epoxy that doesn't scorch, melt or go tacky can soften temporarily under heat. If a hot cup sits in one place for several minutes, the surface dents. Once it cools the epoxy hardens again and the dent stays.

This can't be fully prevented. All clear, unfilled resins — epoxy, polyurethane and polyester — dent under sustained load at raised temperature. An epoxy with higher heat resistance, such as HT Epoxy Resin (up to 140°C), dents noticeably less. There are also heavily filled epoxy systems that hold up beyond 140°C; the drawback is that they are opaque, often filled with aluminium powder.

Bonding with epoxy

Epoxy is a strong resin that adheres well to many substrates. The surface must be clean, dry and dust-free; sanding is recommended.

Epoxy is usually very thin. To bond with it, thicken it with a filler such as talcum powder or fibres. Prepared this way, epoxy can also be used as a filler paste. Using hollow glass spheres as the filler gives you a lightweight epoxy putty of your own making.

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