You're fine.  We all do this.  Standard procedure.

Thanks so much,,, now I can sleep tonight.  Ken.

Epoxy manufacturers often say that a large majority of epoxy problems are caused my inadequate mixing of resin and hardener. With that in mind, I like to mix any fresh epoxy separately in its own container. I think that way I get more thorough mixing than if I just added the correct ratio of additional resin and hardener to an older batch, and mix it all up together. It doesn't really take any more effort. But if you mixed everything very very well you will probably get away with it.

Old Yeller

It only takes one experience of spending several days removing uncured epoxy to make a builder very careful about mixing ratios and thorough mixing. I would never add a new mix to old, even if it means tossing out some aging mixture. -Wes

I will never, never, never, do what I did again. Yesterday I mixed some fresh in with the old,,, that was my 1st mistake. Then,,, I mixed up another batch (fresh) but was having trouble with resin pump. It kept locking down and sometimes didn't come way back up. I though I pulled it up each time except once. So,,,, I added just a dab more resin to the mix. This morning, all batches were stone hard!! I had beginners luck I guess. IT WILL NEVER HAPPEN AGAIN. Thanks everyone for the help.  Ken.

Many builders swear by the pumps, but I've never trusted them. I want to be able to see the relative amounts of resin and hardener with my own eyes before I mix them together, so I measure their volumes with a calibrated dipstick in a parallel-sided flat-bottomed container. Other people like to measure by weight, but the ratios by weight are often slightly different than the prescribed ratios by volume, so that must be considered.

Old Yeller

Klitz - I mis-read what you wrote and didn't register that you said "mustard mix".  I have no problem mixing in new unthickened with a little older (within 1 hour or less) unthickened, but if I run out of mustard or peanut butter mix, I mix up a whole fresh batch, because, as pointed out above, you need to be sure to mix the epoxy thoroughly, and that's hard to do with thickener already in the pot.  But you lucked out, so all is well.  And I tend to trust the pumps to give me a true measure, but if I should get bitten by this trust, then I may re-think my options.  So far, so good.

If you check your pump calibration at the start of each session (5 squirts of resin in one clear plastic cup  and 10 in another and make sure they are the same level) and stir each batch carefully you should be ok. With a 2:1 product a squirt or two one way or the other in a 6oz. batch should not be a problem.

Best practice would be to use up a batch before mixing another. Adding resin and hardener to an already mixed batch may prevent the new product from curing fully. Fortuneately  MAS slow gives you plenty of time to work. If you keep a mass of mixed epoxy in a small container it will tend to heat up and kick (become unworkable) sooner. So it's best to spread what you have and then mix new.

I too no longer use pumps. I find I get more accurate mixes, and a lot more mileage per gallon of resin, using one-ounce measuring cups for small batches, and three-ounce cups for bigger batches. Pouring is also faster than pumping. -Wes

I have utmost respect for those who measure by volume or weight, but I've always used the pumps. Through seven boats and countless other epoxy projects, they have never let me down. If a pump starts "burping," I toss it (and the batch of epoxy it contributed to) and start with a new pump. If a pump starts sticking in the down position (it has always been the resin, not the hardener), I very gently  -- and s-l-o-o-o-o-w-l-y help it back up. And I never mix new with old.

The oddest thing I ever did with epoxy? Last year, a new kind of mosquito made its home in my driveway, a very elegant black and white striped critter, and a very aggressive biter. I caught one and submerged it in a freshly mixed, unthickened wafer of epoxy in the bottom of a yogurt cup. Now I've got a nice little trophy. It's an Asian tiger mosquito, by the way.

Jim

I meant to say 10 squirts of hardener in another cup. After checking the pumps this way  you can add 5 more squirts of resin, dump the two cups together and after you stir for about 4 minuts you have about 3 or 4 ounces.

Also, if you add wood flour or cab-o-sil the mix will heat up faster reducing working time further so it's best to get it on ASAP.