Epoxy getting soft and rubbery

That’s soft and rubbery epoxy!

In any case I’ve been fixing some badly rotted out windows using Loctite two part epoxy as a base and then finished over with MinWax High Performance Wood Filler.

Much to my dismay all the windows started cracking and when I cut out the MinWax Wood Filler I found the epoxy soft and rubbery. I know the epoxy was rock hard before I applied the wood filler and I’m at a loss to understand what’s going on.

Any home improvement Doppers have any ideas what’s causing this?

Damn this whole question sounds like one big double entendre.

Maybe the wood filler has acetone which disolves epoxy? If so, just letting it dry should harden it again.

Thanks for your response Sailor. I couldn’t find a list of ingredients but it says to use acetone to clean it up so I wouldn’t think it would be one its ingredients. Also, even when exposed to the air for a prolonged period the epoxy doesn’t harden up it just stays rubbery.

Now I’m going to have to redo every damn window—I’d like to kill someone. By the way if you’re interested it turns out that the highest rated product for this application is “Bondo Extra-Strength Wood Repair Epoxy.”

Unfortunately, mostly incorrect. The “wood dough” is some sort of filler, probably wood dust or fibers, plus something like lacquer. The solvent used in the lacquer-like substance may or may not be acetone exactly but is very similar (. The paste hardens as the solvent evaporates, or in this case, as it dissipates into the epoxy.


This time first try the composite sandwich (wood, bondo, filler) on something you can through away?

(Normal) Bondo is not an epoxy - it is a polyester composite - and also subject to acetone degradation (I’ve used acetone to loosen bondo used as an adhesive - it works quite well!)

I would suggest using EITHER one product or the other NOT BOTH

That makes sense, but why wouldn’t the epoxy re cure when exposed to air? Does epoxy even need to be exposed to air in order to cure?

This stuff is made by Bondo but it isn’t the stuff you’re thinking of.

Unless the “epoxy” is mixed with more or less equal parts of stuff “A” and stuff “B”, it probably isn’t an epoxy - it is some sort of polyester or vinylester composite - acetone will degrade either.

Again, do not mix chemistry unless either you or the manufacturer know it is safe to do so.

(I use a silicone based wood-hardener (allowed to cure for several days) and then wood filler (built up slowly) when restoring rotted wood).

I use epoxies and polyesters quite extensively in my boat repair business, but not the particular products you mention. However, it is clear to me that the failure of your repair is almost certainly due to applying a polyester resin filler(MinWax High Performance Wood Filler) over a cured epoxy. The rule in my business is never use polyester over epoxy. You can get away with it for example with a west system epoxy( I know because I’ve tested and proven this point), but I’ve experienced exactly your result with other epoxies.

The stuff Ring is using is this, clearly an epoxy. As sailor said, if you’ve gotten acetone on the epoxy, it will dissolve the epoxy. I used to use acetone to clean epoxy parts, and it would eventially rot the epoxy away; the epoxy never re-hardened. Since different varieties of epoxy can react differently, I assume that some types will be more or less sensitive to acetone, and may or may not reharden. If your epoxy stays rubbery for a long time…I think you’re SOL.

I used epoxy, the stuff in a double syringe, to fix my aquarium. Nothing watertight, just to glue pins on the hinged lid. The stuff bent and got all soft, I assume from the warmth and constant humidity. I guess I was using the wrong kind of epoxy, perhaps the epoxy you used wasn’t rated for the weather it would be subjected to.

I’ll have to have an eye test, I thought this thread said “Enjoy getting soft and rubbery”
Ah Well!

Could be the wrong kind of epoxy, sure, but might very well be that the two parts weren’t being mixed in the right proportions. Those double syringes are notorious for cocking to one side and delivering too much of one or the other parts, and you’ve got to get pretty close to the right ratio for the epoxy to cure correctly. I suspect that might be the real reason, particularly if you mixed up only a small batch.

Yes, but the wood filler on top of the “bondo” epoxy is a polyester, which explains the problem to me. I checked out this descriptiom of MinWax High Performance Wood Filler or instructions rather and this exerpt

Now that is a standard ratio for a polyester curing regimen with BPO catalyst, definitely not an epoxy. Epoxy resins to perform as per expectations require rigid adherance to ratios in the order of 6 to 1 down to 1 to 1.

Now with respect to those who claim that acetone might be responsible, I can assure you that it would be foolish to introduce acetone into a polyester filler system. To do so would only exascerbate the problem that polyester putties incur upon cure and that is shrinkage. Now styrene is present in all polyester resins, however most of it becomes part of the polyester chain during the cure rather than evaporate like acetone. Furthermore, to degrade a cured epoxy or polyester for that matter, requires a long period of immersion.

Epoxies work very well in marine applications, used extensively underwater. Water does no degrade epoxy. The epoxies used in coating systems have generally been pasticized, and in this case, Benzyl Alcohol to the tune of 30% is added to Part B of the epoxy bondo. That means that this epoxy is designed to be somewwhat flexible.

Another point to bring up (with reference to the aquarium) is as mention earlier that failure to rigidly apply the specified ratio will result in undercure. That is too little or too much hardener.

Secondly, ambient temperature can be an important factor in the degree of cure in an epoxy system. If this was the case, you might try to improve on the cure by somehow getting the temperature up for several hours.