Epoxy OK In Sunlight?

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.


Well-Known Member
Nov 13, 2003
Reaction score
This seemed like the best section to post this thread under.

My question is this: Is epoxy especially suceptable to UV rays. I know that over time it is weakened, but how much time? Is it ok to take a rocket with external epoxy outside for a launch without first giving it a shot of primer? I began to wonder when I realized I have never seen a High Power rocket without at least primer on it out at launches. Thanks.
Is epoxy especially suceptable to UV rays.

YES very much so!

Extreamly so...it needs to be protected by paint or varnish.

wow this is really really interesting. I didnt know that at all, and it is a good thing to know. Sandman, what happens to the parts of your rocket (say altimtter bay) that arent primed. I know they dont get hit with much UV but will they be okay. Hmm I think I am gonna paint everything that gets exposed to any sunlight next time I build
Direct sunlight is the killer.

A little occasional exposure is harmless.

Epoxy will start to cloud first, then become brittle and weak.

Ok thanks for that info! So if I were to take my un-primed/painted rocket to my clubs launch this saturday...just once, would it be ok?
Most epoxies, sure, you will be OK to use your unpainted rocket for a day. Especially if you keep your rocket under cover most of the time (like in the car?) and just bring it out for a few minutes.

I think you would have to leave your epoxy/project out, exposed to the weather, for something like a couple weeks of 24/7 before you would begin to get serious degradation. People who home-build aircraft with epoxy/fiberglass generally paint their projects before taking them outside, but that is for something that is going to sit out on an airport ramp all the time.
Thanks, that helps. Unfortunately the launch was cancelled.:( Guess it'll have to wait till April...
but at least it gives you plenty of time to paint, and that way you dont need to worry:D
Sure it would!

It sounds like that's what it's made for!

Copied from the West System Epoxy site:

***The function of a finish coating like paint or varnish over an epoxy barrier coat, is to decorate the surface and protect the epoxy from sunlight. In doing so, the finish coating extends the life of the epoxy moisture barrier, which, in turn provides a stable base that extends the life of the finish coating. Together the two form a protective system far more durable than either coating by itself.
Protection from sunlight is a primary consideration in the selection of a finish coating. Long term UV (ultraviolet) protection of the barrier coat depends on how well the finish coating itself resists UV and keeps its pigments, or its shield of UV filters on the surface of the epoxy barrier coat. A high gloss finish reflects a higher proportion of the light hitting the surface than a dull surface. All other thing being equal, a white (especially a glossy white) coating will last the longest.

Most types of coatings are compatible with epoxy. Thoroughly cured epoxy is an almost completely inert hard plastic. This means most paint solvents will not soften, swell or react with it. However, it is still a good idea to build a test panel to assure coating compatibility.

Coating types

Latex paints are compatible with epoxy and they do an adequate job of protecting the epoxy barrier from UV radiation. In many architectural applications latex paint may be the most suitable coating to use. Their durability is limited.

Alkyd finishes-enamel, alkyd enamel, marine enamel, acrylic enamel, alkyd modified epoxy, traditional varnish and spar varnish-offer ease of application, low cost, low toxicity, and easy availability. Their disadvantages are low UV resistance and low abrasion resistance.

One-part polyurethanes offer easy application, cleanup and better properties than alkyds. They are also more expensive and some may be incompatible with amine cure epoxy systems such as WEST SYSTEM epoxy, although 207 Hardener may offer better compatibility. Test first.

Epoxy paints are available in one-part and two-part versions. Two-part epoxies offer many characteristics similar to the higher performance polyurethanes. They are durable and chemical resistant, but offer limited UV protection compared to the linear polyurethanes.

Two-part linear polyurethane (LP) paints offer the most durable protection available. LP's are available as pigmented or clear coatings and offer excellent UV protection, gloss retention, abrasion resistance, plus compatibility with epoxy. However, compared to other types of coatings, they are expensive, require more skill to apply and present a greater health hazard, especially when sprayed.

Bottom paints are available in a variety of formulations. Most bottom paint systems are compatible with epoxy and can be applied directly over a prepared epoxy barrier coat. If you are unsure of compatibility or have curing or adhesion problems with a specific bottom paint, use only a primer recommended for that bottom paint over the barrier coat. Follow the recommendations given for preparation of fiberglass surfaces. Other paints, including marine LP's and primers, are not recommended for use below the waterline.

Primers are usually not needed to help a paint film bond to epoxy, although interfacing primers may be required with some specialized bottom paints and high-build primers are useful for hiding scratches or flaws in the substrate. If the instructions on your paint or varnish recommend a specially primed surface, follow the recommendations given for fiberglass preparation. Self-etching primers do not work well on an epoxy coating because of epoxy's chemical resistance.

Polyester gelcoat is a pigmented version of the resin used to build fiberglass boats and other products. Gelcoat is sprayed into a mold before the glass fabric and resin are applied to provide a smooth pre-finished surface when the part is removed from the mold. It is not often used as a post-production finish coating, but it can be applied over epoxy and is useful in some repair situations. Refer to 002-550 Fiberglass Boat Repair and Maintenance, published by Gougeon Brothers, for detailed information on patching gelcoat over an epoxy repair.***

Sorry for the length. I found it helpful.