The purpose of glass bedding a stock is two fold, first, it forms a hard plastic shape that conforms microscopically perfectly to the action. This is especially important with recoiling guns so that the wood and steel move together as a unit. Second, it can create hard points for mount bolts. The hard points can be oversize holes filled with bedding compound, or metal sleeves bedded into oversized holes.
The bedding compound is thickened epoxy, and it must be prevented from sticking to the steel with tape, wax, and Polyvinyl Alcohol.
One could use 'Bedding Compound' from sources like Brownells
or make your own as I do:
Laminating resin of the type like West Systems sells at boat supplies or I prefer the low amines hard curing product Tap Plastics sells, Marine Grade 314 resin and 102 fast hardener. Here
In addition you need a thickner. Fine sanding dust can be used all by itself, but I combine it with milled glass fiber and Cabosil, composites industry standard thickeners.
Partall #2 wax: here
PVA Mold release (Polyvinyl Alcohol) here
Small Dixie mixing cups
1/4" dowel stir stick
Acetone for cleanup
Plasticine type modeling clay
Plastic packaging tape
Cheap electronic scale like here
Cheap basic math calculator
You can cover the action with plastic packaging tape, and wax and PVA it per below instructions, but then you won't have a perfect fit of the steel to the stock, it will have that slight gap. However, if your action is painted aluminum you certainly want to cover it with tape first, and an expensive steel action that you're nervous about damaging the blueing of in the slightest, it's best to cover it with tape. If you have an extra action that isn't pristine, you can achieve a perfect fit without the tape.
If using an un-taped steel action, clean any oil on the steel with acetone.
Wax with Partall #2 mold release wax. Wax the entire steel, just in case you touch it with a glued finger (the slightest bit of a glue smudge will come right off when cured). The wax for your car has silicates and makes liquids bead. Partall #2 is a non-silicate wax so liquids sheet. We want that property because:
After wax dries apply PVA with wet paper towel. Put it on somewhat thick (but not necessarily with big runs) it will dry to a microscopic size.
Let PVA dry.
Thickly wax bolts. PVA is not necessary and wouldn't survive threading anyway.
If you're squemish about the whole thing, do a small test dab of epoxy on the waxed and PVA'd surface to prove to yourself it comes off totally easy, which it will.
Use coarse sandpaper and or Dremel drum sander to roughen up inside inletting where epoxy will go.
Drill bolt holes over-size in stock if creating hard points.
Tape off outside of stock.
The action will not remove from the stock if there are any features that would create a 'lock'. Use plasticine type modeling clay to shape around any features like cross pin holes that have recesses to prevent any epoxy locks from occuring.
If using Acraglass follow the directions.
If making your own as I do with Tap Marine Grade system:
Put cup on scale. Turn on scale (so the weight of the cup is not reflected).
Add epoxy resin. Take weight and multiply by 1.25.
Add hardner unti weight is what the calculator says it supposed to be.
Mix with 1/4" wood dowel for 1 minute.
Add fine walnut sanding dust (and milled glass fiber and cabosil if you so wish). Ad material until mix has a peanut butter consistency. You want a stiff consistency that's not going to run out but not so stiff you can't work it.
Note: You want the color of the mixed epoxy to be the color of the oiled wood, not bare wood. Bare wood is a light brown, when oil gets on it it turns quite dark. The eopxy will not change color when oiled. Whatever color it is when you mix it is the color it will be on the finished stock. So try to match the oiled color of the wood, not the raw color.
Apply epoxy (bedding compound) everywhere you want the bedding compound to be. Apply compound inside bolt holes.
Place action in compound in the wood. Bolt in place with waxed bolts. Put more glue in the bolt holes or on the bolts as necessary to make sure they're filled.
Let cure. Remove bolts. Use a rubber mallet if necessary to break action loose in stock (just a little vibrating bumping with the hammer from a few directions is usally sufficient...we're not talking going crazy beating it). You might insert a large action mount bolt in it's hole to tap on.
Sand any epoxy that is proud at the top of the forearm.
Clean wax and PVA from action with oil cloth.