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I'm working on some MDF pieces for some design and thought of using both protective (anti-mold, Polyurethane) varnish and epoxy resin together. Problem is, I don't know which order would be best for the coats.

I saw some people using either but never both (yet) so not sure how to approach this.

The design is a bit weird but: this is for a desktop case. I saw a lot of people on different forums and youtube videos making them, but wanted to make my own design.

I planned on MDF (like some other people). Because I noticed molds was a possible enemy, I wanted to coat it with some Polyurethane varnish, to at least alleviate the "future" problem. Then I thought that the constant heat inside the case (even with good cooling, a desktop case can have roughly 20-30°C in temp, and more under heavy loads) might deteriorate the varnish, so then came the epoxy resin.

I wanted to also just put epoxy resin, at the very least because:

  1. it would alleviate the "burned" smell from the pieces I bought since they were laser cut.
  2. It would look better on my eye(s)
  3. I wanted to try and get a heat-resistant epoxy, since I don't think it would withstand desktop case's normal temperature, at least overtime.

After all of that, I asked myself which one I should put on first, especially when I learned that epoxy might not stick on every surface, and need it to be sanded/roughed, etc to be able to stick.

Any ideas or recommendations?

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    Intuition would have me do the resin first (deeper penetration), but that might swell the mdf. So do a test, make sure it works, and then see how the varnish bonds to the epoxy.
    – Aloysius Defenestrate
    Commented Sep 2, 2022 at 12:11
  • I see :) I did plan on still making experiments with 1. varnish only, 2. epoxy, 3. both on small pieces of MDF to see how they would hold/look like, but still wanted to know in advance (aware it could swell/make the wood thicker but that's not an issue on my end) @AloysiusDefenestrate
    – Nordine Lotfi
    Commented Sep 2, 2022 at 12:28
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    Since you mention coats and you're concerned with order presumably the epoxy isn't being used as a filler so this prompts the first question, why?? The bottom line here is that often there's no point in combining two clear finishes, so there really needs to be a good reason (e.g. big difference in drying time or build). "Any ideas or recommendations?" What is it you're trying to achieve? You haven't actually said what the end goal is here.
    – Graphus
    Commented Sep 2, 2022 at 13:46
  • @AloysiusDefenestrate, are you saying epoxy first because it would penetrate deeper than varnish, or it would because it's the first thing applied? Obviously yes if it's the second thing to go on (penetration could be effectively zero if applied over even a thin coating of varnish if that fully wets the surface) :-)
    – Graphus
    Commented Sep 2, 2022 at 13:52
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    I just meant any of the existing woodworking forums out there online, since the standard threaded forum (or the Reddit 'equivalent') is much better geared to asking a question and then going off on a few different tangents simultaneously as the discussion flows. Because of the unknown level of competence/knowledge of the people who'll respond I regret to say that you definitely do still have to do your due diligence (bootstrap yourself) so you don't build houses of cards based on any of a number of poorly understood concepts or outright myths, surprisingly widespread even with the Internet :-(
    – Graphus
    Commented Sep 4, 2022 at 19:30

1 Answer 1

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The discussion was getting carried away, so here we go:

Heat resistance

Polyurethane is very heat resistant -62°C to 93°C with special formulations able to handle temps as high as 150°C, however that's in a solid, cast or molded form.

All other search results I've looked at for "heat resistant polyurethane wood finish" come up with concerns about putting hot items directly from the stove or oven on a woodworking-finish polyurethane-coated table. This Woodworking.SE answer, however, indicates that a poly coat may have been what was used on the question's table that was, in fact, cooking-hot pot heat resistant for several decades.

Considering that the internal case temperatures aren't likely to be anywhere near those of a pot off the stove or casserole dish out of the oven, I'd think that it's quite likely that your finish will hold up to temps into the 40°C range for quite some time.

Alternatively, you are considering an epoxy coating. Epoxy can be had in quite a number of formulations, including very high temperature resistant ones, so it's a simple matter of choosing one that suits your expected conditions. According to this site:

Unfortunately, epoxy resin is not heat resistant. Epoxy resin can only withstand heat up to 302°F. Fortunately, there are many types of hard-curing resins, and each has a different degree of heat resistance.

Note that epoxy can "only" withstand temps up to 302°F (150°C) and that's a fair bit above what your computer components will produce. Right out of the box with epoxy you're at the upper end of what polyurethane can take so you probably won't need to look for any sort of special heat-resistant epoxy coatings - just about any common wood working epoxy should do the trick.

Mold resistance

According to this site:

Mold, Mildew & Fungus Resistance
Most polyether based polyurethanes do not support fungal, mold and mildew growth and are therefore highly suitable for tropical environments. Special additives can also be added to reduce this in polyester materials as well.

To be honest, I don't know if polyurethane based wood finishes are polyether based, so it would be wise to do some research on the particular brand you're considering. By "doing research", I really mean "contact the vendor and explicitly ask", as that would get you the most accurate answer. However, it does appear that most PU finishes are mold resistant, so that shouldn't be a concern for you either.

According to this site:

Master Bond epoxy systems are non-nutrient and do not sustain or support mildew/mold/fungi or biological growth.

Now, this specifically talks about one brand of epoxy, but note that it says "non-nutrient". I can't imagine that there are any ingredients in any epoxy formulation that would actively support the growth of mold. (I take this claim to be much like the "Gluten Free!" label on a package of bacon. Gluten is from wheat, and there shouldn't be any wheat product in bacon.)

Of course, no matter what coating you use, if there are mold spores in the air, they can land on any surface and if they get enough humidity and enough nutrients landing on them from other airborne sources, mold will grow. However, there's no indication that either polyurethane or epoxy will directly support mold growth any more than a piece of steel would.

Summary

This indicates that neither heat nor mold resistance are going to be significant concerns for your computer case. Therefore, you're free to choose your finish treatment as you desire without worry about either of these concerns.

One thing that you should be concerned about but didn't bring up is that MDF is exceptionally vulnerable to water. Anything more than average humidity exposure will cause the MDF to absorb moisture and start to swell. Once this happens, your material is shot and it's time to replace it - there's no repairing swollen MDF. This leads to a decision that provides the most water resistance for the MDF and that would, I would think, point me toward epoxy rather than poly.

Another concern is that MDF is not a particularly pretty material to work with and you'll probably want to hide it. Polyurethane coatings are, basically, clear (slightly yellowing, but basically clear). This has the tremendous disadvantage of showing off the bland MDF below. Epoxy, on the other hand, can be had (or made) in a multitude of colors, allowing you to hide the material beneath, even allowing you to match the case to your decor, should you desire.

All in all, I would use a colored epoxy for this situation, however, that's down to an aesthetic choice which is opinion based and, therefore off-topic for a question here. Functionally, I think either material would work well with a slight advantage to the epoxy.

Note: No endorsement of any of the web sites used as references or their products is implied or intended. They were chosen solely based upon their order of appearance in search results and for their ability to supply supporting data.

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  • this is so detailed..Thanks a lot! :D (also didn't thought of humidity/water damage, very good point)
    – Nordine Lotfi
    Commented Sep 2, 2022 at 17:36
  • Sadly, it took more time for me to write up an answer than it did to search the internet for the information, which is what the OP could easily have done. :/
    – FreeMan
    Commented Sep 6, 2022 at 18:10
  • ah, sorry about that. I did find some good links that explained that using both, or one after the other works in X cases, but I really wanted to see what could be said in this site.
    – Nordine Lotfi
    Commented Sep 6, 2022 at 18:25

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