: Has anyone else had trouble with their plunger-type epoxy pumps? I just had
: another one go bad. During the downward pump stroke, it spits a slug of
: air. There must be a bad seal or check valve that lets air in during the
: up stroke even though the pump is almost new. Is there a fix for this? For
: some reason, it is only the resin pump that I have had trouble with. I
: tried heating the resin and pump thinking that there might be some
: crystalized resin gumming things up, but it didn't help.

This is a common problem with these lotion pumps. They were not designed for accurate volumetric measurement and delivery of fluids. That's why they are so cheap. Several things can go wrong:

A chunk of crystallized resin or foreign matter can jam the check valve;
The higher viscosity of the resin side can make the pump "burp" air past the top seal on the upstroke;
The check valve can fail to reseat itself and resin can drain back down the fill tube;
On the hardener side, the stuff can crust over inside the outlet and partially backflush itself down the fill tube.

They do get gunked up over time, and need to be disassembled and cleaned out with solvent. One way to avoid pumping air on the resin side is to put the whole bottle in a pan of 120* water for 10 minutes to reduce the viscosity. Another is to "ride" the pump on the upstroke with slight downward force from your hand, to slow down the refill and reduce the tendency for the rebound spring to overpower the flow through the check valve and burp air past the top seal. Another is to pump into graduated measuring cups to visually verify that the volume is correct, but that defeats the purpose of the graduated pump; you may as well pour directly into the measuring cup.

I used to pour directly into measuring cups. For example, I would mark the 2 oz. and 6 oz. lines in one cup, pour hardener up to 2, and add resin in the same cup up to 6. This would be for a 2:1 product. Stir in that cup, dispense from that cup, wipe it out with a paper towel and do it again.

But the price has come down on digital scales lately, and for $25 you can buy one from Sunbeam that will measure to 1 gram. So measuring by weight now is more attractive than previously, when a decent scale cost over $100. You turn on the scale, put the cup on the plate, and press the Tare button to zero it out. Than pour in the resin, remember how much is in there, press the Tare button to zero it out again (without removing the cup from the plate) and pour in the appropriate weight of hardener. Mix it in the same cup and go. This is especially helpful when fussing with very small batches.

There is one important thing to remember when measuring by weight though. If the mix is resin-rich, it will be weak and flexible; if hardener-rich, it will be weak and brittle. Either way, the operative word is weak. If the density of the resin and hardener are not identical, some detective work and math are required to adjust the ratios.

With MAS epoxies, the resin and hardener are the same density, so you simply use the 2:1 ratio for volume or weight.

With System 3 epoxies, although the volumetric ratio is 2:1, the weight ratio is 100:43.

Other manufacturers will vary. You may be able to work out the adjustment factor from physical characteristics in the MSDS, but the easiest and most accurate method is to call the company and ask a tech rep.