Lifespan of twopart bedding

[holzwurm]

Some years ago (maybe 4 pr 5) I bought a 4oz kit box of Pro-Bed bedding compound sold by Score HIgh Gunsmithing. I needed to do some patching this week and after carefully measuring and mixing I noticed that the material didn't really get hard like I've seen other products harden. Twelve hours later I could still scrape the stuff with my thumbnail. I kept a warm lamp on the spot over night and finally it got as hard as it going to get.

This got me to wondering if this stuff has a shelf life - should I toss it and get a new batch?

[Ben I. Voss]

Don't know about that particular variety, but yes, epoxy does tend to go bad after a while. Sometimes it will tend to be somewhat granular in appearance when it gets old, too. I'd get a new batch!

[D. Taylor Sapergia]

I've reconstituted epoxy resin ) the hardener stays the same as when new), by putting the bottle into hot water.  It returns to a liquid, and seems as good as before.  this is with two part epoxies like 5, 15 30 minute stuff.  It stays liquid for about a year, but eventually goes granular again, so repeat the operation.  I expect it has a shelf life, but so far I just keep using it without problems.

[Gregory]

I've used the microwave to reconstitute the hardner.  10 seconds at atime until it gets smooth again.

Greg

[Acer Saccharum]

I had some old accraglas that didn't really set up properly. Stayed soft. Ten years old, what can I expect? (the epoxy, not me)

[FL-Flintlock]

All compounds have a shelf-life and each varies according to the mixture, components and storage conditions.  The "rated shelf-life" denotes storage in the original un-opened container in ambient conditions of 65-80°F @ 50%RH. 

I can't speak for all products, only for the ones I use and sell.  The hard-setting gray machinable adhesive/filler epoxy has a nominal rated shelf-life of 24 months but I have had it last in the re-closed container well over 48 months without any special storage concerns, 60 months with optimum storage conditions and I've had some go bad in less than 12 months from adverse storage conditions.  The natural (clear) adhesive epoxy is a different mixture being fast-cure and its shelf-life is rated at 12 months but with optimum storage conditions it will easily last >48 months still sealed and >24 months in the re-closed container.  My flexible bedding compound, again being a different mixture, has a rated shelf-life of 24 months but I've gotten better than 36 months from it in a re-closed container sitting in my shop.

Optimum storage conditions vary with the particular product but in general, the "actual" shelf of a re-closed container can be greatly extended by the following:
Get all air out of the container before re-closing it.
Make sure the container is properly closed.
Wrap the whole container in a heavy plastic bag (I prefer freezer bags) and wrap the bag tightly around the container forcing all the air out of the bag then folding the end of the bag over so as to create a seal and secure the bag with some rubber bands.
Put it in the refrigerator where it'll stay cold but not freeze and where the fan is not blowing directly onto it.
Before use, unwrap and allow it to warm on it's own to the ambient temperature of at least 65°F.

The use of heat sources or IR lamps should be avoided unless the product comes with specific force-cure instructions and a comparison of the resultant mechanical ratings of force-cure verses natural cure.  Every product responds differently to force-curing but as a general rule of thumb is that force-curing products not specifically blended to be force-cured will result in greatly reduced mechanical strength, brittleness and premature bonding failure.  Again, all depending on the composition of the product, the rule of thumb is "throw it out" rather than attempting to de-crystallize.  If the product is capable of being de-crystallized, there will a set procedure for such which is normally warming the product to a specific temperature in either a water bath or an explosion-proof dry convection oven for a specified duration of time.  Extreme caution should be used when warming any adhesive/bedding/potting product as many will release highly toxic fumes and/or corrosive agents.

To expand just a bit on how product specifics can vary, the industrial grade cyanoacrylate (liquid or gel) adhesive I sell has a rated shelf-life of 18 months but if the container is either un-opened or properly re-closed and stored in the refrigerator when not being used, the shelf-life becomes indefinite and it's not uncommon to store it for several years without any ill effects on its performance.  Similar products from other mfg's will crystallize or harden while still in the factory sealed OEM package even when stored under refrigeration in <6 months.  Un-cured portion of my flexible bedding compound can be frozen for 30+ days at or below 0°F after the two parts have been mixed for use.

Since this is a gun building site, I can't stress enough how important it is to only use products that carry high thermal variation stability and UV resistance ratings not only for the cured product itself but more importantly for its bond holding ability.  Two other key factors are the un-cured product's bonding ability and bond-holding resistance of the product to petroleum and solvents.  No product is fool-proof but a lot of problems can be avoided by using the best quality products available for the given application.  I'd like to say, "you get what you pay for" but that isn't always the case.  When I started making reinforced stock blanks for suppository guns, I went through many different adhesives.  The particular two-part adhesive I use costs me $200 for a 1.6 gallon total volume kit yet three others that were absolute junk exceeded $240 wholesale of a 1 gallon total volume kit.  On the other hand, you get a 50ml twin-tube of generic 5 minute epoxy at the hardware store for $5, it's not going to perform anywhere near as good as an industrial 5 minute blend that sells for $15

[flehto]

After having some epoxy not harden until days later, I've been using slightly more hardener and it seems to have corrected the problem. Possibly the "stuff" that took so long to harden was too old, but now I "overdose" the mixture w/ hardener as a matter of course but just wish that the manufacturer would provide more hardener vs the resin....Fred

[holzwurm]

Both the resin and hardner putty are stiff but not christalized. I can scoop proportional amouints on the end of a popcycle stick and they mix together pretty well.  I can't tell any diference in viscosity between when I first bought the kit and when I used it a couple of day's ago.

The reason I bought this batch was because it was brown and I was working on walnut stocks. There is a big difference in shade between sanded walnut and the pro-bond stuff. Is there a good product that can be colored successfully during the finishing step?

[FL-Flintlock]

Holzwurm,

Most epoxy & polyester resins will blend with just about any basic pigment but any pigment that is not part of the structural nature of the resin will have negative effects on the cured compound.  The five most common problems are:
Reduced adhesion - The ability of the resin to make/maintain sufficient bond to the work.
Plasticizing - Pigments & dyes will often cause reductions in hardness in cured hard-setting resin and can increase hardness of soft-setting resin resulting in the cured resin being easily displaced/ruptured like common thermoplastics.
Shrink - Mis-matched pigments can often result in shrink on curing often resulting in microfractures throughout the cured resin as well as loss of adhesion as the resin pulls away from the work as it cures.
Brittleness - As the name implies, the cured resin becomes brittle with considerable loss of strength & durability.  Liken this to substituting chunks of styrofoam in place of the aggregate in concrete, the mortar will cure as normal but the strength is greatly reduced by the weak styrofoam.

Credible resin mfg's will readily provide the technical data for a given cured epoxy/polyester resin in both its natural and pigmented/dyed state.  Depending on the particular type of pigment/dye used, no matter if it comes pre-mixed from the mfg or is mixed on-site, more often than not you will see considerable changes in the cured properties of the compound.  Unless the properties of the OEM pre-mix are within acceptable spec's, I strongly suggest using the natural compound and color matching as needed on the surface only.

Most cured hard-setting epoxy/polyester resins can be painted with minimal preparation of simple scuff sanding.  If you're filling a crack where a given portion of the resin will show and it creates a major color differential, you can get a far better color match using artist oil paints (once cured quality artist paints will not be harmed by BLO or finishes like varnish/polyurethane (if using a lacquer finish, the repair should be color matched using tinted lacquer prior to clear/dye coating the entire piece.  Depending on the particular resin, some will take common wood stain at least to a point sufficient for color blending a repair.  What I have done successfully is put some stain into a small cup and let it sit in open air, stirring occasionally and allowing the carrier solvent to evaporate until the stain becomes a fairly thick paste.  Scuff sand the surface of the cured compound and apply a layer of the stain paste, do not wipe, let it dry on the surface.  Repeat paste application as necessary to obtain the color you want, if it's too dark from the first application, very lightly sand it with 1000-1500 grit until you lighten it enough.

Something I didn't mention above, there's a huge difference between "natural resin color" and "natural colored resin".  The natural resin color is the way certain compounds come from the mfg such as with the reinforced gray adhesive epoxy where it's natural use-mixed color is gray because it takes on the combined color of the blended reinforcing components, one in the resin, the other in the activator.  Natural color resin mixes to an amber color and when applied to bare wood, they cure very close to the look of wood finished with any clear top-coat like BLO, polyurethane, varnish, lacquer.  In most cases, you can repair cracks using a natural color structural epoxy without having it show as nothing more than a variation in the grain color or slight spalting. 

[FL-Flintlock]

After having some epoxy not harden until days later, I've been using slightly more hardener and it seems to have corrected the problem. Possibly the "stuff" that took so long to harden was too old, but now I "overdose" the mixture w/ hardener as a matter of course but just wish that the manufacturer would provide more hardener vs the resin....Fred

That's actually a very bad thing to do.  Two-part resins are designed to be mixed within certain proportions to obtain a given result.  Adding more/less activator to any resin completely changes the properties of the cured material and will more often than not result in creating major problems.  Unless the specific compound is designed for unequal mixing, just a 5% variation in mixing ratio can make for major changes in the cured results.  For example, one of the epoxy resins I work with has a shear rating of 3,100 psi with a Shore D hardness of 85 for a 1:1 mixture.  Adding just 5% more activator reduces the shear rating to just 900 psi and makes the cured compound extremely brittle.  Increasing the amount of activator by just 3% in a particular epoxy compound common to gun work results in increasing the shrinkage rate to a point where it'll tear out the wood fibers it bonded to resulting in failure of the joint.  Going to a different blend like my flexible bedding compound, the amount of activator can be varied ± 10% to allow for adjusting the cured hardness from 50 to 90 on the Shore A scale without changing the shear strength or adhesion properties of the cured compound.  Reducing the amount of activator in the polyester resin I work with by 5% will result in a very weak plasticized cure while increasing it by 5% results in its yield strength being reduced by more than 75%.

[Artificer]

Fl-Flinter has written some excellent posts in this thread.  A HEARTY Well Done to you, Sir.

I've been using two part epoxy "glass bedding" compounds, often on a daily basis, for 23 years while on active duty and then at least once a week since.   In 1973, we had given up on Micro Bed and went with Fenwall.  Even when mixing the entire kit at one time, Fenwall required extremely careful and thorough mixing or you would get soft spots in the glass and many holes in the surface.  In 1974, we switched to Bisonite on The Marine Corps Rifle Team and since that was so much superior, we started using it i the NM Rifle rebuild section around 1976.  In the early 1980's, we were given samples of Traveco's "Marine Tex" to try out and that was better than Bisonite.  We were still using that when I retired in 1997 and I continue to use it today.

We also tried Accraglas and Accaglas Gel, but it didn't last as long and remained tight from the pounding of recoil as long as even Bisonite.   In the early 70's, the Marine Corps Rifle Team fired at least 120 rounds a day  through our rifles from about the 20th of May through to the Nationals at Camp Perry.   The only days we didn't fire at least that many rounds was in actual matches and the single day off we got liberty on the 4th of July. So we learned what stood up and what didn't.

I use Accraglas and Accraglas Gel much more now than I did on active duty - this both in modern rifles and for repairs of antique and reproduction blackpowder firearms.  The regular Accraglas is better as an epoxy and the gel is better as a bedding compound. 

When I want the very best in two part epoxy kits, I use Hysol Epoxy Patch Repair Kits.   We used that since the 1970's for gluing many things that we absolutely had to make sure stayed together.  Hysol comes in various kits, and their clear and optically clear kits can not be beaten by anything that is commonly available.  The problem is that you have to pay an MSDS fee when you buy it and that means you have to buy maybe 10 kits or more to spread out the costs.  I buy it about every two years and take orders from other folks I know so we wind up with about 2 to 3 kits apiece.

Working so much with two part epoxies in both my full time Active Duty Career and in my full time job since, I've found some things to be of supreme importance in their use.  That's because I've seen almost every possible way they could fail.

1.  If you want optimum performance, you can not "eyeball" measure even 1:1 ratio mixes.  You have to measure them.  I do this with heavy gage cooking/baking spoons.  The smallest mix I use is with the 1/4 teaspoon size spoon.  I use a low priced Artist's pallet knife to get the epoxy in and out of the spoons.  Then wipe the spoon and pallet knife with an acetone soaked paper towel prior to putting the second resin or hardener material into the spoon.  That keeps you from getting a bad mix or reaction started in some of the epoxy before you mix it all up.  I don't mix the resin and hardener until after it goes on the mixing plate.  If a large quantity is needed, I do it on a thin plate of aluminum about 6" x 9", though that is just the size plate I found.  Something a little smaller will work.  For small amounts of epoxy, I mix it on the SMOOTH side of Brownell's mixing pads.  When finished, I tear off the top sheet off with any remaining epoxy on it.  That tells me when the epoxy is truly set up.  I finally gave up on using scraps of paper and treated paper some time ago as hardener will often soak into the paper and not mix correctly in the epoxy mix.  Here's a link:

http://www.brownells.com/.aspx/pid=1047/Product/EPOXY_MIXING_PADS

I always figure to mix a little more than I need as it is cheaper to throw away a little excess than it is to not have enough epoxy to do the job or have too soft set up or the epoxy doesn't set up right at all.

2.  I "LIVE BY" the storage and open can or tube shelf life information on the containers.   On cheaper two part epoxy kits I get from the "Box" hardware stores,  I won't use any remaining epoxy that has sit for longer than six months here in Virginia.  They can last up to a year in California or dryer climates, but they won't last longer here.  (That's from actual experience working with them all over the world from Okinawa, to Hawaii, to California through Virginia and even in Somalia and the UK.)   I order Accraglas Gel in larger quantities than most people, but only so the open containers will be around no more than about a year.   Marine Tex can be good for up to three or four years in open containers.  BUT, and this is a HUGE BUT, if your shop is not climate controlled and gets cold in the winter, many opened containers of epoxy will not last that long.  When I worked in a garage shop, I brought my epoxy compounds in the house in October and would take them out to use them and return them to the house afterwards until maybe March or April.  Virginia doesn't get as cold as many states but even our winters are cold enough to screw up epoxy in open containers.

Well, guess I have to continue this in another post.

[Artificer]

3.  I can not emphasize strongly enough that you mix two part epoxies thoroughly.  I had to almost literally pound this into the heads of some of my On The Job Trainee's (apprentices) when I was the Instructor of OJT's at the RTE Shop at Quantico, VA. and my Armorers when I was the NM Rifle Section Chief and later.  I have seen so many mixes of epoxy that were screwed up because people didn't mix them thoroughly I can't count them all.  This from both not measuring correctly and from not mixing correctly.

I taught people to use a small putty knife ground down to no more than about 1" wide at the end.  You fold, spread out and mix and fold, spread out and mix and fold, spread out and mix to ensure the epoxy is mixed right.  This so you will not have soft spots and so you won't have air holes on the surface when the epoxy is set up.  Better to mix a bit too long than not long enough.  You  also use the putty knife to FORCE the mixture into where you want it and pack it in at least a couple of times to keep air holes down to the absolute minimum.

4.  As to colouring agents, I use ONLY what the manufacturer's recommend.  I've actually contacted manufacturers to ask about what they recommend to colour their epoxy or if they advise against using any colouring agent.  The Accraglass dye works well with Accraglass, but I've found you can not trust the color of the mixed batch to what the color will be when it hardens.  It always hardens lighter in color than the mixed batch color.  When I am repairing really expensive stocks or antique stocks, I will mix up small test batches to see what will match the colour of the stocks the best before I use it on the stock.

5.  I have experimented for years with adding sawdust to epoxy for patching of stocks.  I found you have to FILE the sawdust or it won't be fine enough.  I wet the exposed wood at the bottom of a patch area with clear glue to ensure the patch will remain stuck to the stock.  Then I fill a patch with a mixture of glue and saw dust.  You have to really pack it in, mound it higher than the hole and place a piece of wax paper over the mound.  Then use wide tape over the wax paper to force the mound down into the hole and keep it tight against the stock and yes it will spread a bit over the stock.  This ensures you won't have air holes in the surface of a patch.  One thing I must warn about though, is that I have found no way to keep such a patch the same color as the wood around it.  It will dry darker than the wood.  So you have to either accept that or colour the top with paints as FL-Flinter mentioned.

[FL-Flintlock]

Artificer,

Thanks for the compliments.  A few things you touched on brought more to mind.  The most common complaints I've seen on bedding compounds is "pounding-out" where the compound separates from the stock and "loosening" from shrink or displacement caused by recoil.

Up until I started working on my own bedding compound, my experience was primarily with industrial/commercial applications and the subsequent testing that's involved with those applications quickly sort what work from what doesn't.  Commercial structural applications correspond best to gun applications because they incorporate a wide variety exposure and operational concerns such as; thermal cycling, impact, vibration, compression, dissimilar material interaction and so forth.  One series of testing was conducted by the company seeking a suitable product thus eliminating any competitor bias, among the worst performing brands were Devcon, Acraglas and Marine-Poxy.

There is no set standard I know of for testing adhesives, most of the testing procedures are tailored to the application.  Compounds that work very well for a given application may not be worth a hoot for a different application.  Example are those products making hardness claims for compounds containing ceramic, stainless steel, fiber or other reinforcing materials.  Whereas the ball pressure penetration hardness or machinability may be very high, subsequent tests such as; impact, vibration and thermal adhesion stability often fail miserably.

When I started R&D work on Ultra-RVC companies primarily sent low-mass potting compounds for samples and while some had good adhesion, they had very little shear strength.  Some with good shear strength had very little adhesive ability.  It took seven years, three of those years working with a single mfg to get the cured properties right then tweak it to be more user friendly such as the ability to measure by either volume or weight.

There are major issues concerning any adhesive/bedding compound being used on a gun application because these are among some of the most demanding in terms of broad-spectrum durability.  In a gun application you've got everything going on all at once such as; thermal shock & exposure extremes, impact, crush, UV exposure, adhesion, dissimilar material compatibility, elongation, ect...  Several years ago I got into a discussion with a gunsmith who wanted to use a cyanoacrylate product to harden checkering.  He read some sales hype on the internet about spraying the wood with accelerator and absolutely insisted that all cyanoacrylates were the same.  A few weeks later, he happened to get a table right across from mine at a gunshow, I couldn't help but ask him how the stock turned out to which his reply is not fit for writing here but his helper came over to me later on and said, "It turned into a complete mess and he had to make a whole new stock."  It's the same with those who insist all epoxies are the same.  You can't put an epoxy compound with an elongation rating of >1% on a wood stock where the wood has an elongation rating of 3% and expect it to hold-up, like anything else, something is going to give-up.   

I agree with you on the need for complete mixing but would point out that it is necessary to check with the mfg as to measuring by volume or by weight.  The polyester resin I use for certain lamination applications can only be measured to the proper 3:2 ratio by weight.  Also, if the compound is too cold, it will usually not mix properly - the minimum mixing temp varies by the blend but general rule of thumb is for the components to be no less than 65°F.

[MSLitho]

Why not use woodworker's system 3 with these properties:

http://www.jamestowndistributors.com/userportal/show_product.do?pid=1479&familyName=System+Three+T-88+Structural+Adhesive