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There are lots of choices

When it comes to epoxies there are lots of choices depending on your knowledge/comfort level. Clearly the kits are very useful in providing just the right stuff to do the job all in one place. But it's all available as separate components elsewhere if you want.

All epoxies are very similar in their original form. Working time is a function of which combination of resin and hardener is used (i.e. packaged together). In general they all take a week to "fully" cure - that doesn't mean they are not functional well before that time. They all shrink some (some more than others) but in the realtively thin films that we use them I doubt that shrinkage is a deciding factor. Still low shrinkage is a good thing and slow cure epoxies tend to shrink a little less than fast. Thin layers tend to reduce any negative impacts of shrinkage. That is, the aggregate shrinkage of multiple thin layers tends to be less than a single thick layer.

They all liberate roughly the same "heat" by volume during the cure. The issue is not really heat - it's temperature build-up. Thick layers generate more heat than thin due to volume and don't dissipate it as well - so thick layers tend reach higher temperatures than thin layers. Fast cure epoxies generate heat faster than slow cure and therefore tend to reach higher temperatures.

There are all kinds of suitable release agents. Floor wax and shoe polish have been used for years with good result. It's best to let them harden and polish them out just like on floors and shoes. The work if you don't polish them but they won't produce as nice a surface. Two to three layers is the conventional recommendation although six to ten will improve the "finish" of he epoxy surface. The only risk is in leaving bare spots. I quite often use vaseline for small parts but have not ventured into Crisco (at least not gun work). Remember that soft wax and these greases will all leave more residue on the epoxy than the specially formulated release agents and may compromise additional epoxy layers. My favorite mold release is the green PVA (PolyVinyl Alcohol?) liquid that comes in most of the kits. This can be found in pints and quarts at most hobby shops and craft stores ( I think I got mine at a "Michaels"). DO NOT EVER USE A SILICONE PRODUCT. Silicones will compromise all future epoxy and finish work.

The classic Devcon putty (in the original post) has been used successfully for years in all sorts of firearms - including M1 Garands. Another putty that has been widely used is Marine-Tex. The slow cure versions of both are very low shrinkage, bond great and work well for bedding rifles. Their main draw back is that the mixing ratio is something like seven to one which is difficult for small batches. Another product I've used a lot for boat work - not bedding - is "Super-Mend" made by Titan resins. It's available at most home improvement centers here on the west coast. It's a one-to-one ratio so small amounts are easy. It's also white so it's very easy to tint. The thin Devcon referred to the in later posts is just straight epoxy resin (i.e. ""Glue").

The difference in all these products are the additives. For those that want to "roll" your own - typical addives to turn resin into a"putty" are coloidal silica ("Cab-O-Sil") a general purpose thickener, talc a high density filler, milled fibers for improved bonding and micro-balloons for light weight fillers. I tend to use Cab-O-Sil and talc. The micro-balloons result in an easily sanded compound like wood filler so I don't use them for bedding. All of this stuff is available at your local marine store.

I usually use West System epoxy since I keep quart quantities on hand for other reasons. It is also a 7/1 ratio (I think) but the cans are fitted with metering pumps so I always get the correct ratio at about a 1 1/2 tablespoons per squirt - so it's just about right for bedding and other small jobs. The other thing I've used a lot of lately is a West Syestem repair kit. This has a number of pre-measured foil packets of resin and hardener in foil packets plus a small tub of micro-balloons, gloves, mix sticks etc. Each packet is more than enough to bed any rifle including sealing the barrel channel with straight resin. The price of this kit is about what you'd pay for a regular bedding kit but, depending on the size kit, you get enough to do lots of bedding jobs and other chores. For a good discussion of epoxy for home use, additives etc try Gudgeons manual on their "West System" epoxy
 
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